Generally speaking, speci \201 c gravity and osmolality correlate. An excep- tion is when relatively large particles, such as glucose, proteins, and radio contrast media, are presen t in the urine. These produce an i in speci \201 c gravity, with little change in osmolali ty. Uses Investigation of polyuria and hypo-/hypernatraemic states ( b p. 782). Recurrent stone formers can monitor their own urine speci \201 c gravity to maintain a dilute urine that discourages stone development. Isosthenuria CKD leads to a progressive d in the range of urinary osmolality that the kidne ys can generate. In advanced renal insuf \201 ciency, the osmolality becomes relatively \201 xed at 7 300mOsmol/kg ( 7 1.010 speci \201 c gravity) \200\224 close to that of glom erular \201 ltrate. In this situation, the urine cannot be adequately concentrated or dil uted in response to Na + and water deple- tion and overload, respectively. This is termed i sosthenuria. Urinary pH Urinary pH ranges from 4.5 to 8.0 (usually 5.0 \200\223 6.0), depending on sys- temic acid \200\223 base status. Most people (except vegans) pass acidic urine the majori ty of the time. Isolated urinary pH measurements provide very little useful information. Th eir main clinical use is the investigation of systemic metabolic acidosis. In this situatio n, a fall in urinary pH (to around 5) is expected, as acid is excreted. Failure of this res ponse may indicate renal tubular acidosis ( b p. 824). Most urine dipsticks have an indicat or strip for estimation of pH, but, if a tubular disorder is suspected, a pH meter should b e used. In certain situations, the therapeutic manipulation of urinary pH might be useful (see Box 1.6). URINALYSIS: CHEMICAL ANALYSIS 17 Box 1.6 Therapeutic urinary alkalinization ( X see relevant chapters.) \200 Urinary stone disease (cysteine and urate stones) ( b p. 716). \200 Poisoning ( b p. 909): Salicylates. \200 Barbiturates. \200 Methotrexate. Rhabdomyolysis ( b p. 152). \200 AKI s to cast nephropathy in myeloma ( b p. 622). CHAPTER 1 Clinical assessment 18 Urinalysis: further tests Further dipstick tests Leucocyte esterase and nitrites Increasingly used as indicators of infection. Detection of neutrophil ester- ase activity identi \201 es white cells (pyuria), while the nitrite test exploits the ability of some urinary pathogens (though not all \200\224 notably certain G ram +ve organisms, including Strep. faecalis , Staph. albus, Neisseria gonorrhoeae , as wel l as many Pseudomonas spp. and mycobacteria) to reduce nitrate l nitrite. Positivity requir es an adequate dietary nitrate intake as well as suf- \201 cient bladder dwell time (pref erably >4h). When combined, these methods possess good speci \201 city (i.e. take seri- ously if +ve) t hough only modest sensitivity (i.e. treat a \200\223 ve result with caution if infection i s likely clinically). 2 While they can serve as a useful screening test in at-risk populati ons they are not a substitute for micros- copy and culture. Bilirubin and urobilinogen \200 Conjugated ( 6 water-soluble) bilirubin l biliary excreti on l small bowel l converted to urobilinogen l distal reabsorption l partially excreted in the urine. This means: (i) unconjugated (water-insoluble) bilirubin does not pass into the urine ( 6 d ipstick +ve bilirubin indicates hepatic or cholestatic disease); (ii) the absence of dipsti ck urobilinogen in a jaundiced patient suggests biliary obstruction. Glucose Glycosuria results when tubular reabsorptive capacity for glucose is exceeded (plas ma level >10mmol/L). A valuable screening tool, but less useful for diagnosis and monitorin g of DM. \200\230 Renal \200\231 glycosuria occurs when proximal tubular injury leads to a failure to reabsorb \201 ltered glucose ( b p. 76). Causes of a +ve dipstick for ketones \200 Dipsticks semi-quantitatively detect acetoaceta te (but not B -hydroxybutyrate). A +ve test can be seen in: Diabetic ketoacidosis (and occasionally severe intercurrent illness in T2DM). Prolonged fasting and starvation diets (e.g. Atkins \200\231 diet). \200 Alcoholic ketoa cidosis. \200 Severe volume depletion. \200 Isopropyl alcohol poisoning (hand-rubs, sol vents, and de-icers). URINALYSIS: FURTHER TESTS 19 Urinary test strips A variety of test strips for urinalysis are available. Some have a spe- ci \201 c purpose, e.g. Clinistix (glucose), Hemastix (blood), and Albustix (alb umin). Others cast a wider net, with various combinations of the following: \200 Speci \201 c gravity. \200 pH. \200 Leucocytes. \200 Nitrites. \200 Glucose. \200 Urobi linogen. \200 Bilirubin. \200 Ketones. \200 Albumin or protein. * \200 Blood. * Dip sticks able to detect microalbuminuria are also available. CHAPTER 1 Clinical assessment 20 Urinalysis: protein Introduction \200 Urinary protein excretion should not exceed 150mg/d ay, of which less than 20mg is albumin. The remainder consists mainly of non-serum-derived tubular mucoprotein, such as Tamm \200\223 Horsfall/uromodulin. i excretion of albumin is a sensitive marker of renal, particularly glomerular, disease ( b p. 58). Protein excretion can be measured in untimed ( \200\230 spot \200\231 ) or timed (usually 24h) samples. Proteinuria (total protein) and albuminuria (albumin) are not strictly interchangeable term s. When screening for early renal disease, speci \201 c tests for albumin are preferable. Dipsticks Convenient, highly speci \201 c, but less sensitive. Contain pH-sensitive indi- cators that change color when bound to negatively charged proteins. Predominantly detect al bumin (some are albumin-speci \201 c, e.g. Albustix ) and may not identify large amount s of other proteins, e.g. Bence \200\223 Jones. Dipsticks have completely superseded sulph osalicylic acid turbidity testing. A +ve result occurs with protein excretion \211 300mg/ L. Lower amounts of proteinuria, particularly in the context of diabetes, are termed micro- albuminuria ( b p. 60) (de \201 ned as 30 \200\223 300mg/day) and usually measured by EL ISA or radioimmunoassay (although sensitive dipsticks are available). Dipsticks are semi-quantitative. As a rough guide: 70.15 \200\2230.3g/L 70.3g/L 71g/L 72.5 \200\2235g/L >10g/L Trace + ++ +++ ++++ (cid:2) Changes in urinary concentration affect the result. If volumes are high and the urine dilute, large amounts of protein can go undetected (speci \201 c gr avity may be a clue). Concentrated morning samples are 6 preferable. 2 If +ve dipstick test of \211 1+, repeat after 1 \200\223 2wk. If persistent, verify with a protein/creatinin e ratio (uPCR) or albumin/creatinine ratio (uACR). Timed collections Historically, 24h urine collections were preferred to spot urine samples. However, they suffered from signi \201 cant drawbacks, including inaccurate collection (s ee Box 1.7) and the time required for lab processing. URINALYSIS: PROTEIN 21 Box 1.7 uACR and uPCR (see also b p. 197) \200 Untimed ( \200\230 spot \200\231 ) urine samples are now used virtually universally for the detection and monitoring of proteinuria. Urinary excretion of creatinine generally remains constant ( 7 10mmol/day). uACR and uPCR correct for variations in urinary concentration (caused by changes in hydration) and correlate well with measurements obtained from timed c ollections. A \201 rst morning urine specimen is preferable. \200 2 The relationship between uACR a nd uPCR is non-linear (see Table 1.3). It can be helpful to multiply uPCR x 10 to provide an estimate of 24h excretion, e.g. uPCR of 125mg/mmol approximates to 1.25g/24h. In the USA and in labs where urinary Cr is measured in mg/dL, rather than mmol/L, uACR is expressed as mg/g. \200 Normal: <30mg/g. \200 Microalbuminu ria: 30 \200\223 300mg/g. \200 Overt proteinuria: >300mg/g. uACR and uPCR will underestimate proteinuria when Cr excretion is high (black people, muscu lar build) and overestimate when Cr excretion is low (elderly, cachectic). However, they ar e useful for serial monitoring in individual patients. uACR is more sensitive than uPCR if screening for early disease in high-risk groups (especially diabetes), but total uPCR is an adequate alternative in most c ircumstances. Dipsticks that aim to estimate uPCR await further validation. Table 1.3 Relationship between uACR and uPCR uACR (mg/mmol) uPCR (mg/mmol) g/24h Interpretation 3 \200\22330 <0.3 70.5 71 73.5 Microalbuminuria 750 7100 7350 30 Overt proteinuria 70 300 Nephrotic range Timed 24h collection: what to tell the patient \200 Non-acidi \201 ed, clearly labeled c ontainer. \200 Pick a convenient day with minimum commitments. \200 Discard \201 rst urine void on that day. \200 Start collection \200\224 all subsequent urine into the con tainer (including overnight). First void the following day into the collection. \200 Since Cr excretion, or creatinine clearance (CrCl) ( b p. 33), should be similar in two successive samples from the same pati ent, their measurement in \200\230back to back \200\231 24h collections may enhance relia bility (i.e. disregard the result if CrCl differs greatly between the two). CHAPTER 1 Clinical assessment 22 Urinalysis: red blood cells Introduction Haematuria is de \201 ned (arbitrarily) as the pr esence of two RBCs per high-powered \201 eld (hpf) in spun urine. The amount determines w hether it is visible to the naked eye (macroscopic haematuria) or requires a dipstick or mi croscopy for detection (microscopic haematuria). Causes and inves- tigation are considered later ( b p. 62). Dipstick Hb induces a color change (usually green) in a dye linked to organic per- oxide. D ipsticks detect as little as two RBCs per \201 eld and are at least as sensitive as micro scopic examination (though appreciable false \200\223 ve rate). However, if a dipstick is +ve, it is still desirable to perform con \201 rmatory microscopy. Dipsticks detect Hb and 6 remain +ve, even after RBC lysis. They also detect haemoglobinuri a from intravascular haemolysis as well as myoglo- bin from muscle breakdown ( l though red cells will be absent on micros- copy in both situations). Urine sediment Regrettably, a dying art, but microscopic examination of the urine sedi- men t remains a valuable investigation (see Box 1.8). Urinary RBCs may have variable morphology (see Fig. 1.3). Those that have passed through th e glomerular basement membrane and suffered osmotic stress in the tubules may have an abnor mal appearance, best appreciated under phase contrast microscopy (see Box 1.8). In expert hands, the presence of these \200\230dysmorphic \200\231 red cells can help dis tinguish glomerular from lower urinary tract bleeding. (cid:2) However, the presence of dys morphic cells does not rule out a lower tract lesion. Non-glomerular bleeding: Red blood cells have normal morphology Glomerular bleeding: Red blood cells that pass into the urine through an inflamed or damage d glomerulus may show budding, spiculation or other surface irregularities Fig. 1.3 Morphology of red blood cells. URINALYSIS: RED BLOOD CELLS 23 Box 1.8 Examination of the urine sediment \200 Tell the patient to discard the \201 rst few mL of urine and then collect 20mL into a universal container. Process and analyze the sample within a few hours (red cell lysis!). \200 Centrifuge a 10 mL aliquot at 400 g for 10min. \200 Remove 9.5mL of supernatant with a pipette. \200 Re suspend the pellet in the remaining 0.5mL of urine (gently!). \200 Transfer a drop of res uspended urine to a slide. \200 Cover the sample (unstained) with a coverslip. \200 Exa mine (preferably) with a phase contrast microscope at 160 x and 400 x magni \201 cation. Cellular elements are quantitated as number per high-powered \201 eld. \200 Look also f or casts and other elements. \200 Use polarized light to identify crystals. \200 Clean the microscope, and discard all the urine! \200 In an urgent situation, e.g. for rapid di agnosis of UTI, an unspun sample may be examined. It may be necessary to acidify the urine to prevent precipitation of (view-obscuring) phosphate crystals. CHAPTER 1 Clinical assessment 24 Urinalysis: cells, organisms, and casts Leucocytes Neutrophils: leucocytes are a prominent feature of urinary infection but may also be present in in \202 ammatory renal conditions (GN, TIN). Sterile pyuria refers to the situation wherein leucocytes are seen consistently on microscopy, but subsequent culture is sterile. Causes of sterile pyuria \200 Partially treated UTI or fastidious organism (e.g. Chlamydi a ). \200 Calculi. \200 Prostatitis. \200 Bladder tumor. \200 Papillary necrosis. \200 TIN. \200 TB (send 3 x EMUs, b p. 696). \200 Appendicitis. Lymphocytes: may be a feature of chronic tubulointerstitial disease. Eosinophils: identi \201 ed with Hansel \200\231 s or Wright \200\231 s stain. Associated with TIN ( b p. 580 ) but also occasionally present in other conditions, including RPGN, prostatitis, and ather oemboli. Renal tubular cells: large, oval cells. Present in normal urine but i in tubular d amage (ATN or TIN). Squamous epithelial cells: large cells with small nuclei, often of uret hral origin (or skin/ vaginal contaminant). Transitional epithelial (urothelial) cells: sug gest cystitis. Malignant cells: special stains, immunocytochemistry, and \202 ow cytometr y all assist detection ( b p. 751). Microorganisms \200 Bacteriuria: normal urine is sterile. Simultanous presence of leucocytes suggests true infection, rather than contamination. Gram staining enables initia l identi \201 cation and cell count while culture and sensitivity results are awaited. Fungi: Candida species are most frequently encountered. Typical appearance is that of a small pale green cell, often with visible budding. May result from genital contamination. Risk factors for colonization are: indwelling foreign body (ureteric stents, bladder catheter), DM, antibiotic therapy, and immunosuppression. Trichomonas: oval and \202 agellate (motile if alive). Usually a genital contaminant. Schistosoma haematobium: ova detection is an important technique in endemic areas. URINALYSIS: CELLS, ORGANISMS, AND CASTS 25 Urine culture M,C+S differentiates contamination from true infection and guides treat- ment . A pure growth of >10 5 colony-forming units (cfu)/mL is the conven- tional diagnostic cri terion for urinary tract infection ( b p. 707). Casts Casts are plugs of Tamm \200\223 Horsfall mucoprotein within the renal tubules, with a characteristic cylindrical shape. They are classi \201 ed according to appearance and t he cellular elements embedded in them. Though pro- duced in normal kidneys, they can be val uable clues to the presence of renal disease (see Fig. 1.4). Non-cellular casts \200 Hyaline casts: mucoprotein alone and virtually transparent. A non -speci \201 c \201 nding that occurs in concentrated urine. Granular casts: granular material (aggregates of protein or cellular remnants) is embedded in the cast. Often pathological but non-speci \201 c. Broad or waxy casts: hyaline material with a waxy appearance under the microscope. Form in dilated, poorly functioning tubules of advanced CKD. Cellular casts \200 Red cell casts: 2 virtually diagnostic of GN. \200 White cell casts : characteristic of acute pyelonephritis \200\224 may help to distinguish upper from lower tract infection. Also occur in TIN. Epithelial cell casts: sloughed epithelial cells embedded in mucoprotein. A non-speci \201 c feature of ATN. Also found in GN. \200 Fatty casts: cont ain either lipid- \201 lled tubular epithelial cells or free lipid globules. Distinguished from other casts by \200\230Maltese cross \200\231 appearance under polarized light. Occ ur in the lipid-laden urine of the nephrotic syndrome. Lipids may also appear as droplets o r crystals. When clumped, these are referred to as oval fat bodies . \200 Other casts: un der the right conditions, any constituent of the urine (microorganisms, crystals, bilirubin , or myoglobin) may become entrapped in a mucoprotein cast. CHAPTER 1 Clinical assessment 26 (a) (b) Fig. 1.4 a) Granular cast. b) Red cell cast. Reproduced from Barrett, J, Harris, K, Topham, P. Oxford Desk Reference of Nephrology (2008), with permission from Oxford University Pres s. URINALYSIS: CELLS, ORGANISMS, AND CASTS 27 CHAPTER 1 Clinical assessment 28 Urinalysis: crystals Detected by examining the urine under polarized light (see Fig. 1.5). (cid:2) Most crystals are clinically irrelevant. Uric acid Usually lozenges with a yellow-brown hue. Precipitate at acid pH. A few may be no rmal (e.g. high meat intake), but i quantities may indicate hyperuricosuria. May be present in acute urate nephropathy (tumor lysis syndrome, b p. 160). Calcium oxalate May be monohydrated (ovoid) or bihydrated (pyramidal \200\224 like the bac k of an envelope). Prefer an acidic pH but not always. A few may be normal (spinach and cho colate ingestion) but can denote hypercalciuria or hyper- oxaluria ( b p. 721). A diagnosti c clue in ethylene glycol poisoning in both real life and exams ( b p. 910). Calcium phosphate Heterogeneous in their appearance ( l needles, prisms, stars). Favored by alkaline pH. Might be a risk factor for calcium stone formation. Magnesium ammonium phosphate (triple phosphate) Birefringent prisms ( \200\230 cof \201 n lids \200\231 ). Prefer alkaline pH. If present, exclude a Proteus UTI. Amorphous phosphates Unattractive clumps in (alkaline) urine cooled for storage. No clinica l signi \201 cance. Cystine Hexagonal. Cystine is not a constituent of normal urine so always signi \201 - can t. Prefer acid urine. A marker of cystinuria ( b p. 719). Cholesterol Thin plates with sharp edges. Occur with heavy proteinuria. Drug-induced crystalluria Many drugs can precipitate in the renal tubule. In severe cases t his may cause AKI. \200 Antibiotics: sulfadiazine, amoxicillin. \200 Antiviral agents: aciclovir, valaciclovir, famciclovir, ganciclovir, valganciclovir, and indinavir. Methotrexate. \200 Primidone (a barbiturate). \200 Triamterene. \200 Vitamin C (calci um oxalate deposition). URINALYSIS: CRYSTALS 29 Uric acid Calcium oxalate Bihydrated Monohydrated Calcium phosphate Magnesium ammonium phosphate Cystine Fig. 1.5 Urinary crystals. CHAPTER 1 Clinical assessment 30 Determining renal function Introduction Several aspects of renal function can be measured. The most important is glomerular \201 ltration rate (GFR). Glomerular \201 ltrate refer s to the ultra \201 l- trate of plasma that crosses the glomerular barrier into the urinar y space. GFR is measured per unit time (usually expressed mL/min) and represents the sum of \201 ltration rates in all functioning nephrons ( 6 a surrogate for the amount of functi oning renal tissue). GFR is relatively constant in an individual. d GFR may result from a reduction in nephron n umber or a reduction in the GFR of single nephrons ( d SNGFR). A reduction in number can be compensated for by an increase in SNGFR through elevated glomerular capillary pressure or glomerular hypertrophy ( (cid:2) this means signi \201 cant kidney damage may not initiall y be associated with d GFR). GFR is useful for: \200 Providing a consistent measure of kidney function. \200 Monitor ing progression of CKD (and response to treatment). \200 Forecasting the need for dialysi s and transplantation. \200 Determining appropriate drug dosing in renal impairment ( b p . 874). It provides no information regarding the cause of renal insuf \201 ciency. Measurement of GFR \200 Measured indirectly by evaluating clearance from plasma of a (ren ally excreted) marker substance. Clearance: the volume of plasma from which this substance is removed per unit time. Suitable markers require certain characteristics shown in the following list and may be endogenous (e.g. Cr) or exogenous (e.g. inulin). Inulin, a fructose polysaccharide for which (along with \202 atulence) we have the Jerusalem artichoke to thank, remains the gold standard. Cost and technical consid erations (it requires a continuous infusion) prohibit routine use. Characteristics of an ideal clearance marker \200 Safe to administer, economical, and eas y to measure. \200 Freely \201 ltered at the glomerulus. \200 Not protein-bound (able to distribute in the extracellular space). \200 Present at a stable plasma concentration . \200 No extra-renal elimination. \200 Not reabsorbed, secreted, or metabolized by the kidney. DETERMINING RENAL FUNCTION 31 In clinical practice, GFR is estimated by one of the following means: \200 Serum Cr (and, to a lesser extent, urea). \200 Formulas based on the serum creatinine (estimated or eGF R). \200 Creatinine clearance (CrCl) from a 24h urine collection. \200 Isotopic clearan ce (e.g. EDTA-GFR or DTPA-GFR). Aspects of renal function \200 Glomerular \201 ltration rate (GFR). \200 Tubular func tion (including Na + and K + handling and urinary concentrating/diluting capacity). Acid \200\223 base balance. \200 Endocrine function: Renin \200\223 angiotensin system (RAS). \200 Erythropoietin production. \200 Vitamin D metabolism. Not measured in clinical practice \200 Autocrine ( l production of endothelins, prostagla ndins, natriuretic peptides, nitric oxide). Protein and polypeptide metabolism (e.g. insulin catabolism). CHAPTER 1 Clinical assessment 32 Creatinine Serum creatinine \200 Convenient and inexpensive \200\224 the most commonly u sed indirect measure of GFR. It is estimated that it is measured over 300 million times annually in the USA. Generated from non-enzymatic metabolism of creatine in skeletal muscle. Production is proportional to muscle mass (20g muscle l 7 1mg Cr). Little short-ter m variation in an individual. 7 25% is derived from dietary meat intake (creatine mostly; creatinine if the meat is stewed). d muscle mass (elderly, cachectic) l i Cr production l overestimation of GFR. U cr x V is relatively constant in the formula for CrCl (where V is volume). CrCl = (urineC r x V)/plasmaCr CrCl = constant/plasmaCr So Becomes Hence, serum Cr varies inversely with GFR: d GFR l i SCr (until a new steady state is reach ed). Cr meets many, but not all, of the criteria for a clearance marker. Shortcomings: \200 It is secreted by the proximal tubule ( 7 10 \200\223 20% when GFR is normal), so the amount excreted in the urine exceeds the amount \201 ltered. As GFR falls , there is a progressive i in tubular secretion until saturation occurs at SCr 7 132 \200 \223 176 mol/L (1.5 \200\2232.0 mg/dL); beyond this point, SCr rises as expected (see Fi g. 1.6). It undergoes extra-renal elimination by secretion and degradation in the GI tract. This becomes more important as GFR falls. When d GFR is rapid, it takes time for a steady state to be reached and for SCr to i , i.e. SCr may initially be normal after a catastrophic renal insult. Traditionally measured using the Jaff alkaline pictrate colorimetric assay. However, interference by non-creatinine chromogens created a tradition of overestima ting Cr. Enzymatic methods on modern auto-analysers are generally more accurate, with less variability. \200 Certain substances interfere with Cr, either through competitive inhibition of tubular secretion (cimetidine, trimethoprim, amiloride, spironolactone, triam terene) or assay interference (in the Jaff reaction: bilirubin, ketoacids, vitamin C, gl ucose, and cephalosporins). CREATININE 33 Creatinine clearance CrCl is calculated as: CrCl x plasma creatinine (P Cr ) = urine creatinine (U Cr ) x volume (V) 6 CrCl = (U Cr x V ) / P Cr Example \200 Serum Cr = 106 mol/L \200 Urine Cr = 8800 mol/L \200 Urine volume = 1.2L \200 CrCl = (U Cr x V)/P Cr \200 CrCl = (8800 x 1.2) 106 = 99.6L/day \200 C onventionally, CrCl is shown in mL/min 6 x 1000/1440 (99.6 x 1000)/1440 = 69.2mL/min (normal range: \231\200 : 95 9 20mL/min; \231\202 : 120 9 25mL/min) (cid:2) Limitations \200 Tubular secretion of Cr means that GFR is overestimated. \200 Requires accurate uri ne collection. \200 Heir to all the pitfalls inherent in Cr measurement. \200 Even if c ollections are accurate, there is marked serial variation (15 \200\223 20%). 8 L d / g m 6 , e n n i t a e r c i 4 a m s a l P 2 0 0 40 80 120 160 Glomerular filtration rate, mL/min Fig. 1.6 The relationship between Cr concentration and GFR (measured as inulin clearance) i n 171 patients with glomerular disease. The hypothetical relationship between GFR and Cr is shown in the continuous line, assuming that only \201 ltration of Cr takes place. The br oken horizontal line represents the upper limit of normal of serum Cr (1.4mg/dL or 115 m ol/L). It can be seen that, because of Cr secretion, serum Cr consistently overestimates GF R (reproduced from Shemesh O, Golbertz H, Kriss JP, et al . (1985) Limitations of creatinin e as a \201 ltration marker in glomerulopathic patients. Kidney Int 28 : 830 \200\223 8, with permission from Nature Publishing Group. CHAPTER 1 Clinical assessment 34 eGFR Introduction (see also b p. 196) In the last few years, it has become routine to estim ate and classify kid- ney function and disease using equations based on the serum Cr. This is termed estimated or eGFR. Such equations attempt to correct for the confounding effects of body weight, age, sex, race, and muscle mass. Limitations Still based on Cr and do not take into account tubular secretion, extra-renal e limination, or differences in production between individuals of the same age and sex, or th e same individual over time. Cockcroft \200\223Gault (CG) 1.2 x {140 \200\223 age (yr)} x weight (kg) eGFR (mL/min) = Cr ( mol/L) Multiply x 0.85 in \231\200 to correct for reduced creatinine production. MDRD Widely used internationally and the current basis of CKD classi \201 ca- tion. Develo ped from data in the Modi \201 cation of Diet in Renal Disease (MDRD) study. However, rema ins poorly validated in children (age <18 years), elderly (age >70 years), pregnancy, ethni c groups other than Caucasians and African Americans, and those without CKD. eGFR , in mL/min per 1.73m 2 = (170 x (P Cr [mg/dL]) exp[ \200\223 0.999]) x (Age exp[ \200\223 0.176]) x ((S Urea [mg/dL]) exp[ \200\223 0.170]) x ((Albumin [g/dL]) exp[+0.318] ) Multiply x 0.762 if the patient is \231\200 \200 Multiply x 1.180 if the patient is blac k. \200 Simpli \201 ed version: eGFR = 186.3 x ((serum creatinine) exp[ \200\223 1.154]) x (Age exp[ \200\223 0.203]) x ( 0.742 if \231\200 ) x (1.21 if African American) Notes: (i) to convert Cr from mg/dL to mol/L, x 88.4. To convert urea from mg/dL to mmol /L, x 0.357; (ii) exp = exponential. (cid:2) CG tends to overestimate and MDRD underestimate GFR. Up to 25% of patients will be misclassi \201 ed where either is used to categorize patients according to the KDOQI CKD s cheme ( b p. 192). eGFR 35 CKD-EPI equation \200 The CKD Epidemiology Collaboration has pooled data from multiple studies to produce the CKD-EPI equation. Uses the same variables as MDRD. \200 The study population included individuals with and without kidney disease over a wide range of GFRs. The equation provides a more accurate assessment of GFR in individuals with normal or only slightly d GFR (particularly GFR >60mL/min), resulting in: \200 Lower estimates of population CKD prevalence. \200 More accurate prediction of adv erse outcomes. There are still important limitations: 7 50% will have an eGFR that is >16mL/min different from true measured GFR (although this represents a slightly improvement over MDRD). Although most laboratories currently report an MDRD eGFR, a move to adopt CKD-EPI for routi ne clinical use is likely to be increasingly advocated. The eGFR formulas are available as web-based and downloadable calculators. \200 M http://www.kidney.org/professionals/tools \200 M http://www.renal.o rg/eGFRcalc Several useful Apps are also available, e.g. National Kidney Foundation eGFR calculator, MedCalc, QxMD, MedMath, Epocrates. Reciprocal of plasma creatinine \200 There is an inverse relationship between GFR and SCr : CrCl = constant / SCr Plotting the reciprocal of SCr (1/SCr) against time will often, though not always, produce a straight line, the slope of the curve representing change in G FR with time. A logarithmic plot of SCr can be used in a similar way. \200 This may be useful in two se ttings: Extrapolation of the line can help predict when CKD is likely to reach ESRD and 6 assist timely planning of RRT. A change in the slope of the curve can be used to monitor treatment: a d in the slope indicates slowed progression, while an i may indicate a 2nd ins ult (acute-on-chronic kidney injury). CHAPTER 1 Clinical assessment 36 GFR measurement: other methods Isotopic GFR Several radiopharmaceuticals ( 51 Cr-EDTA, 99m Tc-DTPA, 125 I-iothalamate) compare favorably to inulin for the measurement of GFR. Isotopi c tech- niques are expensive. Method A single IV injection, followed by venous sampling at regular intervals (intervals i , as expected GFR d ). Post-injection, plasma isotope activ- ity d rapidly, as it distribu tes throughout the ECF. A slower exponential decline (renal elimination) then follows, allo wing GFR to be determined. Protocols based on urine collection are also described. With DTPA, renal elimination is often measured with a gamma cam- era positioned directly ov er the kidneys. While not as precise as venous sampling, it enables assessment of each indi vidual kidney \200\231 s contribution to total GFR (termed \200\230split function \200 \231 ) ( b p. 54). Cystatin C \200 The variable nature of Cr production has encouraged the search for altern ative endogenous compounds that might help estimate GFR. Cystatin C, a 13kDa cysteine protease inhibitor, is the most developed of these. Produced by all nucleated cells and freely \201 ltered at the glomerulus before tubular r eabsorption and metabolism. Unlike Cr, it does not undergo tubular secretion, but a degree of extra-renal elimination is likely. Unfortunately, despite early optimism, several important \200\230non-GFR \200\231 determi nants of cystatin C concentration have been identi \201 ed. \200 Advancing age, male sex , diabetes, i BMI, i albumin, and i CRP are all associated with increased levels. Relative high or low cell turnover, e.g. i / d thyroidism or steroid treatment ( (cid:2) transplant patients!), also cause variability. \200 Several cystatin C-based eGFR equations have been formulated (as have equations that incorporate both cystatin C and SCr). \200 At present, there remains considerable uncertainty as to any advantages for GFR estimation, both in the general population and existing CKD. Available d ata are frustratingly inconsistent. \200 A shorter half-life and smaller volume of distri bution led to an expectation that it might be a more useful marker in the non-steady state, particularly AKI . However, results have proved variable. \200 There also appears to be no clear-cut advan tage in the context of low muscle mass. Nonetheless, cystatin C appears to be a better predictor of adverse outcomes than either SCr or eGFR based on the SCr. There is also signi \201 cant current inter-assay and inter-laboratory variability, although progress is being made toward standardization. Overall, it remains to be seen whether it will yet prove itself in selected circumstances a nd 6 move into the clinical mainstream. GFR MEASUREMENT: OTHER METHODS 37 Urea \200 Synthesized in the liver as a means of ammonia excretion. \200 Rate of produc tion not constant (unlike SCr). \200 Inverse relationship with GFR. \200 In \202 uence d by a number of factors independent of GFR (see Table 1.4). Freely \201 ltered at the glomerulus but reabsorbed in the tubules: Urea movement is linked to water (under vasopressin in \202 uence) in the distal nephron. d renal perfusion l i urea reabsorption l disproportionate i urea compared to SCr. May help to differentiate \200\230pre-renal \200\231 renal dysfunction ( b p. 96). Table 1.4 In \202 uences on urea i Ur \200 High dietary protein intake \200 GI bleeding \200 Catabolic states \200 H emorrhage \200 Trauma \200 Corticosteroids d Ur \200 Low protein diet \200 Liver disease \200 Pregnancy Tetracyclines CHAPTER 1 Clinical assessment 38 Renal function in the elderly The kidneys suffer signi \201 cantly at the hands of the age ing process, with senescence associated with progressive glomerular and tubulointerstitial scarring, nephron loss, declining renal function, and downstream conse- quences for systemi c haemodynamics. These changes begin (depressingly) in the 4th decade and hasten during the 5th and 6th, with increasing clinical relevance. Pathology Macroscopic features include thinning of the renal cortex and an overall decrease in renal size. Glomerular Basement membrane thickening, mesangial matrix expansion, capillary loop changes , periglomerular \201 brosis, glomerular hypertrophy, glomeru- losclerosis ( 7 30% by age 80). Tubular Dilatation and atrophy (particularly in outer medulla), macrophage in \201 l tra- tion, and collagen deposition l interstitial \201 brosis. Vascular Intimal hyperplasia ( l stiffness) of interlobular arteries and afferent arteriole s. Pathophysiology \200 Glomerular hyper \201 ltration and hypertrophy l glomerular scleros is and scarring l further glomerular hyper \201 ltration and hypertrophy. \200 Intrarena l RAS activation l i A2 l intrarenal (including podocyte) injury. Endothelial dysfunction l d NO production l hypoxic and ischaemic damage. TGF- B overexpression l \201 brosis. \200 i oxidative stress l tissue damage. \200 AG E accumulation. \200 Arteriolar hyalinosis l failure of autoregulation l glomerular injur y. \200 General biological senescence: telomere shortening, mitochondrial loss, and enhanced apoptosis. Effect on renal function GFR The Baltimore longitudinal study (1958 \200\223 1981) used se rial measurements of CrCl to show a d GFR of 0.75mL/min/year. Recent studies have used inul in clearances to show that, although GFR progressively falls in older age groups ( \231 \202 > \231\200 ), it is not necessarily an inevitability. There may be no demonstrable de cline in normotensive, otherwise healthy, individuals, so it remains important to consider the effect associated comorbidities ( i BP, vascular disease, CCF, etc.) may be having over and above age itself. RENAL FUNCTION IN THE ELDERLY 39 Assessment of GFR SCr is less useful in the elderly because of (i) d muscle mass and (ii) d urinary Cr excretion. This means that formulas based on SCr (including MDRD, Cockcroft \200\223 Gault, and CKD-EPI) tend to underestimate GFR in those age >65 (and are not actual ly robustly validated in this population). Additional effects of renal ageing \200 d renal plasma \202 ow. \200 Microalbuminuria and overt albuminuria more common. \200 Regulation of Na + balance is impaired ( d excre tion l contributes to i BP in the context of a high-salt western diet). i K + more common ( d excretion and d aldosterone). The elderly are 6 vulnerable to the eff ect of any medication interfering with K + excretion (such as ACE-I). Water handling (both diluting and concentrating) ability is impaired. Potential consequences: \200 Nocturia is very common. \200 Inappropriately dilute urine (and reduced thirst response) despite i plasma osmolality l dehydration. Inadequate excretion following water load l hyponatraemia. Impaired distal tubular acidi \201 cation ( l mild acidosis). \200 i EPO production (alt hough the increase in response to a low Hb is actually blunted). Effects on mineral metabolism: i PTH, d calcitriol, and d PO 4 . Clinical relevance \200 d muscle mass in the elderly l d SCr. \200 2 i SCr in an elderl y patient usually represents a signi \201 cant d GFR. \200 Although age-related d GFR is common anyway, it will be exaggerated in the presence of comorbidity, such as i BP and vascular disease. X CKD is common in the elderly: In NHANES (1999 \200\223 2004), nearly 7 45% of elderly subjects had CKD ( b p. 194) by cu rrent criteria, with those aged >70 accounting for 50% of CKD overall. However, is early CKD relevant in this age group? For example, GFR 50 \200\223 59mL/min does not increase CV mortality in comparison to >60mL/min? In view of this, many believe CKD is an unhelpful term in the majority of individuals in this age group. The incidence and prevalence of ESRD is higher in the elderly (mean age of RRT is mid-60s). AKI and acute-on-chronic kidney disease are both more common. \200 The elderly are more v ulnerable to the effects of nephrotoxic drugs. \200 The elderly are also more prone to el ectrolyte imbalances occurring through disease or inappropriate medical management (e.g. indisciplined perioperative \202 uid therapy). CHAPTER 1 Clinical assessment 40 Immunological and serological investigation Introduction Immunological and serological test ing are widely (perhaps too widely) undertaken in the work-up of many forms of renal diseas e. Although help- ful, such tests are rarely diagnostic in isolation and must always be int er- preted in the context of a detailed clinical assessment. There are a few common scenarios where the \200\230renal screen \200\231 is often performed (see Table 1.5): \200 Unexplained AKI ( b p. 119) or progressive CKD. \200 Fo r correlation with histology after a renal biopsy ( b p. 82). \200 During follow-up to as sess disease activity or response to treatment. Table 1.5 Immunological testing in kidney disease Clinical context Investigation ANCA (b p. 640) Anti-GBM (b p. 656) ANA, anti-dsDNA (b p. 659) Complement com ponents C3, C4 AKI where RPGN a possibility (b p. 71). Consider: ANCA, anti-GBM Microscopic haematuria Consider: ANA, anti-dsDNA C3, C4 With consent: HBV, HCV, HIV serology Age >40: Serum protei n electrophoresis (SPEP) Urine protein electrophoresis (UPEP) Serum free light chains (SFLC ) Proteinuria Proteinuria with microscopic haematuria As above, plus: ANCA, anti-GBM ANA, anti-dsDNA, C3, C4 Age >40: SPEP, UPEP, SFLC With consent: HBV, HCV, HIV serology Nephrotic syndrome (with or without evidence of a multisystem disease) Unexplained CKD (particularly older age groups) SPEP, UPEP Suspected lymphoproliferative disorder SPEP, UPEP, SFLC, cryoglobulins Thrombotic microangiopathy (diarrhea-negative) Anti-ADAMTS13 IMMUNOLOGICAL AND SEROLOGICAL INVESTIGATION 41 Complement and the kidney (See Tables 1.6 and 1.7.) \200 Common investigations are immuno assays for C3 and C4 and a functional assay for the total haemolytic component (CH50). \200 C3 and C4 measure circulating amount, while CH50 assesses activation (it measures the 50% red cell haemolysing dose of complement). C3 and C4 can be used to screen for classical and alternative pathway abnormalities, with d iseases associated with high levels of circulating immune complexes associated with hypocom plementaemia. Low C3 implies tissue injury, with activation of either the classical or alternative complement pathway. Low C4 implies activation of the classical pathway (or a hereditary null allele). CH50 is a good screen for overall complement activity (e.g. a patient with recurrent infections).