Evidence of functional renal hypertrophy becomes apparent very soon ater conception. Glomeruli are larger, although cell numbers do not grow (Strevens, 2003). Pregnancy-induced intrarenal vasodilatation develops, and both aferent and eferent resistances decline. This leads to greater efective renal plasma flow and glomerular filtration (Helal, 2012; Hussein, 2014). By 12 weeks' gestation, the glomerular filtration rate (GFR) is already augmented 20 percent above nonpregnant values (Hladunewich, 2004). Ultimately, plasma low and GFR rise by 40 and 65 percent, respectively. Consequently, serum concentrations of creatinine and urea drop substantively across pregnancy, and values within a nonpregnant normal range may be abnormal for pregnancy (Appendix, p. 1257). Other alterations include those related to maintaining normal acid-base homeostasis, osmoregulation, and fluid and electrolyte retention.
• Assessment of Renal Function During
Urinalysis results are essentially unchanged during pregnancy, except for occasional glucosuria. Although protein excretion normally rises, it seldom reaches levels that are detected by usual screening methods. Higby and colleagues (1a994) reported 24-hour protein excretion in pregnancy to be 11a5 mg/d with a 95-percent confidence level of 260 mg/d. Values did not signiicantly difer by trimester (Fig. 4-14, p. 67). Albumin constitutes only a small part of total protein excretion and ranges from 5 to 30 mg/d. Airoldi and Weinstein (2007) concluded that proteinuria must exceed 300 mg/ d to be considered abnormal. Many consider 500 mg/d to be important with gestational hypertension. Investigators have correlated a urinary proteinto-creatinine ratio of :0.3 in a spot urine sample-ideally from a irst morning void-with a 24-hour protein excretion rate of :300 mg (Kuper, 2016).
In one study, 3 percent of 4589 nulliparas screened before 20 weeks had idiopathicihematuria, deined as 1 + or greater blood on urine dipstick (Stehman-Breen, 2002). These women had a twofold risk of developing preeclampsia. In another study of 1000 women screened during pregnancy, the incidence of dipstick hematuria was 15 percent (Brown, 2005). Most women had only trace levels of hematuria, and the falsepositive rate was 40 percent.
If the serum creatinine level in pregnancy persistently exceeds 0.9 mg/dL (75 LmollL), then intrinsic renal disease is suspected. In these cases, some determine the creatinine clearance as an estimate of the GFR. Of other assessment tools, sonography provides imaging of renal size, relative consistency, and elements of obstruction (see Fig. 53-1). Magnetic resonance (MR) imaging of renal masses provides excellent anatomic information (Putra, 2009). Full-sequence intravenous pyelography is not done routinely, but injection of contrast media with one or two abdominal radiographs may be indicated by the clinical situation. The usual clinical indications for cystoscopy are followed. Ureteroscopy is another available tool when indicated.