At a Glance Why Get Tested? To determine whether the osmolality
of the blood (or urine) is within normal limits When to Get
Tested? When a patient cannot produce concentrated urine,
particularly following a head injury; when Diabetes Insipidus is
suspected; when symptoms of inappropriate anti-diuretic hormone
secretion are present; when an explanation of a low serum sodium
is being sought A blood sample drawn from a vein in your arm; a
urine sample taken at the same time usually helps the doctor to
interpret the results The Test Sample What is being tested?
Osmolality is a measure of the number of particles dissolved in
a kilogram of fluid. Osmolarity is the number of particles in a
litre of fluid. Osmolality and osmolarity values are
approximately the same. Normally, the major particles
contributing to osmolality are sodium and potassium salts,
glucose and urea and the osmolatity is approximately: This
simple calculation is useful because sodium, potassium, glucose
and urea are more frequently and more easil