22 - Urine Osmolality

Urine osmolality is an important test for the concentrating ability of the kidney. Interpretation of urine osmolality must always be made in the light of the appropriate physiological response to the state of hydration of the patient. The test is useful in the following areas:

• For determining the differential diagnosis of hyper- or hyponatraemia.
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• For identifying SIADH (urine osmolality > 200 mmol/kg, urine sodium > 20 mmol/L, low serum sodium, patient not dehydrated and no renal, adrenal, thyroid, cardiac or liver disease or interfering drugs)
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• For differentiating pre-renal from renal kidney failure (high urine osmolality is consistent with pre-renal impairment, in renal damage the urine osmolality is similar to plasma osmolality).
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• For identifying and diagnosing diabetes insipidus (low urine osmolality not responding to water restriction).