The urine-to-plasma osmolality ratio can help in the diagnosis of renal dysfunction and polyurias. It is more accurate than urine osmolality or urine specific gravity alone in the distinction of prerenal azotemia from acute tubular necrosis. It is related to the osmolal clearance.
Specimen Collection:
• serum and random urine with normal fluid intake
• serum and random urine after 12 hour fluid restriction
urine-to-plasma osmolality =
= (urine osmolality in mOsm per kg water) / (plasma osmolality in mOsm per kg water)
urine-to-serum osmolality =
= (urine osmolality in mOsm per kg water) / (serum osmolality in mOsm per kg water)
Interpretation:
• urine osmolality depends on urine intake; with normal fluid intake it is normally 300-900 mOsm per kg H2O but after 12 hour fluid restriction it is normally > 850
• serum osmolality is normally 275-295 mOsm/kg H2O, with values down to 266 in neonates and up to 301 in persons over 60 years of age
• during normal fluid intake, the ratio is normally 1.0 - 3.0
• after fluid restriction, the ratio is 3.0 - 4.7
• in prerenal azotemia, there is an increase in urine specific gravity and urine osmolality resulting in an increased ratio; values of urine specific gravity > 1.030 and of urine osmolality > 500 mOsm/(kg water) are diagnostic.
• in acute renal failure due to causes other than prerenal conditions, the urine osmolality is usually < 350 mOsm/(kg water) so that the ratio is decreased
• in diabetes insipidus, the ratio before and after fluid restriction is in the order of 0.2-0.7, with the urine osmolality decreased
• in diabetes insipidus with severe dehydration, the ratio may be > 1.0 due to increased serum osmolality (> 320 mOSm/kg H2O)
• in polyuria of neurogenic origin, the ratio before fluid restriction will be normal but the ratio will increase after fluid restriction
• in water intoxication the ratio is about 0.5, since the serum osmolality is decreased
• after diuretic therapy the ratio is about 1.0
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Diagnosis
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Without Fluid Restriction
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With Fluid Restriction
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normal
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1.0-3.0
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3.0-4.7
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prerenal azotemia
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elevated, rarely < 1.1
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acute tubular necrosis
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0.9-1.05, rarely > 1.5
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polyuria of diabetes insipidus
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0.2-0.7
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0.2-0.7
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diabetes insipidus with severe dehydration
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may be > 1.0
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polyuria of neurogenic origin
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normal
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increases
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water intoxication
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0.5
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increases
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