Laboratory Findings
- Risk factors, history, and results of physical examination will help in the selection of appropriate laboratory tests.
- Urinalysis is the most important noninvasive diagnostic test.
- Thorough examination of a freshly voided or bladder-catheterized urine sample helps distinguish prerenal from renal causes.
- Normal or near-normal urinalysis, with few cells, few or no casts, or little or no proteinuria, is seen in prerenal disease, obstruction, and some cases of acute tubular necrosis.
- A sample showing muddy-brown granular casts and epithelial cell casts strongly suggests acute tubular necrosis.
- Erythrocyte casts are diagnostic of glomerulonephritis.
- Proteinuria indicates glomerular disease.
- Urinary indices important for diagnosis of oliguria
- Urinary sodium
- < 10 mEq/L in oliguria resulting from intravascular volume depletion
- Neonates: prerenal disease is associated with urine sodium concentration < 20–30 mEq/L.
- Specific gravity
- > 1020 in prerenal oliguria
- Creatinine
- Urine/plasma creatinine ratio > 40 in prerenal oliguria
- Urine/plasma creatinine ratio < 20 if renal cause
- Osmolality
- Urine/plasma osmolality > 1.5 in prerenal oliguria
- Urine/plasma osmolality < 1.5 if renal cause
- Fractional excretion of sodium
- < 1% suggests reabsorption of almost all filtered sodium in response to decreased renal perfusion (prerenal).
- In acute tubular necrosis: > 2%
- Blood urea nitrogen (BUN) and serum creatinine
- In prerenal oliguria, increased BUN level is marked and the BUN/serum creatinine ratio is > 20.
- BUN/creatinine ration of 10–15 suggests intrinsic renal damage.
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