Acute tubular necrosis laboratory findings

CBC, urinalysis with sediment microscopy, urine electrolytes, osmolarity, serum electrolytes, blood urea nitrogen and serum creatinine, and urine dipstick are commonly performed in patients to evaluate acute tubular necrosis and other causes of acute renal failure. Urine sediment may show tubular epithelial cells and epithelial cell casts or brown muddy granular casts. Increased urine sodium concentration >40 mEq/L, urine fractional excretion of sodium greater than 2 percent along with elevated serum creatinine concentration at a rate greater than 0.3 mg/dL/day may be found in acute tubular necrosis. However, these tests may have some limitations.

Patients with acute tubular necrosis may show following findings on laboratory tests. These include:

• Complete blood count (CBC):
  • Anaemia may be due to decreased erythropoietin production.
  • Abnormal platelets
• Urinalysis:^[1][2]
  • Urine sediment microscopy may show muddy brown granular casts from necrotic tubular cells binds with tamm horsfall protein or epithelial cell casts, and renal tubular epithelial cells. Acute tubular necrosis caused by rhabdomyolysis and hemolysis, urinalysis may show heme-positive urine with out erythrocytes.
  • Urine sodium concentration:^[3] Urine sodium concentration is high >40 mEq/L
  • Urine fractional excretion of sodium concentration:^[4] > 2%
    • Limitations of the fractional excretion of sodium percent: Some conditions include, radio contrast media induced nephropathy, acute tubular necrosis in the presence of cirrhosis and congestive heart failure, and rhabdomyolysis may have <1%.
  • Urine osmolarity:^[3] <350 mosmol/kg
• Serum electrolyte abnormalities: Hyperkalemia, hyperphosphatemia, hyponatremia, hypocalcemia, and hypomagnesemia may be found.^[5]
• Urine/plasma creatinine ratio: <20^[3]
• Fractional excretion of urea: >35%
• Urine/plasma urea nitrogen:<3
• Ratio of BUN/sr.creatinine: 10 to 15:1 and is normal.
• Rate of creatinine concentration in serum rise: Serum creatinine levels are increased progressively at a rate greater than 0.3 to 0.5 mg/dL/day.
• Specific gravity: Low <1.020
• Urine dipstick test: Commonly performed for blood pigments, proteins, WBC, nitrites, and glucose in all patients with suspected renal failure to exclude certain etiologies such as infection, hemolysis, myoglobinuria, and rhabdomyolysis.

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