Diagnostic value of urine microscopy for differential diagnosis of acute kidney injury in hospitalized patients.

BACKGROUND AND OBJECTIVES: Urine microscopy is the oldest and one of the most commonly used tests for differential diagnosis of acute kidney injury (AKI), but its performance has not been adequately studied in the setting of AKI.

DESIGN, SETTING, PARTICIPANTS, & MEASUREMENTS: Fresh urine samples were obtained from 267 consecutive patients with AKI, and urinary sediment was examined. The cause of AKI was assessed at two time points: (1) Before urine microscopy diagnosis and (2) after patient discharge or death (final diagnosis). A urinary scoring system also was created on the basis of casts and renal tubular epithelial cells (RTEC) to differentiate acute tubular necrosis (ATN) from prerenal AKI.

RESULTS: The urinary sediment scoring system was highly predictive of the final diagnosis of ATN. In patients with a high pretest probability of ATN (initial diagnosis of ATN), any casts or RTEC (score > or =2) resulted in very high positive predictive value and low negative predictive value for a final diagnosis of ATN. In patients with a low pretest probability of ATN (initial diagnosis of prerenal AKI), lack of casts or RTEC on urinary sediment examination had a sensitivity of 0.73 and specificity of 0.75 for a final diagnosis of prerenal AKI. The negative predictive value of lack of casts or RTEC in patients with low pretest probability of disease was 91%.

CONCLUSIONS: Urine sediment examination is a valuable diagnostic tool for confirming the diagnosis of ATN. A score of > or =2 on an ATN urinary sediment scoring system is an extremely strong predictor of ATN.