Diagnostic Utility of Serial Microscopic Examination of the Urinary Sediment in Acute Kidney Injury

Background: Microscopic examination of the urinary sediment (MicrExUrSed) is an established diagnostic tool for AKI. However, single inspection of a urine specimen during AKI is a mere snapshot affected by timing. We hypothesized that longitudinal MicrExUrSed provides information otherwise not identified in a single inspection.

Methods: MicrExUrSed was undertaken in patients with AKI stage ≥2 and suspected intrinsic cause of AKI seen for nephrology consultation over a 2-year period. MicrExUrSed was performed on the day of consultation and repeated at a second (2-3 days later) and/or third (4-10 days later) interval. Cast scores were assigned to each specimen. Chawla scores (CS) 3-4 and Perazella scores (PS) 2-4 were categorized as consistent with acute tubular injury (ATI), whereas CS 1-2 and PS 0-1 were categorized as nondiagnostic for ATI (non-ATI). Nonrecovering AKI was defined as a rise in serum creatinine (sCr) ≥0.1 mg/dl between microscopy intervals.

Results: At least two consecutive MicrExUrSed were performed in 121 patients (46% women, mean age 61±14, mean sCr at consult of 3.3±1.9 mg/dl). On day 1, a CS and PS consistent with non-ATI was assigned to 64 (53%) and 70 (58%) patients, respectively. After a subsequent MicrExUrSed, CS and PS changed to ATI in 14 (22%) and 16 (23%) patients. Thus, 20%-24% of patients only revealed evidence of ATI after serial MicrExUrSed was performed. Patients with nonrecovering AKI were more likely to change their PS to the ATI category (odds ratio, 5.8; 95% CI, 1.7 to 19.3; P=0.005 and positive likelihood ratio, 2.0; 95% CI, 1.3 to 2.9).

Conclusions: Serial MicrExUrSed revealed diagnostic findings of ATI otherwise not identified in a single examination. A repeat MicrExUrSed may be warranted in patients AKI of unclear etiology that are not recovering.