Urine Sediment Findings in Patients With COVID-19-Associated Renal Injuries

Patients with COVID-19 can suffer from renal complications, especially acute kidney injury (AKI). AKI is a frequent complication in patients with critically severe COVID-19, and a meta-analysis report has shown that the average incidence of AKI was 11% among all subjects with COVID-19 and 23% among critically ill patients.

Urinalysis and urine sediment examinations are the oldest and most commonly performed tests for assessing the pathophysiological condition of the kidney/urethral system. Urinalysis findings such as proteinuria, hematuria, and leukocyturia are reportedly associated with COVID-19-associated renal injuries and the severity of COVID-19.

Clinical Laboratory Scientists at the University of Tokyo Hospital (Tokyo, Japan) collected residual urine samples from 88 subjects with COVID-19 (total: 279 samples; average: three samples per patient; range: 1–16 samples) diagnosed using RT-PCR. The also collected urine samples from 78 subjects (total: 147 samples; average: two samples per patient; range: 1–9 samples) with acute renal injuries caused by non-COVID-19 factors, who were admitted to an emergency or intensive care unit (ICU).

The urine sediment examination was performed using manual microscopy. For the measurement of urinary clinical markers, the team used the following reagents: urinary creatinine (uCr) was measured using an enzyme assay (L-type Wako CRE • M; FUJIFILM Wako Pure Chemical, Osaka, Japan); total protein (TP) (Micro TP-test Wako) was measured using the pyrogallol red method; microalbumin (µAlb) (Auto Wako microalbumin; FUJIFILM Wako) was measured using a turbidimetric immunoassay (TIA); N-acetyl-β-D-glucosaminidase (Ltype Wako NAG) was measured using an enzyme assay; α1-MG (LZtest ‘Eiken’ α1-M; Eiken Chemical, Tokyo, Japan) was measured using latex agglutination turbidimetry;

Liver type fatty acid-binding protein (L-FABP) (NORUDIA L-FABP; SEKISUI MEDICAL, Japan) was measured using latex agglutination turbidimetry; and NGAL (U-NGAL Abbott; Abbott Park, IL, USA) was measured using a chemiluminescence immunoassay (CLIA). Urinary specific gravity (SG) was measured using an Eiken Chemical automated urine chemical analyzer US-3500 and urinary sodium was measured using an electrode method.

The investigators reported that that the number of urine sediment particles and the levels of N-acetyl-β-D-glucosaminidase, α1-microglobulin, liver type fatty acid-binding protein, and neutrophil gelatinase-associated lipocalin were associated with the severity of COVID-19. In addition, they observed that the number of granular casts, epithelial casts, waxy casts, and urinary chemical marker levels were lower in the subjects with COVID-19 than subjects without COVID-19 with acute renal injuries when the subjects were classified according to their renal function.

The authors concluded that the urinary sediment findings were milder in subjects with COVID-19 when compared according to their renal functions, suggesting that pre-renal injury factors might be largely involved in the pathogenesis of COVID-19-associated renal injuries than non-COVID-19-associated renal injuries in patients with AKI arising from surgery or sepsis. The study was published on April 1, 2022 in the International Journal of Infectious Diseases.

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