Tubular Damage Biomarkers Are a Useful Tool for Identifying Early Renal Injury in Long COVID.

Patients without overt glomerular dysfunction may develop tubular injury, referred to as subclinical acute kidney injury. The burden of COVID-19-related renal damage may therefore be underestimated, as current KDIGO criteria do not include tubular damage biomarkers (TDBs). This study evaluated kidney injury in patients with long COVID by assessing TDBs alongside glomerular biomarkers, proteinuria (UPCr) and albuminuria (UACr). In this cross-sectional study, 75 patients without prior chronic kidney disease were recruited from a long COVID outpatient clinic and stratified according to the time since SARS-CoV-2 infection into 6-, 12-, and 24-month post-COVID-19 groups (referred to as 6-, 12-, and 24-MPC, respectively). Overall, 49.3% of patients had normal estimated glomerular filtration rate (eGFR >90 mL/min/1.73 m2), 34.7% showed mildly reduced eGFR (90-60), and 16% exhibited marked eGFR reduction (<60). Among patients with normal eGFR, the combined mean prevalence (mean ± SD) of abnormal TDBs, UACr, and UPCr was 29.7 ± 4.9%, indicating early tubular injury. Temporal analysis revealed a higher prevalence of TDB abnormalities at 6-MPC, whereas glomerular dysfunction was more pronounced at 24-MPC. These findings suggest that renal injury in long COVID is more prevalent than previously recognized and that TDB assessment may improve early detection of kidney damage.