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

## TL;DR

This study shows that measuring tubular damage biomarkers can detect early kidney injury in long COVID patients, even when other kidney tests appear normal.

## Contribution

The study introduces the use of tubular damage biomarkers to detect early kidney injury in long COVID, revealing higher prevalence than previously recognized.

## Key findings

- 49.3% of long COVID patients had normal eGFR but showed signs of early tubular injury.
- Tubular damage biomarkers were abnormal in 29.7% of patients with normal eGFR.
- Tubular injury was more common at 6 months post-infection, while glomerular dysfunction increased at 24 months.

## Abstract

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.