Fig 2: uNGAL is associated with the duration and severity of acute tubular injury in patients with COVID-19: (a) uNGAL, but not uKIM-1, was associated with sAKI (meeting AKIN criteria for ≥72 hours) in patients with COVID-19. No AKI and transient AKI (<72 hours) levels are found for comparison. (b) uNGAL, but not uKIM-1, was associated with the severity of AKI (AKIN stage); bars represent medians. Notably, mean levels of uKIM-1 were equally elevated in all 4 groups, including in patients with COVID-19 with AKIN stage 0 (no elevation of SCr). (c) ROC curves for uNGAL (shades of red) and uKIM-1 (shades of blue), by ascending AKI severity (AKIN 1–3 vs. 0, AKIN 2–3 vs. 0–1, AKIN 3 vs. 0–2). (d) In non–COVID-19 AKI biopsies, NGAL (LCN2) mRNA is expressed in the distal nephron including AQP2+ collecting ducts (top panel), whereas KIM-1 (HAVCR1) is expressed in proximal tubules and not in AQP2+ collecting ducts (bottom panel). Bars = 50 μM. (e) COVID-19–positive kidney biopsies with widespread acute tubular injury (80%–100% of tubules) had extensive expression of NGAL in a noncanonical distribution, in LRP2+ and KIM-1+ proximal tubules (top panels), whereas COVID-19 kidney biopsies with limited ATI (30% of tubules) exhibited limited NGAL-KIM-1 overlap (bottom right panel). KIM-1 was expressed only in AQP2− proximal tubules (bottom left panel). Tissue sections were counterstained with hematoxylin. Bars = 20 μM. AKI, acute kidney injury, AKIN, Acute Kidney Injury Network; ATI, acute tubular injury; NS, not significant; ROC, receiver operating characteristics; sAKI, sustained acute kidney injury; SCr, serum creatinine; uKIM-1, urinary KIM-1; uNGAL, urinary neutrophil gelatinase-associated lipocalin.

Fig 3: Comparison of the CKD population with healthy controls for urinary (A) calprotectin, (B) NGAL, (C) KIM-1, and (D) ACR. Data are presented as scatter plots (logarithmic Y-axis, medians are indicated by horizontal lines). Significant differences were ***p < 0.001 and **p < 0.01 by Mann–Whitney testing. NGAL: Neutrophil gelatinase-associated lipocalin; KIM-1: kidney injury molecule-1; ACR: albumin/creatinine ratio; CKD: chronic kidney disease.

Fig 4: Higher urinary NGAL levels are associated with critical illness and death in patients with COVID-19: (a) Urinary NGAL levels were associated with AKI and sustained AKI (>72 hours) after adjustment for age, sex, race, and ethnicity (minimally adjusted model, blue) and baseline SCr and preexisting comorbidities (fully adjusted model, red; n = 371). Urinary NGAL levels were also associated with secondary outcomes of death, dialysis, shock, and respiratory failure in both minimally and fully adjusted models, N = 440. In contrast, uKIM-1 level was not associated with AKI or any secondary outcomes except for respiratory failure. ORs and HRs are expressed per 1 unit of SD of biomarker distribution; 95% CI. (b) Kaplan-Meier survival analysis reveals survival differences by tertile of urinary NGAL levels measured by ELISA or (c) by 3 levels of urinary NGAL dipstick test (unadjusted P values provided for both b and c; N = 440). AKI, acute kidney injury; ELISA, enzyme-linked immunosorbent assay; HR, hazard ratio; NGAL, neutrophil gelatinase-associated lipocalin; OR, odds ratio; SCr, serum creatinine; uKIM-1, urinary KIM-1.

Fig 5: Classification accuracy of the AKI peptide marker pattern and the other AKI predictive markers NGAL, KIM-1 and EuroSCORE II for the AKI-associated endpoints nonocclusive mesenteric ischemia (NOMI) and death in comparative ROC analysis. Shown is a receiver operating curve (ROC) comparison of the different prognostic markers for the endpoints NOMI (upper left panel) and death (upper right panel) and a table summarizing the corresponding ROC-derived area under the curve (AUC) values of the different prognostic markers for these two AKI-associated endpoints. AKI acute kidney injury, KIM-1 kidney injury molecule-1, NGAL neutrophil gelatinase-associated lipocalin