To investigate the relationship between the urinary complement component 3a (C3a) and the risk of acute kidney injury (AKI) and 90-day mortality in critically ill patients, and its potential predictive significance.

The prospective study included 221 patients with critical illness admitted to the ICU of Baoji People's Hospital from March 2020 to December 2022. Enzyme-linked immunosorbent assay was used to detect the urinary C3a level of patients at admission. The primary endpoint was the incidence of AKI within 48 hours after admission to the ICU. According to whether AKI occurred, all critically ill patients were divided into an AKI group and a non AKI group. The secondary endpoints included all-cause mortality at ICU and 90 days, dialysis dependence at discharge, ICU length of stay, and total length of stay.

The median urinary C3a level in the AKI group was significantly higher than that in the non AKI group [7.73 (6.02, 10.14) μg/L vs. 3.68 (2.72, 5.32) μg/L, Z = 7.199, P < 0.001]. The higher urinary C3a level was an independent influencing factor for the occurrence of AKI in critically ill patients [odds ratio = 1.083, 95% confidence interval (CI) (1.014, 0.155), P = 0.017]; the area under the receiver operating characteristic curve for predicting AKI was 0.791 [95%CI (0.729, 0.852), P < 0.001], the optimal cutoff value was 6.06 μg/L, and the sensitivity and specificity were 75.31% and 80.00% respectively. In terms of short-term prognosis analysis, the higher urinary C3a level was also an independent influencing factor for the 90-day mortality in critically ill adult patients [hazard ratio = 1.046, 95%CI (1.023, 1.069), P < 0.001]. Patients were divided into a low-level urine C3a group (≤ 5.28 μg/L, 110 cases) and a high-level urine C3a group (> 5.28 μg/L, 111 cases) based on the median value. Compared with patients with low urinary C3a levels, patients with high urinary C3a levels had a lower 90-day survival rate (log rank χ² = 50.668, P < 0.001). In addition, compared with patients with low urinary C3a levels, patients with high urinary C3a levels had a higher ICU mortality rate [10.91% (12/110) vs. 47.75% (53/111), χ² = 36.110, P < 0.001], a higher proportion of dialysis dependence [2.73% (3/110) vs. 9.91% (11/111), χ² = 4.804, P = 0.028], longer ICU duration [8.00 (5.00, 14.00) d vs. 18.00 (12.75, 38.00) d, Z = 2.587, P = 0.010], and longer total hospital duration [10.00 (6.00, 18.00) d vs. 24.50 (17.00, 44.00) d, Z = 2.647, P = 0.008].

The higher urinary C3a levels at admission are associated with an increased risk of AKI and 90-day mortality, and have the potential to serve as a predictive biomarker for AKI and 90-day mortality risk in critically ill patients.