Association between urine microscopy and severe acute kidney injury in critically ill patients following non-cardiac surgery: a prospective cohort study

Background: Acute kidney injury (AKI) is a common and adverse complication following non-cardiac surgery. Evidence have shown urine microscopy could help early detection, differentiating the causes and predicting the progression of AKI. However, little evidence is available on AKI after non-cardiac surgery. Thus, we investigated the association between urine microscopy and severe AKI in critically ill patients after non-cardiac surgery. Methods: This was a single-center prospective cohort study. The primary outcome was severe AKI, defined as stage 2 or 3 according to maximal KDIGO criteria within 7 days following non-cardiac surgery. Urine microscopy immediately, 6 and 12 hours after surgical intensive care unit (SICU) admission were examined and graded by a urine microscopy score (UMS) based on the observed quantification of renal tubular cells and casts in the sediment. Then, multivariate Logistic regression models were used to analyze the associations between UMS and postoperative severe AKI. Results: From May 20, 2019 to November 24, 2020, 661 patients were enrolled with 147 patients (22.2%) developing postoperative severe AKI. Multivariate Logistic regression model showed elevated UMS (>= 1) 6 and 12 hours after SICU admission were independently associated with postoperative severe AKI (OR 2.200, 95% CI: 1.182-4.095, P=0.013 and OR 2.949, 95% CI: 1.657-5.248, P<0.001, respectively). Furthermore, higher UMS 6 hours after SICU admission demonstrated correlation with greater risk of severe AKI with OR 3.887 (95% CI: 1.430-10.563) for UMS >= 3 and OR 2.429 (95% CI: 1.237-4.770) for UMS =1-2. The specificity and sensitivity of UMS >= 1 for severe AKI was 93.8% (95% CI: 91.7-95.9%) and 15.6% (95% CI: 9.7-21.5%), respectively. While the negative and positive predictive value was 79.5% (95% CI: 76.3-82.7%) and 41.8% (95% CI: 28.8-54.8%), respectively. In addition, patients with higher UMS (>= 3, 1-2 and 0) had significantly more postoperative complications and longer SICU stay; and they also showed a trend toward other adverse postoperative outcomes. Conclusions: Early urine microscopy was independently associated with severe AKI in critically ill patients following non-cardiac surgery with higher UMS related to greater risk.