Association of postoperative proteinuria with AKI after cardiac surgery among patients at high risk

Abstract

Background and objectives: Preoperative proteinuria is associated with a higher incidence of postoperative AKI. Whether the same is true for postoperative proteinuria is uncertain. This study tested the hypothesis that increased proteinuria after cardiac surgery is associated with an increased risk for AKI.

Design, setting, participants, & measurements: This prospective cohort study included 1198 adults undergoing cardiac surgery at six hospitals between July 2007 and December 2009. Albuminuria, urine albumin-to-creatinine ratio (ACR), and dipstick proteinuria were measured 0-6 hours after surgery. The primary outcome was AKI, defined as a doubling in serum creatinine or receipt of acute dialysis during the hospital stay. Analyses were adjusted for patient characteristics, including preoperative albuminuria.

Results: Compared with the lowest quintile, the highest quintile of albuminuria and highest grouping of dipstick proteinuria were associated with greatest risk for AKI (adjusted relative risks [RRs], 2.97 [95% confidence interval (CI), 1.20-6.91] and 2.46 [95% CI, 1.16-4.97], respectively). Higher ACR was not associated with AKI risk (highest quintile RR, 1.66 [95% CI, 0.68-3.90]). Of the three proteinuria measures, early postoperative albuminuria improved the prediction of AKI to the greatest degree (clinical model area under the curve, 0.75; 0.81 with albuminuria). Similar improvements with albuminuria were seen for net reclassification index (0.55; P<0.001) and integrated discrimination index (0.036; P<0.001).

Conclusions: Higher levels of proteinuria after cardiac surgery identify patients at increased risk for AKI during their hospital stay.

Figure 1.

Final selection of study population from patients meeting inclusion and exclusion criteria.

Figure 2.

Receiver-operating characteristic curve of AKI with first measure of postoperative proteinuria. AKI was defined as the receipt of acute dialysis or the doubling of serum creatinine during the hospital stay. Urine albumin-to-creatinine ratio: to convert from mg/g to mg/mmol, multiply by 0.113. ACR, albumin-to-creatinine ratio; AUC, area under the receiver-operating characteristic curve; LR+, positive likelihood ratio; LR−, negative likelihood ratio; NPV, negative predictive value; PPV, positive predictive value.

Figure 3.

Trend of albuminuria and urine creatinine over time. AKI is defined as receipt of acute dialysis or a doubling in serum creatinine during the hospital stay. Median time to AKI was 3 days (interquartile range, 2–3.5 days). Day 1 is the day of surgery, with time 0 representing the point when the patient arrived in the postoperative intensive care unit. Median values for urine albumin and urine creatinine are presented.