Preoperative factors (age, sex, race, height, weight, medical comorbidities, and preoperative laboratory values), intraoperative factors (surgical duration and procedure type), and complications (progressive renal insufficiency and acute renal failure) were extracted from NSQIP and included in this study. NSQIP collects data for 30 days postoperatively, therefore all complications including AKI are within one month after surgery. Body mass index (BMI) was calculated using height and weight.
The eGFR was calculated using the following equations, utilizing the preoperative sCr taken closest to the time before surgery:

MDRD II equation [11 (link)]: eGFR = 186 × sCr − 1.154 × Age − 0.203 × (0.742 if female) × (1.210 if African − American)

Re-expressed MDRD II equation [12 (link)]: eGFR = 175 × sCr − 1.154 × Age − 0.203 × (0.742 if female) × (1.210 if African − American)

CG equation [13 (link)]: eGFR = [(140 − Age) × Weight/(72 × sCr)] × (0.85 if female)

This equation is adjusted for body surface area: (1.73 m2 × CG)/BSA，where BSA = 0.007184 × weight 0.425 × height 0.725

Mayo equation [14 (link)]: eGFR = exp [1.911 + 5.249/sCr − 2.114/sCr2 − 0.00686 × Age − (0.205 if female)], if sCr < 0.8 mg/dL then sCr = 0.8

CKD-EPI Equation [15 (link)]: eGFR = 141 × min (sCr/κ, 1)α × max (sCr/κ, 1) − 1.209 × 0.993Age × 1.018 [if female] × 1.159 [if African − American], where κ is 0.9 for males and 0.7 for females, α is –0.411 for males and –0.329 for females, min demonstrates the minimum of sCr/κ or 1, and max demonstrates the maximum of sCR/κ or 1 [15 (link)].

The preoperative eGFRs calculated by the five different equations were stratified into categories based on KDIGO classification: Stage 1: ≥ 90, Stage 2: < 90–60, Stage 3a: < 60–45, Stage 3b: < 45–30, Stage 4: < 30–15, and Stage 5: < 15 mL/min/1.73 m2 [6 (link)].
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