Back-calculating baseline creatinine with MDRD misclassifies acute kidney injury in the intensive care unit

Abstract

Background and objectives: The purpose of this study was to assess the viability of back-calculation with the Modification of Diet in Renal Disease (MDRD) formula to determine baseline creatinine on the basis of acute kidney injury (AKI) metrics, RIFLE criteria, and Acute Kidney Injury Network (AKIN) criteria for the purpose of clinical trial outcomes or epidemiology.

Design, setting, participants, & measurements: This study was a retrospective analysis of prospectively collected data from patients with measured baseline creatinines before entry to the intensive care unit (ICU). The AKI status was determined using five different baseline creatinines: the measured creatinine (the standard) and an estimated creatinine determined by back-calculation using MDRD assuming a GFR of 75 ml/min (epCr75), 100 ml/min (epCr100), randomly generating a value on a lognormal curve (epCrRnd), and choosing the lowest creatinine value within the first week in the ICU (epCrlow). A subgroup of patients without chronic kidney disease (CKD) was similarly analyzed.

Results: Of 224 patients, 70 (31%) had AKI according to RIFLE and 93 (42%) according to AKIN. The epCr75 and epCr100 distributions greatly overestimated the proportion with AKI. The epCrlow overestimated AKI according to AKIN but correctly estimated AKI according to RIFLE. The mean of 1000 epCrRnd distributions correctly estimated AKI according to RIFLE and AKIN. Each estimated distribution performed better in the non-CKD population with the exception of epCrRnd. However, only the epCrlow distribution accurately determined the proportion with AKI.

Conclusions: A measured rather than estimated value should be used for baseline creatinine in trials or epidemiologic studies of AKI.

Figure 1.

Bland–Altman plots of the measured creatinine (pCr_m) against the difference between the measured (pCr_m) and estimated baselines: (A) epCr₇₅, (B) epCr₁₀₀, (C) epCr_Rnd (an example of the 1000 random distributions calculated), and (D) epCr_low (n = 224, men n = 123, women n = 101). A perfect agreement between pCr_m and an estimated baseline distribution would mean all points lying alongside the y = 0 line. Total bias and SD shown are for the entire distribution (men + women).

Figure 2.

Dependency of patients classified as AKI using RIFLE criteria R, I, and F depending on method of selecting baseline creatinine. Each bar represents the total number of patients. The lower (unshaded) portion of the bar represents the number of CKD patients. Note that CKD patients contribute disproportionally to this overestimation.