An Evaluation of Commonly Used Surrogate Baseline Creatinine Values to Classify AKI During Acute Infection

Introduction: Classification of acute kidney injury (AKI) requires a premorbid baseline creatinine, often unavailable in studies in acute infection. Methods: We evaluated commonly used surrogate and imputed baseline creatinine values against a reference creatinine measured during follow-up in an adult clinical trial cohort. Known AKI incidence (Kidney Disease: Improving Global Outcomes [KDIGO] criteria) was compared with AKI incidence classified by (1) back-calculation using the Modification of Diet in Renal Disease (MDRD) equation with and without a Chinese ethnicity correction coefficient; (2) back-calculation using the Chronic Kidney Disease- Epidemiology Collaboration (CKD-EPI) equation; (3) assigning glomerular filtration rate (GFR) from age and sex-standardized reference tables; and (4) lowest measured creatinine during admission. Back calculated distributions were performed using GFRs of 75 and 100 ml/min. Results: All equations using an assumed GFR of 75 ml/min underestimated AKI incidence by more than 50%. Back-calculation with CKD-EPI and GFR of 100 ml/min most accurately predicted AKI but misclassified all AKI stages and had low levels of agreement with true AKI diagnoses. Back-calculation using MDRD and assumed GFR of 100 ml/min, age and sex-reference GFR values adjusted for good health, and lowest creatinine during admission performed similarly, best predicting AKI incidence (area under the receiver operating characteristic curves [AUC ROCs] of 0.85, 0.87, and 0.85, respectively). MDRD back calculation using a cohort mean GFR showed low total error (22%) and an AUC ROC of 0.85. Conclusion: Current methods for estimating baseline creatinine are large sources of potential error in acute infection studies. Preferred alternatives include MDRD equation back-calculation with a population mean GFR, age-and sex-specific GFR values corrected for good health, or lowest measured creatinine. Studies using surrogate baseline creatinine values should report specific methodology.

Automated summary

The study found that equations using an assumed glomerular filtration rate (GFR) of 75 ml/min underestimated the incidence of AKI in studies of acute infection. Back-calculation with the CKD-EPI equation and an assumed GFR of 100 ml/min most accurately predicted AKI but misclassified all AKI stages and had low levels of agreement with true AKI diagnoses.