The comparison of the commonly used surrogates for baseline renal function in acute kidney injury diagnosis and staging

Baseline serum creatinine (SCr) level is frequently not measured in clinical practice. The aim of this study was to investigate the effect of various methods of baseline SCr determination measurement on accuracy of acute kidney injury (AKI) diagnosis in critically ill patients.

This was a retrospective cohort study. All adult intensive care unit (ICU) patients admitted at a tertiary referral hospital from January 1, 2011 through December 31, 2011, with at least one measured SCr value during ICU stay, were included in this study. The baseline SCr was considered either an admission SCr (SCr_ADM) or an estimated SCr, using MDRD formula, based on an assumed glomerular filtration rate (GFR) of 75 ml/min/1.73 m² (SCr_GFR-75). Determination of AKI was based on the KDIGO SCr criterion. Propensity score to predict the likelihood of missing SCr was used to generate a simulated cohort of 3566 patients with baseline outpatient SCr, who had similar characteristics with patients whose outpatient SCr was not available.

Of 7772 patients, 3504 (45.1 %) did not have baseline outpatient SCr. Among patients without baseline outpatient SCr, AKI was detected in 571 (16.3 %) using the SCr_ADM and 997 (28.4 %) using SCr_GFR-75 (p < .001). Compared with non-AKI patients, patients who met AKI only by SCr_ADM, but not SCr_GFR-75, were significantly associated with 60-day mortality (OR 2.90; 95 % CI 1.66–4.87), whereas patients who met AKI only by SCr_GFR-75, but not SCr_ADM, had a non-significant increase in 60-day mortality risk (OR 1.33; 95 % CI 0.94–1.88). In a simulated cohort of patients with baseline outpatient SCr, SCr_GFR-75 yielded a higher sensitivity (77.2 vs. 50.5 %) and lower specificity (87.8 vs. 94.8 %) for the AKI diagnosis in comparison with SCr_ADM.

When baseline outpatient SCr was not available, using SCr_GFR-75 as surrogate for baseline SCr was found to be more sensitive but less specific for AKI diagnosis compared with using SCr_ADM. This resulted in higher incidence of AKI with larger likelihood of false-positive cases.