Distinguishing AKI from CKD: outcomes and characteristics of patients with abnormal serum creatinine and no known baseline

BACKGROUND: Comparison of a patient's abnormal serum creatinine result to an earlier value is fundamental to differentiating Acute Kidney Injury (AKI) from Chronic Kidney Disease (CKD), and is the first step in electronic AKI detection systems. For those patients in whom a baseline serum creatinine is unavailable, some systems generate a warning message to highlight the elevated serum creatinine but without distinguishing AKI from CKD (a "?AKI?CKD" warning). We aimed to determine demographic characteristics of this group, the proportion who had a first presentation of AKI, their clinical outcomes, and how these alert messages translate into subsequent biochemical testing and follow-up. METHODS: We performed a retrospective cohort analysis of adult patients with serum creatinine testing at University Hospitals of Leicester during 2019. Using the NHS England AKI detection algorithm, we identified patients with AKI Warning Test Scores (WTS) and "?AKI?CKD" warnings. The "?AKI?CKD" cohort was classified as probable AKI, probable CKD, or no follow-up result, based on subsequent serum creatinine measurements. Survival (90-day and 1-year) was analysed with Kaplan-Meier methods. RESULTS: Among 3,464 patients with "?AKI?CKD" warnings, 8.5% were probable AKI, 59.4% probable CKD, and 32.0% had no follow-up test. Probable AKI patients were younger (median age 71 versus 76 years) and more often hospitalised at warning time (56% versus 15%). One-year survival was lower in probable AKI (72%) compared to probable CKD (88%) or no follow-up (89%). Probable AKI survival was similar to AKI WTS stage 1 but better than stages 2-3. Extending baseline serum creatinine look-back to 426 days changed categorisation minimally (≤ 2%). CONCLUSIONS: These findings highlight that the major feature of the "?AKI?CKD" classification is not simply misclassification between AKI and CKD, but the variability of clinical response, with one-third of patients receiving no subsequent serum creatinine test. Most patients flagged as "?AKI?CKD" likely have CKD rather than AKI, and this, coupled with comparable outcomes of the probable AKI group to early-stage AKI, suggests minimal missed population-level AKI detection. However, one-third lacked follow-up testing, highlighting missed opportunities to identify CKD. CLINICAL TRIAL NUMBER: Not applicable.