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

## TL;DR

This study examines patients with abnormal creatinine levels and no prior baseline, finding that most likely have CKD rather than AKI, and highlights missed opportunities for follow-up testing.

## Contribution

The study quantifies the clinical characteristics and outcomes of patients flagged with a '?AKI?CKD' alert and evaluates the effectiveness of current detection systems.

## Key findings

- Only 8.5% of '?AKI?CKD' patients were classified as probable AKI, with the majority (59.4%) as probable CKD.
- Probable AKI patients had higher 1-year mortality (28%) compared to probable CKD (12%) and no follow-up (11%).
- One-third of '?AKI?CKD' patients received no follow-up creatinine test, indicating missed care opportunities.

## Abstract

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.

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.

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%).

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.

Not applicable.

The online version contains supplementary material available at 10.1186/s12882-026-04758-8.

• One-third of flagged patients have no repeat serum creatinine test, which is a potential missed opportunity for early diagnosis.

• Patients with an abnormal serum creatinine result but no baseline most often have undiagnosed or existing chronic kidney disease (CKD), rather than acute kidney injury (AKI).

• Patients with probable AKI in this group have higher short-term mortality and are more often hospitalised than those with probable CKD.

The online version contains supplementary material available at 10.1186/s12882-026-04758-8.

What was known

This study adds

• Missing baseline serum creatinine slows down automated AKI detection and differentiation from CKD.

• AKI detection algorithms commonly generate a “?AKI?CKD” flag with elevated serum creatinine to avoid misclassification when no prior serum creatinine exists.

• The proportion of AKI versus CKD in this flagged group, and their clinical outcomes, were not previously well established.

• Only about 8.5% of “?AKI?CKD” patients meet criteria for probable AKI; the majority are probable CKD.

• Probable AKI patients have higher early mortality and hospitalisation rates than probable CKD.

• One-third received no follow-up serum creatinine test, identifying gaps in recommended care pathways.

The online version contains supplementary material available at 10.1186/s12882-026-04758-8.

Supports maintaining “?AKI?CKD” as a separate alert category to limit AKI overdiagnosis.

Stresses the importance of prompt repeat serum creatinine testing in uncertain kidney function cases.

Identifies the need for targeted clinical follow-up systems to address missed testing opportunities that will increase CKD detection, which is particularly important with the expanding therapeutic landscape for these patients.

The online version contains supplementary material available at 10.1186/s12882-026-04758-8.