BACKGROUND: The impact of small increases in serum creatinine after surgical aortic valve replacement (SAVR) that fail to meet the acute kidney injury stage 1 criteria is unknown. The aim of this study was to investigate prognosis after primary SAVR in patients with small increases in postoperative serum creatinine. METHODS: This observational cohort study included all adult patients who underwent primary SAVR in Sweden from 2009 to 2022. The primary outcome was all-cause mortality. Secondary outcomes were chronic kidney disease and heart failure. Regression standardization addressed confounding. RESULTS: In 16,766 patients, 4074 (24.2%) had no change in postoperative serum creatinine, 5764 (34.3%) had a small increase in postoperative serum creatinine (0.06 mg/dL ≤ Δserum creatinine <
0.3 mg/dL), and 2753 (16.4%) fulfilled the Kidney Disease Improving Global Outcomes acute kidney injury stage 1 criteria. The mean age was 67 years, and 31% of patients were female. No significant difference in long-term all-cause mortality was observed in the no change group at 13 years compared with the small increase group (absolute survival difference, 2.3% [95% CI, 0%-4.6%]). A stepwise increase in the risk of 30-day mortality was observed with increasing changes in serum creatinine. At 13 years of follow-up, there was a significant difference in the risk of chronic kidney disease (absolute difference, 2.8% [95% CI, 1.0%-4.5%]) and heart failure (absolute difference, 3.5% [95% CI, 1.3%-5.7%]) between the no change and small increase groups. CONCLUSIONS: A small increase in postoperative serum creatinine after SAVR was associated with an increased risk of adverse outcomes. The acute kidney injury definition may benefit from including more reliable and specific biomarkers together with small creatinine increases to detect kidney injury.