Low haematocrit during cardiopulmonary bypass is a known risk factor for acute kidney injury, but it is unclear whether high haematocrit also increases acute kidney injury risk. We examined if a haematocrit range associated with a reduced risk for acute kidney injury could be identified.

This observational study included cardiac surgery patients operated on between 2016 and 2020, using prospectively collected data from the SWEDEHEART registry and a local cardiopulmonary bypass database. Acute kidney injury was defined by registry-available data as a postoperative serum‑creatinine increase >1.5 times baseline or postoperative initiation of renal replacement therapy. For each predefined haematocrit threshold, weighted exposure was calculated as the accumulated time‑and‑magnitude by which haematocrit rose above or fell below the thresholds. Associations between weighted exposure and acute kidney injury were analysed using multivariable logistic regression adjusted for established acute kidney injury risk factors.

A total of 3798 patients were included. Postoperative acute kidney injury occurred in 598 of 3798 patients (15.7%). After adjustment, a non‑linear association between haematocrit weighted exposure during aortic cross‑clamp and acute kidney injury was observed. AKI risk increased with haematocrit below 25% (adjusted odds ratio [aOR] 1.006 per 1 [%×min]; 95% CI 1.002-1.009; P < .001) and above 38% (aOR 1.006 per 1 [%×min]; 95% CI 1.001-1.011; P = .019).

In this cohort, haematocrit exposure below 25% and above 38% during aortic cross-clamp was associated with higher postoperative acute kidney injury risk. Further studies are needed to confirm these findings.