BackgroundWhile clinical guidelines recommend that residual kidney function (RKF) is measured in peritoneal dialysis (PD) patients, 24-h urine collection is cumbersome and prone to errors. We wished to determine whether an equation using serum β2-microglobulin (β2M) could prove of clinical benefit in estimating RKF and identifying patients who could start PD with incremental prescriptions.MethodsWe measured serum β2M in consecutive PD outpatients recently starting dialysis with continuous ambulatory PD (CAPD) or automated PD (APD), attending a single tertiary hospital for their routine clinical visit. RKF was defined as the mean of 24-h urine clearances of creatinine and urea. An equation estimating RKF (eRKF) was generated based on serum β2M levels on a randomly selected modelling group.ResultsWe included 511 patients, of whom 351 in the modelling group and 150 in the validation group. Mean age was 58.7 ± 15.8, 307 (60.0%) were men and median RKF value was 4.5 (2.4–6.5) mL/min/1.73 m2. In the validation group, an equation based on β2M, creatinine, urea, age and gender showed minimal bias of − 0.1 mL/min/1.73 m2to estimate RKF. Area under the receiving operator characteristic curve was 0.915 to detect RKF ≥ 2 mL/min/1.73 m2.ConclusionAn equation based on serum β2M concentration would not be able to replace 24-h urine collection as the standard of care when an exact measurement of RKF is required. However, it could prove useful in identifying patients suitable for an incremental PD prescription and for monitoring RKF in individuals unable to reliably collect urine.Graphic abstract