Haemodialysis recirculation detected by the three-sample method is an artefact.

The efficiency of haemodialysis may be limited by recirculation of blood between venous and arterial needles. Recirculation can be detected directly using a saline dilution method but is most commonly calculated from the urea concentrations of simultaneous samples from venous and arterial lines and a peripheral vein (three-sample method). The methods detect markedly different rates of recirculation in similar study populations. To investigate the possibility that the methods detect different phenomena, we performed both tests on 16 haemodialysis patients at various extracorporeal blood flow rates (Qb). The saline dilution method showed no recirculation in any of the patients, whereas the three-sample method indicated recirculation in all patients. The three-sample method indicated a mean recirculation fraction of 12.5% (SD 6.1) and was not influenced by changing Qb, suggesting that it was not detecting fistula recirculation. The three-sample method detects a solute concentration difference between arterial blood and peripheral blood during dialysis. There appears to be a disequilibrium between a central pool, represented by the arterial sample, and a poorly perfused peripheral pool, relatively isolated from the dialysis process, represented by the peripheral venous sample. The three-sample method for detecting recirculation should be abandoned.