Metabolic acidosis frequently complicates end-stage renal failure. In haemodialysis patients its severity is usually monitored by measurement of the total CO 2 (TCO 2 ) level. Samples from ‘satellite dialysis’ patients are often stored prior to analysis. We investigated the affect of storage of 21 samples for 24 h under different conditions prior to analysis. If samples were stored at room temperature the TCO 2 fell from 22.7±4.2 mmol/l to 21.6±3.7 mmol/l ( P = 0.001). If the same samples were spun and stored at 4 °C the TCO 2 was 22.4±3.9 mmol/l ( P = not significant). We conclude that the magnitude in the fall of TCO 2 stored at room temperature for 24 h is unlikely to be clinically significant and can be prevented by spinning the sample and refrigerating it.