In patients with chronic renal failure, blood samples for laboratory analysis are often taken via dialysis catheters. This report describes a case of gross spurious hypernatraemia in a blood sample collected from a patient undergoing haemodialysis. After centrifugation of the blood sample in question, the separator gel formed the topmost layer, with the serum in the middle and the clot at the bottom. Subsequent analysis of the serum showed severe hypernatraemia (serum sodium, 744 mmol/litre). It was established that the blood sample had been taken from the patient’s dialysis catheter into which 3 ml of Citra-Lock™ (46.7% trisodium citrate) had been instilled previously as a “catheter locking” solution. The hypernatraemia seen in this case was recognised immediately as an artefact, but it was found that even minimal contamination of blood samples with Citra-Lock may significantly affect sodium concentrations. This contamination may be missed, with potentially adverse consequences for patient management.