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We report the case of a 20-year-old man who suffered an artefactual elevation of creatinine after consuming a creatine water supplement. Before this event, the patient was regularly seen in our clinic to monitor progression of a secondary paroxysmal nocturnal haemoglobinuria clone complicating childhood aplastic anaemia, for which he was previously treated with immunosuppression. Aside from his asymptomatic, stable, moderate thrombocytopenia (platelet count, 50-60 × 10 9 /L), there had never been evidence of haemolysis or thrombosis. The patient took no regular prescription medications and had previously documented normal renal function (Table). Blood tests performed 1 day before the patient's admission to hospital showed a significantly elevated creatinine level of 196 μmol/L (reference range, 40-120 μmol/L), with an estimated glomerular filtration rate of 38 mL/min, calculated using the MDRD (modification of diet in renal disease) formula. 1 The patient reported no recent systemic illnesses or symptoms. However, on specific questioning, he reported using a creatine water supplement at 1.0-2.5 L/day for the previous 2-3 months— an intake greater than that recommended in the product packaging information (3 g creatine in 500 mL of water daily). Physical examination was unremarkable and fluid status was clinically euvolaemic. The patient was admitted to hospital for investigation of apparent acute renal dysfunction. Repeat blood tests confirmed a disproportionately elevated creatinine level of 206 μmol/L relative to the urea level, which was normal (6.9 mmol/L; reference range, 2.1-7.1 mmol/L). All electrolyte levels were within normal limits, including a potassium level of 4.1 mmol/L. A full blood count examination demonstrated stable thrombocytopenia with a platelet count of 58 x 10 9 /L. Other parameters, including glycated haemoglobin level, white cell count and neutrophil count, fell within their reference ranges. Results of further directed investigations did not show any significant abnormalities to account for the apparent renal impairment. These included a total protein level of 82 g/L, an albumin level of 47 g/L, and a creatine kinase level of 331 U/L. Although above the reference range, at this level the creatine kinase would not be associated with elevated creatinine. Also within reference range were the patient's levels of negative antinuclear antibodies, extractable nuclear antigens, anti-double-stranded DNA, antinuclear cytoplasmic antibodies, antiglomerular basement membrane antibody, antistreptolysin serology, HIV, and hepatitis B and C serology. Midstream urine analysis was unremarkable, being within the reference range for cells, casts, protein and myoglobin. Renal tract ultrasound with Doppler studies showed normal-sized kidneys with no evidence of renal artery or vein thrombosis, or obstruction. The patient was initially treated with intravenous normal saline and cessation of the creatine water supplement. Further blood tests performed 2.5 hours later showed a reduction in his creatinine level to 152 μmol/L; and those performed 17 hours later showed that the level had normalised to 81 μmol/L. In view of the rapid return to a normal creatinine level, a renal biopsy was not performed. The patient was clinically well throughout his hospital stay and, since discharge, has experienced normal renal function.