Introduction

Creatinine is released into the blood following non-enzymatic hydrolysis of creatine in skeletal muscle, and excreted into urine depending on glomerular filtration. The serum creatinine concentration is widely used as a marker of glomerular function because it increases in patients with decreased glomerular filtration rate (GFR). Although GFR is costly to measure, an estimated GFR (eGFR) can be calculated from the serum creatinine concentration. Since 2006, laboratories have calculated eGFR values using the isotope dilution mass spectrometry traceable modification of diet in renal disease equation.1 This standardisation allows direct comparison of creatinine and eGFR results from different laboratories. Reporting of eGFR is a significant advance, but it should be remembered that results are affected by interferences affecting the creatinine assay.

Serum creatinine was initially measured for clinical purposes using alkaline picrate (Jaffé reaction),2 but this method is affected by numerous interferents, including bilirubin, ketones, protein and non-creatinine chromogens.3 Enzymatic assays were later introduced, which are less susceptible to interference3 and more specific being unaffected by non-creatinine chromogens.4 However, interference has been reported from 5-fluorocytosine, ethamsylate, dopamine, dobutamine, nitromethane, creatine, sarcosine and ascorbic acid. Hummel et al 5 reported positive interference in enzymatic creatinine measurement caused by monoclonal IgM in three patients with Waldenström's macroglobulinaemia. IgM can precipitate and interfere in the detection step of the reaction. This interference can be avoided by ultrafiltration to remove protein before measuring creatinine. At the time …