A 67-year-old woman had left nostril obstruction for several weeks, which was thought to be due to a simple nasal polyp. Routine laboratory tests showed anaemia (haemoglobin 7.2 g/l (reference range 11.6–14.0)) and a negative serum iron level (−68.2 μmol/l (7.2–26.9)) was detected with an automatic analyser (Hitachi 7600, Hitachi, Tokyo, Japan). Interference in the direct method of iron measurement was suspected and an investigation of the abnormal absorbance pattern (fig 1) was conducted. We combined the patient’s serum with pooled serum at dilution factors 2, 3, 5 and 10, and measured the iron levels. The iron levels were −1.8, 3.4, 5.4 and 5.4 μmol/l, respectively, confirming …