We report a case of a female neonate whose pulse oximetry screening for congenital heart disease at 40 h of life was positive. The pregnancy was uneventful with no relevant family history. The neonate presented with bluish discolouration of the skin lasting until day 15. Cardiovascular examination and chest radiography were normal. Septic screening was negative. Oxygen therapy was started with poor response; investigations revealed a methaemoglobinaemia of 7.4%. The methaemoglobin level reached a peak of 15% on day 10, falling thereafter. The infant was discharged by day 20 with a normal physical examination and a methaemoglobinaemia of 11.4%. By 2 months of age this had fallen to 2.4%. Further investigation revealed a haemoglobin M variant: a heterozygous mutation of the γ globin gene known as Hb F-M Viseu. The mutation occurs in the γ chain, therefore the methaemoglobinaemia is transitory, resolving with the transition from fetal to adult haemoglobin.

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