RESUMEN
Methemoglobin (MetHb) is an oxidized form on hemoglobin, which is unable to bind oxygen and consequently carry it to the tissues. Normally present in small quantities (<1%) without detrimental effects, its elevation produces hypoxemia which can be profound and even lethal. Methemoglobinemia is an abnormal increase of MetHb (>3%) of total hemoglobin. It can be classified in two types: hereditary and acquired. Acquired form is caused by exogenous oxidizing agents, such as nitrites or certain medications, while hereditary types of disease are the result of genetic deficiency in cytochrome B5 reductase, an enzyme responsible for MetHb reduction to hemoglobin. Little data is available on the epidemiology of methemoglobinemia. In general population only sporadic cases are described, while some isolated ethnic populations have increased incidence, possibly inherited from a common ancestor. We present a case of congenital methemoglobinemia in which detection of MetHb was hampered by faulty initial blood gas spectrometry results. A short literature review is also included.