This post is an answer to the Case – Dark Brown Serum
A 53-year-old man presented to the emergency department after a syncopal episode. He was a regular smoker and recreational drug user, and had taken heroin, cannabis, and benzodiazepine during the previous week. He had a history of glucose-6-phosphate dehydrogenase (G6PD) deficiency.
On examination he had grey pallor and was dyspnoeic at rest (respiratory rate 23 breaths per minute). His oxygen saturation was 70% on 100% inspired oxygen and his blood sample, which showed anaemia (haemoglobin 54 g/L; normal 130–180 g/L), was dark brown (image). Arterial blood gas analysis showed pH 7·40 (7.35–7.45) and methemoglobin 26% (<2%).
Oxidant agents like benzocaine or novocaine, which are sometimes added to street heroin and cocaine, oxidise haemoglobin from its ferrous (Fe2+) to ferric state (Fe3+), and can cause methemoglobinemia. This altered form of haemoglobin loses its ability to transport oxygen, and can cause hypoxaemia.
Dark brown blood is seen in methemoglobinemia. Unlike deoxyhaemoglobin, methaemoglobin does not brighten on exposure to atmospheric oxygen. In our patient, haemolysis associated with G6PD deficiency caused release of the methaemoglobin from within the red blood cells into the serum.
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