An asymptomatic 81-year-old woman was referred by her general practitioner regarding a pulse-oximetry oxygen saturation (SpO2) of 74%. An arterial blood gas analysis (ABG) on air showed PaO2 12.9 kPa, oxygen saturation 80%, with normal pH, PaCO2, methaemoglobin and carboxyhaemoglobin levels. After a normal chest x-ray, tinzaparin was administered empirically for possible occult pulmonary embolus. This diagnosis was subsequently excluded with an unremarkable computed tomography pulmonary angiogram (CTPA). She was further investigated as an out patient. DNA globin-gene analysis identified a variant haemoglobin revealed to be haemoglobin Saint Mande (HbSM). Following reassurance regarding the benign nature of her condition, she has remained well.