Pulse oximetry is widely used perioperatively to detect respiratory events. Abnormal haemoglobins such as methemoglobin can result in inaccurate readings. Here we present the clinical challenges of managing a patient with congenital methemoglobinemia who went for surgery involving injection of patent blue dye and subsequently had a prolonged duration of poor saturation readings post-operatively. Differentials include desaturation due to respiratory causes, worsening of methemoglobinemia and delayed excretion of patent blue dye.