Exploring haemoglobin Köln: a rare cause of pulse oximetry and arterial oxygen saturation discrepancy

Pulse oximetry guides the management of respiratory conditions through non-invasively monitoring arterial oxygen saturation. Unstable haemoglobin variants may produce inaccurate readings. We describe a man in his 60s with chronic obstructive pulmonary disease admitted to hospital with a lower respiratory tract infection and pulmonary emboli. His pulse oximetry saturation was 88% on 3 L nasal cannula. However, an arterial blood gas (ABG) showed his oxygen saturation (sO₂₎ at 96.7% and his partial pressure of carbon dioxide raised to 6.0 kPa (45.0 mm Hg). His oxygen was removed, and his pulse oximetry reading dropped to 79%-80%. A simultaneous ABG showed an sO₂ of 91.3%, demonstrating ≈10% saturation gap in oxygen saturation between the pulse oximetry and ABG readings. Subsequent genetic testing confirmed haemoglobin Köln. We recommend performing an ABG and investigating for unstable haemoglobinopathies when the pulse oximetry and arterial oxygen readings are discordant. This is important for patients with respiratory diseases, as over-oxygenation can lead to decompensated type 2 respiratory failure.