Abstract Olive S (2016) Using pulse oximetry to assess oxygen levels. Detecting low oxygen levels in patients is important but not always easy; central cyanosis--when a patient's lips, tongue and mucus membranes acquire a blue tinge--can be missed, even by skilled observers, until significant hypoxaemia is present. Pulse oximetry can be undertaken to measure a patient's oxygen levels and help identify earlier when action must be taken. This article outlines the procedure and its limitations, as well as the circumstances in which it should be used.