Which of the following tests measures the oxygen saturation in a patient's blood?

The measurement of oxygen saturation in a patient's blood is primarily accomplished through pulse oximetry. This non-invasive test utilizes a sensor typically placed on a fingertip or earlobe to detect the color of the blood, which correlates with the levels of oxygen bound to hemoglobin. By emitting light wavelengths through the skin, the device measures the light absorption differences between oxygenated and deoxygenated hemoglobin. This provides an immediate readout of oxygen saturation percentages, which is crucial for assessing a patient's respiratory function and overall oxygenation levels without the need for more invasive procedures.

The other methods mentioned serve different purposes: the peak flow test primarily measures lung function for conditions like asthma; spirometry assesses breathing patterns and lung capacity; and arterial blood gas tests provide detailed information regarding blood pH, oxygen, and carbon dioxide levels through direct sampling from an artery, making them more invasive than pulse oximetry. Each of these tests plays a valuable role in respiratory and circulatory assessments, but when specifically focused on oxygen saturation levels, pulse oximetry is the most suitable choice.