The finger test is a term refereed to in measuring the oxygen saturation of the arterial blood using a device called a pulse oximeter in which a sensor is attached to the end of a finger. The reading represents the percentage of hemoglobin that is saturated with oxygen in the arterial blood. As one climbs higher, less oxygen is available to fill the oxygen-binding sites on the hemoglobin molecule. There is an expected range for oxygen saturation in the normal individual at particular altitudes and in the person that is acclimatized to that altitude. Newcomers will have lower readings that will increase after a stay at that altitude. Individuals with HAPE will have lower readings. At sea level the normal reading is 96 percent or above, while at 15,000 feet (4570 meters) it is around 86 percent, dropping to about 76 percent around 20,000 feet (6100 meters). At the summit of Everest 29,029 feet (8850 meters )it drops to approximately 58 percent. Some groups at altitude carry a pulse oximeter (today’s models are small and light in weight) in an attempt to gauge how well individuals are acclimatizing and hoping to diagnose HAPE if necessary. Cold fingers as well as exercise can give falsely low readings. And readings may be normal with carbon monoxide poisoning. I feel carrying such a device is unnecessary, but in these technological times, a number of groups do.