The bad PI in an oximeter refers to a low perfusion index (PI) reading, which indicates poor peripheral perfusion. The PI is a measure of the strength of the pulse and is calculated as the ratio of pulsatile blood flow to non-pulsatile static blood flow in a patients peripheral tissue. A normal PI value ranges from 0.02% to 20%, with a higher value indicating better peripheral perfusion and a lower value suggesting poor perfusion. A PI value of 0.4% or below indicates weak pulse strength, and the oximeter reading may be unreliable. Common reasons for poor perfusion include diabetes, obesity, blood clots, and peripheral artery diseases. In neonatal acute care, a low PI has been shown to be an objective indicator of severe illness, and it is superior to qualitative approaches such as foot warmth. Conversely, a high PI number generally indicates an optimal monitoring site for pulse oximetry. The PI is also used as an objective indicant of pain levels in patients, especially in those unable to communicate their discomfort to the clinician.