Can a Pulse Oximeter Read Normally Even When Oxygen Levels Are Low?

I’ve seen people question whether a fingertip pulse oximeter can give a reassuring number when someone is actually low on oxygen. How much do factors like movement, cold hands, nail polish, or skin tone affect pulse oximeter readings?

I’m not looking for a diagnosis from a home device; I’m trying to understand how much confidence to place in one reading and when symptoms should matter more than the number.