Alot of different factors can affect a pox reading but should correlate w/ an abg 2-3%.

A continuous accurate reading(correlating w/pts pulse and reading for more than 5 beats and pox displays reading and thats it)is important. Smokers will read higher if just smoked(pts carboxyhemoglobin is whats measured via pox) movement will cause false readings, nail polish (of red/black/purple,false nails)
Perfusion issues(shock/hypovolemia)

Remember to assess the whole picture and the pt. but your pulse ox should be pretty close to the arterial oxygen saturation that results on the arterial gas....