Thanks for writing about this Keith, and for your response Tom! I agree that finger temperature, changes in peripheral circulation from an overdose (decreased oxygenation), dirt, and polish can all affect a reading.
A good way to confirm if a oxygen percent (SpO2 %) is accurate is to check the radial/wrist or carotid/side of throat pulse and count to see if this pulse matches that of the reading for HR on the oximeter. Sometimes because of the above factors a SpO2 reading will be much lower than what it actually is. Always remember to assess the person and their symptoms first and foremost before trusting a machine. For example, if someone is barely breathing and their lips and/or fingertips are blue then their oxygen saturation is likely below 75% and they need breathing support.