blood level oxygen

When I test my blood level oxygen with the finger monitor it reads 96 - 99 all day long however I went to a doctor who drew blood from my artery and said it was 55 and has put me on immediate oxygen 24hours a day.  Why does the finger monitor read good levels...what is the difference.  I am very worried and scared.  Thank you

William Withrow

[National Jewish Health]

Oxygen is a gas.  The reading from the finger monitor, called an oximeter, of 96% to 99% is a normal value.  This is a measure of how much oxygen is actually attached to your red blood cells.  The result from the blood test, which is called an arterial blood gas (ABG), of 55 mm Hg is a measure of gas pressure, like the pressure of air in your tires.  This should be 90 to 100 mm Hg at sea level, so your result of 55 mm Hg is abnormally low.  This discrepancy suggests that one of these measurements is wrong.  Both the oximetry and the arterial blood gas determinations should be repeated, before the final decision is made on your oxygen needs.

In addition, you should ask your doctor, what is the cause of this low value of 55 mm Hg, and if there could be any possibility of carbon monoxide toxicity.