do you need to fast for a pulse oximeter test (blood oxygen test) and if so for how long?

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How are blood Oxygen levels determined via a Pulse Oximeter?

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How exactly does a pulse oximeter measure the amount of oxygen in your blood?

Especially since its applied externally over the finger? And why the index finger? [eba kw="best pulse oximeter" num="1" ebcat=""] [powered by Yahoo! Answers]

Oximeter Plus Clip on C21 Finger Heart Rate & Blood Oxygen Rate Monitor

Dual color HIGH RESOLUTION easy to read OLED display Display SpO2, Pulse Rate ,Pulse bar and Plethysmogram 6 display modes for easy reading from all sides. Low power consumption, automatic power off. Approximately 30 Hour normal operation. Product Description- Battery-low indicator – 2 AAA alkaline or rechargeable batteries – 10 setting adjustable brightness – Accommodates [...]