Eli5: how does the finger clip with the laser at the doctors measure my oxygen level?

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Eli5: how does the finger clip with the laser at the doctors measure my oxygen level?

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From the FDA: “A pulse oximeter is a device that is usually placed on a fingertip. It uses light beams to estimate the oxygen saturation of the blood and the pulse rate. Oxygen saturation gives information about the amount of oxygen carried in the blood. The pulse oximeter can estimate the amount of oxygen in the blood without having to draw a blood sample”.
[LINK](https://www.fda.gov/medical-devices/safety-communications/pulse-oximeter-accuracy-and-limitations-fda-safety-communication)

Blood cells with oxygen in them are redder than blood cells without oxygen. The light reflects more from those cells, and when it detects a pulse of blood, it sees “how red” the pulse is.

Oxygen turns your blood bright red, without oxygen your blood is very dark. The light passes through your finger and a sensor on the opposite side measures the color of the light to determine how much oxygen it contains.

One side of the clip has a light and the other has a sensor. When you clip it on your finger, it shines both red visible light and infrared light through you finger and it hits the detector on the other side after passing through your finger. Hemoglobin is the protein in red blood cells that carries oxygen. Saturated hemoglobin (full of oxygen) and unsaturated hemoglobin (not full of oxygen) absorb different amounts of infrared and visible light, so by comparing the ratio that the sensor detects to the ratio that was emitted, it can tell how much of the hemoglobin in your blood is saturated (carrying oxygen) and how much isn’t.

I’m a paramedic.

The little thing is a Pulse Oximeter. It displays your SpO2. SpO2 is the Saturation of partial Oxygen. A red blood cell has a protein called hemoglobin. There are 4 hemoglobin sites on a normal RBC. The oximeter reads the percentage of hemoglobin that is saturated. It uses Infrared light spectrum to assess this not a laser. It also reads the pulse wave in the fingertip.

While in most people, this reading correlates to oxygen levels, but in some cases, the reading is wrong. Any reduction in the blood flow to the finger will give a false reading (cold, having your blood pressure taken on that arm, low blood volume, sickle cell crisis, etc). Any white or visible light in the sensor will give a false reading. Now, Carbon Monoxide will pull oxygen off the hemoglobin and bind itself to the hemoglobin. An oximeter will read 100%, even though your oxygen is depleted. Cyanide prevents oxygen from disengaging from the RBC and blocks O2 from entering the cells. You will read 100%, but will have signs and symptoms of hypoxia (without oxygen).