How can I test a pulse oximeter to make sure it works at home?
May 1, 2020 7:57 AM   Subscribe

I bought a couple of pulse oximeters. They were cheap and shipped from China. They have no FTC or Health Canada statement on them. They will live in two separate households, but for now they are together (which I figure may help test). How do I test them to make sure they actually reliably estimate blood oxygenation?

I put them on...they registerd between 97-99 (I think the display is actually only two digits, so 99 may be max).

I tried holding my breath but I suck at that and can't really hold it very long. How long would I have to hold my breath for blood oxygenation to drop? It looked like it wasn't working, but then maybe 10 seconds AFTER i held my breath (and was already thinking it didn't work), the blood oxygen dropped to 94. Is that what one would expect? It quickly came back up, which I suppose would make sense since I stopped holding my breath? I don't think it stayed at 94 for more than a second or two.

If I put some medium-dark/bright nail polish on it should register lower if it's working properly, right? Might that be a good way to test? Other ideas?
posted by If only I had a penguin... to Health & Fitness (6 answers total) 2 users marked this as a favorite
A couple of answers:

1) How long would you have to wait after holding your breath before O2 saturation drops
The functional residual capacity (FRC) of your lungs (the amount of air in your lungs) is about 2 litres. Since oxygen is 21% of air, there should be about 400 ml of pure oxygen (the rest being mostly nitrogen). You consume about 200-250ml of O2 per minute at rest, so this reserve will give you somewhere between a minute and a half to 2 minutes before you start to see O2 saturations drop

2) Should there be a lag?
Pulse oximeters report a O2 saturation that is a running average of the previous 30 seconds or so, so a lag is to be expected.

3) How can i be sure it's working?
Short of your little experiment, the only way to make sure that your meter is working properly would be to take an arterial sample of your blood and run it through a machine to compare the PaO2 (the partial pressure of oxygen) in your blood to what the machine is reading.

These gadgets are pretty simple and just rely on the ratio of absorption of two wavelengths. Generally, even these cheapo ones should be reasonably good. Sounds like yours is working.
posted by reformedjerk at 8:17 AM on May 1, 2020 [1 favorite]

I don't think there's any way to tell for sure unless you have a fancy reference one that you know is right (and something tells me that hospitals wouldn't be thrilled about you showing up to ask to use their equipment).

However, I'd be reassured if they 1) gave numbers in the high 90s when you were feeling fine, 2) dropped when you held your breath and 3) recovered after you stopped holding your breath.
posted by Betelgeuse at 8:40 AM on May 1, 2020 [2 favorites]

The only time I've seen a oximeter drop a lot was a CES demo by an Olympic cyclist - if you get on a bike trainer and sprint like hell for a minute or so you should definitely see oxygen levels drop.
posted by GuyZero at 9:15 AM on May 1, 2020 [1 favorite]

The good news is, the worse shape you are in, the easier it is to do GuyZero's test. The other good news is that the difference in a few points doesn't matter that much.
posted by advicepig at 10:03 AM on May 1, 2020

My partner uses a pulse ox and I'm her caregiver so I'm very familiar with how it works. That lag is normal and expected. Saturation for a healthy person is 97-99. Sounds like yours are working fine. (Don't put any polish on it or you'll risk damaging the sensor.)
posted by E3 at 10:16 AM on May 1, 2020

Instead of trying to hold your breath, which sounds both dangerous and difficult, you could figure out a way to reduce blood flow to one limb of your body.

For example, when I sit on the toilet and lean over sharply (when I'm hunched over my phone), my legs start getting tingly and start getting weak. Or on a very soft mattress, lay on your side until your arm falls asleep.

If you know someone with some medical training, you could tourniquet your arm. I wouldn't recommend this without training or supervision because done improperly, you could do permanent damage.
posted by meowzilla at 11:48 AM on May 1, 2020

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