I am a regular blood platelet donor - apheresis. I am also a runner (6 marathons in the last 4 years). Occasionally, I get rejected because of low hematocrit. Today, I got sent home with an all time low of 36 - the cut off (if I remember) is 38. Three weeks ago, I was at a respectable 42. I have not made any significant changes in my running, diet, or anything else I can think of in that period.
So my question is- how accurate (or reliable) is the test they give you at a blood bank? In my case, I’m talking about a finger stick, where they take a drop of blood into a little tube, then centrifuge it for a minute or so, then they place it in some kind of calibrated slot and measure - well, I dunno exactly what they’re looking at. Since I’m often at the edge, I’m wondering how much variation there might be in this procedure. If I just came back in 30 minutes, might I pass?
I’ve asked my doctor about this, and she kind of poo-poohed the blood bank’s test, and ran her own, which she said was fine (health-wise for me, that is).
Are you on your period?
Just kidding…
I work in a hospital, and we all know that a fingerstick is never quite as accurate as a ‘real’ blood test. There is a margin of error so to speak of a couple of points. Clinically, 2 pts may not matter much, but the lower you are the more it matters, and everyone has to have a protocol, to account for everyone donating, young and old alike. But just like a real bloodtest, the same sample could be run 3 different times w/ slightly different results. If they swab your finger w/ alcohol, make sure it is totally dry before they stick you, cause that can dilute the result and make your HCT a little lower. Patients who get a lot of IV fluid (I mean ALOT) can have a diluted result as well. But that doesn’t apply to you probably.
Are you on your period?
Just kidding…
I work in a hospital, and we all know that a fingerstick is never quite as accurate as a ‘real’ blood test. There is a margin of error so to speak of a couple of points. Clinically, 2 pts may not matter much, but the lower you are the more it matters, and everyone has to have a protocol, to account for everyone donating, young and old alike. But just like a real bloodtest, the same sample could be run 3 different times w/ slightly different results. If they swab your finger w/ alcohol, make sure it is totally dry before they stick you, cause that can dilute the result and make your HCT a little lower. Patients who get a lot of IV fluid (I mean ALOT) can have a diluted result as well. But that doesn’t apply to you probably.
Even a “real” blood test has some margin of error, and your 36 could easily be a 40 if repeated (38 +/- 2). While a fingerstick test is probably less accurate, I wouldn’t have thought the differences would usually be that dramatic.
One problem I’ve seen with fingersticks is too vigorous squeezing of the finger forces out some tissue fluid which can dilute the blood causing a falsely low result. It doesn’t take too much since you’re dealing with such a small sample volume overall.
I’ve been to blood donation sites where they just stick your finger, and put a drop of blood in a vial with some kind of blue fluid. I guess, whether your blood sinks or doesn’t determines whether they take you - how “accurate” is that?
I know it’s less accurate. In that test, they’re measuring the density of your blood, which is an indirect way to measure hematocrit. The liquid they use (I’m guessing some sort of copper solution) must be calibrated correctly and be the right temperature. I donated blood a lot in college, and my diet wasn’t always the best. There were a couple times where they’d put a drop in and it would sink only halfway. Then they’d do the little centrifuge test. Sometimes I’d get rejected for it being too low, but sometimes I was in the correct window.
Hematocrit is defined by what percentage of your blood is red blood cells. So what they’re measuring is how much of that little tube is filled with blood cells compared to the overall volume.
FWIW, I was advised to take iron pills for about a week before I went in for a donation to get my HCT up. I haven’t gotten rejected since, but I don’t know whether that’s due to the supplements or a better diet in general.