# What do they need so much blood for? **URL:** \ **Category:** Factual Questions\ **Created:** [March 20, 2003, 7:09am UTC](https://boards.straightdope.com/t/what-do-they-need-so-much-blood-for/162758 "2003-03-20T07:09:46Z")\ **Posts on this page:** 12\ **Page:** 1
**Author:** ![sonicsink](https://avatars.discourse-cdn.com/v4/letter/s/f17d59/32.png) [@sonicsink](https://boards.straightdope.com/u/sonicsink)\ **Post date:** [March 20, 2003, 7:09am UTC](https://boards.straightdope.com/t/what-do-they-need-so-much-blood-for/162758/1 "2003-03-20T07:09:46Z")
So the Dr. tells me she wants to run some blood tests. ok fine. She mentions about 4 different things she wants checked, so I go to the hospital for the blood test and they take 7,yes 7, big honkin’ test tubes of my blood.By the time the last one was being filled, I could feel the blood being drained from my brain. Now really,I’m not a lab technician,but what the heck are they doing with all my blood? When I see little spots on TV about blood tests they usually put a little spot of blood on a glass slide and look at it under a microscope. Why do they need so much of my blood? Also,question within a question here…How much blood is in the human body? Would it be enough to fill a 2-litre coke bottle or is there more? I need a visual reference here. Thank you for taking the time to read my post 🙂 ---
**Author:** ![skaterboarder87](https://avatars.discourse-cdn.com/v4/letter/s/9de0a6/32.png) [@skaterboarder87](https://boards.straightdope.com/u/skaterboarder87)\ **Post date:** [March 20, 2003, 7:16am UTC](https://boards.straightdope.com/t/what-do-they-need-so-much-blood-for/162758/2 "2003-03-20T07:16:46Z")
Maybe they’re vampires? 😉 ---
**Author:** ![sciguy](https://avatars.discourse-cdn.com/v4/letter/s/e68b1a/32.png) [@sciguy](https://boards.straightdope.com/u/sciguy)\ **Post date:** [March 20, 2003, 7:27am UTC](https://boards.straightdope.com/t/what-do-they-need-so-much-blood-for/162758/3 "2003-03-20T07:27:53Z")
IANAD, but I’d guess first off they’d want an excess in case a test gets messed up. Second, some tests are done on specific components of the blood (plasma, platelets, white blood cell count, etc), which I’d figure would require centrifuging a blood sample to separate it. This would obviously need a larger sample than a spot on a microscope slide. As for the amount of blood in a human body, it’s around 10 or 12 pints. For a visual reference, go to a bar with 4-5 friends, and each order two beers. Drink the beers, then hang one of them by their ankles, slit their jugular, then you’ll be able to fill all the empty glasses. For a less graphic illustration, it’s 2.5 to 3 gallons, equivalent to about six 2-litre coke bottles. ---
**Author:** ![Smeghead](https://avatars.discourse-cdn.com/v4/letter/s/f1d935/32.png) [@Smeghead](https://boards.straightdope.com/u/Smeghead)\ **Post date:** [March 20, 2003, 7:33am UTC](https://boards.straightdope.com/t/what-do-they-need-so-much-blood-for/162758/4 "2003-03-20T07:33:56Z")
Well, I work in a lab testing blood samples. There are several reasons. Some of the tests in our lab only need 200 microliters (read “good-sized drop”) of blood, but for others, we need a full 3 milliliters (typically one full tube) to get all the DNA we need, depending on your white blood cell count. Then, if the DNA concentration we get is low, and/or the test needs repeating, we may need to reextract another tube. Then, in other labs, they may need their own separate tube so they can make sure it’s not contaminated. Like, we can’t open a tube, stick a pipette into it to get what we need, then send it to bacteriology, because we might have gotten bacteria in the blood. Then you may have a lab test that needs to be run on plasma, so they’d have to take a full tube and spin it down… Etc, etc, etc. FTR, only a few of our labs even HAVE a microscope. Mine doesn’t. Basically, the person drawing the blood will look at some sort of reference that shows what the sample requirements are for each test, and draw accordingly. ---
**Author:** ![BobT](https://avatars.discourse-cdn.com/v4/letter/b/8e8cbc/32.png) [@BobT](https://boards.straightdope.com/u/BobT)\ **Post date:** [March 20, 2003, 7:34am UTC](https://boards.straightdope.com/t/what-do-they-need-so-much-blood-for/162758/5 "2003-03-20T07:34:31Z")
I think I’ve had blood tests where there have been at least five tubes filled with blood. But when the doctor orders tests on many different things, I suppose you need them. I imagine that some tests take longer than others. A CBC is probably done quickly, but I was also tested for some more unusual stuff, like myasthenia gravis (which I don’t have thankfully) and those took longer. ---
**Author:** ![Achernar](https://avatars.discourse-cdn.com/v4/letter/a/e274bd/32.png) [@Achernar](https://boards.straightdope.com/u/Achernar)\ **Post date:** [March 20, 2003, 7:39am UTC](https://boards.straightdope.com/t/what-do-they-need-so-much-blood-for/162758/6 "2003-03-20T07:39:56Z")
10-12 pints is only 1.25-1.5 gallons, or about two and a half to three two-liter bottles. ---
**Author:** ![dwyr](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/dwyr/32/363_2.png) [@dwyr](https://boards.straightdope.com/u/dwyr)\ **Post date:** [March 20, 2003, 7:52am UTC](https://boards.straightdope.com/t/what-do-they-need-so-much-blood-for/162758/7 "2003-03-20T07:52:00Z")
Different tests have different requirements. Some need whole, anticoagulated blood for instance while others require only the serum portion of blood. You may have noticed some of the tubes have different colored stoppers. This indicates the specific preservative agent (or lack of one) in that particular tube. Another thing to consider is whether or not the lab on the premises will be doing all the tests ordered. Specialized tests are usually sent out to a reference lab and since you lose a little specimen each time you transfer it to another tube it’s good to have extra. Tests that are done in batches may only be run a couple of times a week so some blood is either refrigerated or frozen for these. And at our hospital at least, the doctors frequently think of something else they want after the blood is drawn and want the lab to add that. Also there are the rare lab accidents where a tube will break in the centrifuge. Or the instrument will decide to break down losing your entire freaking run and causing you to have to run everything again. I could go on but you get the idea. Over all it’s better to draw somewhat more blood than needed if this were a perfect world rather than get stuck twice. -a lab flunky ---
**Author:** ![sciguy](https://avatars.discourse-cdn.com/v4/letter/s/e68b1a/32.png) [@sciguy](https://boards.straightdope.com/u/sciguy)\ **Post date:** [March 20, 2003, 7:56am UTC](https://boards.straightdope.com/t/what-do-they-need-so-much-blood-for/162758/8 "2003-03-20T07:56:12Z")
Sorry, that’s what I get for trusting Google for a converter. Looks like the [site](http://www.escapeartist.com/Conversions/Gallons_Liters.html) I used uses the conversion for _quarts_ when the field is clearly labeled pints, and I can never remember the liquid imperial conversions. I always remember the blood volume in pints since that’s the standard blood donation amount. So **sonicsink** , listen to **Achernar** for the gallon/liter amount. Still, the beer glass analogy works better for me anyway 🙂 ---
**Author:** ![Nanoda](https://avatars.discourse-cdn.com/v4/letter/n/ce73a5/32.png) [@Nanoda](https://boards.straightdope.com/u/Nanoda)\ **Post date:** [March 20, 2003, 10:39am UTC](https://boards.straightdope.com/t/what-do-they-need-so-much-blood-for/162758/9 "2003-03-20T10:39:09Z")
Oh! No wonder I had to get the waitress to chip in. ---
**Author:** ![irishgirl](https://avatars.discourse-cdn.com/v4/letter/i/8edcca/32.png) [@irishgirl](https://boards.straightdope.com/u/irishgirl)\ **Post date:** [March 20, 2003, 11:01am UTC](https://boards.straightdope.com/t/what-do-they-need-so-much-blood-for/162758/10 "2003-03-20T11:01:15Z")
standard blood content of an adult male is 5 litres or 7-8 pints. at least that is what i learned for my exams last week, and i passed. 1 litre is the standard loss when donating blood at a blood drive. my guess would be that some of the blood is going with one type of anti-coagulant (eg heparin) and some of the blood is going with another type (eg EDTA) as the anti-coagulants can affect some tests. also, eg for liver function tests, they might need quite a bit of blood for the enzyme assays. ---
**Author:** ![Boyo\_Jim](https://avatars.discourse-cdn.com/v4/letter/b/87869e/32.png) [@Boyo\_Jim](https://boards.straightdope.com/u/Boyo_Jim)\ **Post date:** [March 20, 2003, 12:08pm UTC](https://boards.straightdope.com/t/what-do-they-need-so-much-blood-for/162758/11 "2003-03-20T12:08:12Z")
Sheesh, people can’t even agree on how metric conversions? 1 liter is just a little less (about 12%) than one quart. 5 liters is just over a gallon, or a bit more than 8 pints. In the US, the standard blood donation amount is 1 pint, or just over half a liter. sciguy’s conversion site is mislabelled. To the right of gallons, the labels SHOULD be quarts, pints cups, in order. ---
**Author:** ![Boyo\_Jim](https://avatars.discourse-cdn.com/v4/letter/b/87869e/32.png) [@Boyo\_Jim](https://boards.straightdope.com/u/Boyo_Jim)\ **Post date:** [March 20, 2003, 12:10pm UTC](https://boards.straightdope.com/t/what-do-they-need-so-much-blood-for/162758/12 "2003-03-20T12:10:53Z")
Sheesh, I can’t even get a sentence right. Take out the “how” in my forst sentence.