# When They Draw Blood for Blood Tests, Why Do They Draw So Much Blood? **URL:** \ **Category:** Factual Questions\ **Created:** [September 5, 2018, 10:36pm UTC](https://boards.straightdope.com/t/when-they-draw-blood-for-blood-tests-why-do-they-draw-so-much-blood/820838 "2018-09-05T22:36:38Z")\ **Posts on this page:** 19\ **Page:** 1
**Author:** ![PastTense](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/pasttense/32/14550_2.png) [@PastTense](https://boards.straightdope.com/u/PastTense)\ **Post date:** [September 5, 2018, 10:36pm UTC](https://boards.straightdope.com/t/when-they-draw-blood-for-blood-tests-why-do-they-draw-so-much-blood/820838/1 "2018-09-05T22:36:38Z")
Considering how small blood cells are… ---
**Author:** ![steatopygia](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/steatopygia/32/3132_2.png) [@steatopygia](https://boards.straightdope.com/u/steatopygia)\ **Post date:** [September 5, 2018, 10:49pm UTC](https://boards.straightdope.com/t/when-they-draw-blood-for-blood-tests-why-do-they-draw-so-much-blood/820838/2 "2018-09-05T22:49:14Z")
It isn’t really that much, it does\* look\* like a lot though. Most blood testing tubes are three to 6 ml. The largest commonly used tubes are 10 ml. Depending on what kind of tests are being done, there will be between 1 and 5 tubes drawn (usually). A high average would be 30 ml. That is 2 tablespoons. ---
**Author:** ![Jackmannii](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/jackmannii/32/311_2.png) [@Jackmannii](https://boards.straightdope.com/u/Jackmannii)\ **Post date:** [September 5, 2018, 10:51pm UTC](https://boards.straightdope.com/t/when-they-draw-blood-for-blood-tests-why-do-they-draw-so-much-blood/820838/3 "2018-09-05T22:51:52Z")
> [@PastTense](#): > > WhenThey Draw Blood for Blood Tests, Why Do They Draw So Much Blood? Elizabeth Holmes, is that you? ---
**Author:** ![Exapno\_Mapcase](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/exapno_mapcase/32/1051_2.png) [@Exapno\_Mapcase](https://boards.straightdope.com/u/Exapno_Mapcase)\ **Post date:** [September 5, 2018, 11:36pm UTC](https://boards.straightdope.com/t/when-they-draw-blood-for-blood-tests-why-do-they-draw-so-much-blood/820838/4 "2018-09-05T23:36:10Z")
Have you ever tried to test a single blood cell? I’m always amazed so that many tests can be done on so little blood. ---
**Author:** ![gotpasswords](https://avatars.discourse-cdn.com/v4/letter/g/c57346/32.png) [@gotpasswords](https://boards.straightdope.com/u/gotpasswords)\ **Post date:** [September 6, 2018, 12:01am UTC](https://boards.straightdope.com/t/when-they-draw-blood-for-blood-tests-why-do-they-draw-so-much-blood/820838/5 "2018-09-06T00:01:35Z")
You need a certain amount for the blood to stay liquid. If the lab collected just the one or two drops that they actually need, they’d dry up pretty quickly. Also, the “extra” gives some insurance against minor mishaps like “Oh, that was supposed to be a creatine level, not cholesterol? Good thing there’s more in the tube…” ---
**Author:** ![Joey\_P](https://avatars.discourse-cdn.com/v4/letter/j/919ad9/32.png) [@Joey\_P](https://boards.straightdope.com/u/Joey_P)\ **Post date:** [September 6, 2018, 12:21am UTC](https://boards.straightdope.com/t/when-they-draw-blood-for-blood-tests-why-do-they-draw-so-much-blood/820838/6 "2018-09-06T00:21:29Z")
> [@gotpasswords](#): > > You need a certain amount for the blood to stay liquid. If the lab collected just the one or two drops that they actually need, they’d dry up pretty quickly. Also, the “extra” gives some insurance against minor mishaps like “Oh, that was supposed to be a creatine level, not cholesterol? Good thing there’s more in the tube…” Not that this is an argument for drawing a minuscule amount, but those vials do have an anticoagulant in them. ---
**Author:** ![Projammer](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/projammer/32/559_2.png) [@Projammer](https://boards.straightdope.com/u/Projammer)\ **Post date:** [September 6, 2018, 1:46am UTC](https://boards.straightdope.com/t/when-they-draw-blood-for-blood-tests-why-do-they-draw-so-much-blood/820838/7 "2018-09-06T01:46:27Z")
There is also the occasional possibility that the results of one test reveals the need for additional test(s). It’s a whole lot easier to put a vial back on the conveyor than to call the patient up and ask them to come back in to collect another sample. ---
**Author:** ![Terminus\_Est](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/terminus_est/32/3087_2.png) [@Terminus\_Est](https://boards.straightdope.com/u/Terminus_Est)\ **Post date:** [September 6, 2018, 3:10am UTC](https://boards.straightdope.com/t/when-they-draw-blood-for-blood-tests-why-do-they-draw-so-much-blood/820838/8 "2018-09-06T03:10:21Z")
Note that red blood cells are mostly just bags of hemoglobin. Almost all of the substances tested for in a blood test are found in the plasma. This includes glucose, amino acids, cholesterol, fatty acids, hormones, carbon dioxide, urea, lactic acid, antibodies, and various narcotic substances. ---
**Author:** ![Nava](https://avatars.discourse-cdn.com/v4/letter/n/da6949/32.png) [@Nava](https://boards.straightdope.com/u/Nava)\ **Post date:** [September 6, 2018, 3:34am UTC](https://boards.straightdope.com/t/when-they-draw-blood-for-blood-tests-why-do-they-draw-so-much-blood/820838/9 "2018-09-06T03:34:26Z")
And the “other” blood cells get counted too. Platelets, white cells divided by kind. ---
**Author:** ![Qadgop\_the\_Mercotan](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/qadgop_the_mercotan/32/83_2.png) [@Qadgop\_the\_Mercotan](https://boards.straightdope.com/u/Qadgop_the_Mercotan)\ **Post date:** [September 6, 2018, 12:40pm UTC](https://boards.straightdope.com/t/when-they-draw-blood-for-blood-tests-why-do-they-draw-so-much-blood/820838/10 "2018-09-06T12:40:58Z")
Most blood tests aren’t looking at the cells anyway, they’re looking at substances in the plasma, and sometimes that particular assay takes a lot of plasma. ---
**Author:** ![mcgato](https://avatars.discourse-cdn.com/v4/letter/m/ac8455/32.png) [@mcgato](https://boards.straightdope.com/u/mcgato)\ **Post date:** [September 6, 2018, 12:47pm UTC](https://boards.straightdope.com/t/when-they-draw-blood-for-blood-tests-why-do-they-draw-so-much-blood/820838/11 "2018-09-06T12:47:31Z")
> [@Jackmannii](#): > > Elizabeth Holmes, is that you? I chuckled. I’m in the middle of reading the book on that, Bad Blood. Definitely an interesting read. ---
**Author:** ![PoppaSan](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/poppasan/32/189_2.png) [@PoppaSan](https://boards.straightdope.com/u/PoppaSan)\ **Post date:** [September 6, 2018, 1:26pm UTC](https://boards.straightdope.com/t/when-they-draw-blood-for-blood-tests-why-do-they-draw-so-much-blood/820838/12 "2018-09-06T13:26:36Z")
> [@Projammer](#): > > … put a vial back on the conveyor … ? 😕 ---
**Author:** ![DesertDog](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/desertdog/32/11503_2.png) [@DesertDog](https://boards.straightdope.com/u/DesertDog)\ **Post date:** [September 6, 2018, 1:28pm UTC](https://boards.straightdope.com/t/when-they-draw-blood-for-blood-tests-why-do-they-draw-so-much-blood/820838/13 "2018-09-06T13:28:36Z")
> [@Joey\_P](#): > > Not that this is an argument for drawing a minuscule amount, but those vials do have an anticoagulant in them. Some do, some don’t. That’s why Vacutainer tube tops have all those [purty colors](http://www.labppl.com/which-tube-do-i-draw.html) on them. Red/black have no preservatives at all in them. ---
**Author:** ![Nava](https://avatars.discourse-cdn.com/v4/letter/n/da6949/32.png) [@Nava](https://boards.straightdope.com/u/Nava)\ **Post date:** [September 6, 2018, 1:31pm UTC](https://boards.straightdope.com/t/when-they-draw-blood-for-blood-tests-why-do-they-draw-so-much-blood/820838/14 "2018-09-06T13:31:50Z")
A lot of the tests on blood are done by automated machines, **PoppaSan**. They actually have vertical conveyors; some models let you put in whole racks of vials, older models you still need to fill the intake area manually. It’s been over 20 years so both techniques and protocols will have changed, but back when I did a stage in my local hospital’s lab I was told an ‘extra’ tube was taken in order to be able to do retests without doing redraws, and also as safety in case some klutz dropped a tube. ---
**Author:** ![kayaker](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/kayaker/32/441_2.png) [@kayaker](https://boards.straightdope.com/u/kayaker)\ **Post date:** [September 6, 2018, 1:45pm UTC](https://boards.straightdope.com/t/when-they-draw-blood-for-blood-tests-why-do-they-draw-so-much-blood/820838/15 "2018-09-06T13:45:32Z")
> [@Qadgop\_the\_Mercotan](#): > > Most blood tests aren’t looking at the cells anyway, they’re looking at substances in the plasma, and sometimes that particular assay takes a lot of plasma. Additionally, while some tests can be done on plasma (the liquid component of anticoagulant containing blood), some require serum (the liquid component of blood that has been allowed to clot). ---
**Author:** ![CookingWithGas](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/cookingwithgas/32/485_2.png) [@CookingWithGas](https://boards.straightdope.com/u/CookingWithGas)\ **Post date:** [September 6, 2018, 1:52pm UTC](https://boards.straightdope.com/t/when-they-draw-blood-for-blood-tests-why-do-they-draw-so-much-blood/820838/16 "2018-09-06T13:52:19Z")
I don’t know anything about blood tests but math says that when you are testing a sample for a quantity of anything, the larger the sample, the more accurate the results (up to a point). It could be that the statisticians have determined the optimal blood volume for quant testing. ---
**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:** [September 6, 2018, 9:49pm UTC](https://boards.straightdope.com/t/when-they-draw-blood-for-blood-tests-why-do-they-draw-so-much-blood/820838/17 "2018-09-06T21:49:23Z")
As people have already said, the lab is testing the serum or plasma portion of the blood. I’d like to add that approximately half of any blood sample from a non-anemic person will consist of cells - red, white, and platelets - leaving the other half being what is actually used to test (most of the time). Also, in my experience, some doctors are very fond of adding on additional tests as they think of them. Sometimes days later - we store all specimens in the refrigerator for 7 days. A few physicians at my hospital prefer the “shotgun” approach and order one of everything. The phlebotomists usually draw a “rainbow” (one tube of every color) so they don’t have to stick the patients multiple times. And it is a rare event but tubes do sometimes break in the centrifuge so extra is nice. ---
**Author:** ![Johnny\_L.A](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/johnny_l.a/32/1084_2.png) [@Johnny\_L.A](https://boards.straightdope.com/u/Johnny_L.A)\ **Post date:** [September 6, 2018, 9:51pm UTC](https://boards.straightdope.com/t/when-they-draw-blood-for-blood-tests-why-do-they-draw-so-much-blood/820838/18 "2018-09-06T21:51:32Z")
Snacks. For later. ---
**Author:** ![TokyoBayer](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/tokyobayer/32/13989_2.png) [@TokyoBayer](https://boards.straightdope.com/u/TokyoBayer)\ **Post date:** [September 6, 2018, 11:34pm UTC](https://boards.straightdope.com/t/when-they-draw-blood-for-blood-tests-why-do-they-draw-so-much-blood/820838/19 "2018-09-06T23:34:25Z")
> [@Johnny\_L.A](#): > > Snacks. For later. For those doubters: Well, have you seen any vampires lately? So it must be working.