Why 3 tubes of blood are needed for blood test?

[v0031]

When we go to the hospital sometimes, the doctor ask us to test blood to find the cause of the illness. I wonder why 3 or 4 tubes of blood are needed for blood test? I think a tube is far enough since the test is done by one computer.

So how many tubes of blood are needed when you guys go to the doctor for common disease?

[Grumpier Than Uncle Joe]

I'm with you, v, but it's not uncommon here either.

[Torrain]

As the Theranos scandal showed - the technology to diagnose based on tiny volumes of blood just isn’t there yet.

A major part of the problem is that often, when blood is taken, a whole panel of tests are done, and each must be done separately. So each test uses a portion of the blood which is mixed with a specific reagent and must then be discarded (as the reagents are often mutually exclusive).

Developing tech that can perform multiple tests, using smaller volumes of blood, will probably arrive eventually- but Elizabeth Holmes may have made it harder to get people to invest, now that the field is tainted with scandal.

[dead0man]

along with the above, they like to take more than they need because:
1.they already have the needle in your arm, it's cheap to pull another vile
2.more importantly, you are already there.  Easier to get it now than making you come back to get stuck again
3.thus it's better to have too much than too little, so they get a little more than they need in case there is a problem

[True Federalist (진정한 연방 주의자)]

Also, the first vial is often just a starter used to ensure the blood is being drawn appropriately before filling the vial(s) that will be tested.

[v0031]

This is cared by many people.

[vitoNova]

I never understood why they just couldn't give you a quick poke-in-the-ass and obtain all the information they needed that way, without draining your blood like dracula.