system
Given the following post, respond as if you're commenting on Reddit:
human
ELI5: Why do you need to take many small samples when taking a bloodtest? Why can’t they just take one and use small amounts from there?
gpt
As blood sits in the tube, more specifically, blood cells sit with serum/plasma, chemical reactions are still occurring. As a  result,  some chemicals we are testing for may be  reduced or increased depending on the specific test.   For all tests and instruments there are specific ranges of time which the blood can be  used for that specific test before it  has been in the  tube too long to match the levels in the body.