The following question is presented in my biology textbook:

You are required to draw blood from patient and keep it in a test tube for analysis of blood corpuscles and plasma. You are provided with the following four types of test tubes. Which one of them will you not use for the purpose?

  1. Test tube containing calcium bicarbonate
  2. Chilled test tube
  3. Test tube containing heparin
  4. Test tube containing sodium oxalate

In thinking through the question, I reasoned that since we are collecting blood from a patient for biochemical assay or hematocrit, we would want it to be in an anticoagulated stage. Sodium oxalate and heparin are both anticoagulants and therefore should serve my purpose well. I don't think a chilled test tube would have any effect directly, except the fact that low temperatures delay clotting. So, I had chosen the test tube containing calcium bicarbonate to be unsuitable, as $\ce{Ca++}$ is one of the factors required for clotting and hence would accelerate the clotting procedure.

The textbook disagrees with me, and gives (3) as the answer.

Is this an error in the textbook key, or a flaw in my reasoning? Which should be the correct answer, and why?


Your reasoning is sound and correct. The answer key is wrong.

An unclotted blood sample needs something to prevent clotting. Extracellular calcium is required for both the coagulation cascade and platelet activation. It even has its own name in this context, Factor IV. This why EDTA, a calcium chelator, is used in some blood collection tubes to delay clotting.

So, as you say, the most correct answer would be (1): Test tube containing calcium bicarbonate. That is the opposite of the kind of tube you should use for this type of sample.

The answer given by the key, (3): Test tube containing heparin, is incorrect. Heparin tubes are a good choice for plasma and whole blood analysis, as you indicated. Heparin is a good anticoagulant, both in vitro and in vivo.

See this nicely laid out list from UCI medical school for an example of specimen requirements for specific tubes.


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