Understanding Blood Draw Tubes: Choosing the Right Size for Accurate Results

Question
Hi Samantha,

I have a family member who has to have laboratory blood tests on a regular (or semi-regular) basis for his heart condition. He is also anemic and the nurse/doctor have told us to always let the lab personnel know to use `small' or pediatric tubes. However when at the lab last time they told me it isn't possible to test for certain things in small tubes. Specifically, they said the smallest volume they could take for a CBC is a short draw (and she said it is about 3ml of blood). For electrolytes and creatinine etc she said labs can't do short draw and it must be 5ml.
-Could you please tell me the least volume of blood needed
for these tests?
-As well, can you please tell me if microtubes (not sure of exact terminology) are effective for such tests. The patient weighs about 50 kg (elderly).
Thank you so much for your help.

Answer
Hi Chris, I'd be glad to try and answer your question.  I see you're writing from Canada.  I assume blood collection is about the same as here in the U.S. but I'm not sure.  Here in the U.S. just recently (within the last few years) they have discontinued the "pediatric" tubes and now call them "short draw".  They are all the same width now, the "pediatric" tubes used to be alot thinner.  Some tubes now are shorter than others.  I believe the lab told you correctly.  Especially the CBC tube.  It does need to be about 3 mL because of the anticoagulant in the tube.  If the volume of blood is not even with the amount of anticoagulant in the CBC tube, results can be affected.  Electrolytes and creatinine require a different type of tube, one that doesn't have any anticoagulant.  But the amount of blood needed depends on the tube used, and also the machine that runs the test.  As for microtubes, I'm not sure what you mean by that.  Do you mean the tiny "tubes" that are used for very small draws such as from a newborn from its heel or a child from its finger?  If that's what you mean, then that's not an option.  The blood from a finger/heel is totally different from venous blood (from an arm), and alot of tests cannot be run from this type of sample.  Hope this helps, Chris.

Samantha