Should blood volume be considered when interpreting test results?

[MECarer54]

This is very much a layman’s question.

I’ve read that blood volumes in ME/CFS patients with POTS can be substantially lower than normal. I would like to know whether reduced blood volume directly reduces the effective “dose” of agents that are carried in the bloodstream, such as thyroid hormones.

If I understand correctly, hormone testing typically involves identifying the level of a specific hormone in a sample volume of blood, and then comparing the ratio found against typical ratios found in healthy subjects.

But if the patient’s blood volume is (say) 25% below normal, then presumably the total number of molecules of the hormone present in the patient’s body will also be 25% below normal. And, if it is the total quantity of hormone rather than the hormone: blood ratio that is significant in hormone effectiveness, then presumably a “normal” ratio of hormone will have a reduced effect in people with low blood volumes.

Does anyone know the science around this issue? As someone with zero scientific qualifications, I would welcome comments from those with greater knowledge, as well as pointers to any relevant research.

 

[Pyrrhus]

But if the patient’s blood volume is (say) 25% below normal, then presumably the total number of molecules of the hormone present in the patient’s body will also be 25% below normal.

Not quite. A "low blood volume" typically refers mostly to the amount of water and electrolytes in the circulation.

An easy way to think of it is to consider that "low blood volume" roughly means "de-hydrated". This is why you can increase blood volume simply by drinking a high-quality electrolyte drink, or by an intravenous saline infusion.

Therefore, it is conceivable that the concentration of hormones might be slightly higher in the dehydrated state. However, I don't know of anyone who corrects for blood volume when interpreting test results. Perhaps because blood volume doesn't fluctuate that greatly on a percentage basis, or perhaps because it is just too difficult to quantify.

Good question, though.