Blood Volume Sampling

A simple question has a simple answer, right? Of course, I would argue there is no such thing as a simple question. Take blood sampling, for example. A common question, if not simple, goes like this: what is an acceptable amount of blood to be taken in a study?

There is not a volume of blood that is “acceptable” or not acceptable. There is risk. The numbers that are used as benchmarks (and for qualifications for approval by the expedited process) come from blood donation amounts. This is related to the routine experiences of healthy people, who donate blood in their everyday lives, i.e., minimal risk. Although probably not necessary, let’s be clear on our definitions. Blood donation is usually defined as up to 550 mL from a healthy person (who has been medically screened), no more than every 8 weeks. Changing any of those (health status, including age and screening results, amount and time from last blood collection, etc.) voids the comparison.

So, any blood collection is a risk. Less than about 500 mL in a healthy person is minimal risk, but still a risk. More blood than that or pretty much any sampling in someone who is not “healthy” has at least possibly more risk. To be acceptable, all risk must be minimized (to the extent possible) and then balanced by benefit. There is no upper limit of acceptable risk, theoretically, but in reality there obviously is.

Risk and benefit is almost always the key. What is an acceptable amount of blood to be taken? The amount that produces a risk (after being minimized) that is less than the benefit of the study. If the study needs every drop of blood to be able to reach its objectives, and the possible benefits of those objectives are more than the total minimized risks, that amount is acceptable. Let’s look at it from the other perspective. One drop less than the study needs to reach those objectives is too little. If the whole study fails because there was not enough blood collected, there was no benefit.

So, please propose to draw as much blood as your objectives require. Plan for errors and possible missed collections if you choose. Be ambitious or cautious. But whatever you do, be clear. Think transparency, think open lines of communication. Be simple and redundant when you note what you are doing with the blood and why you need each drop.

How much blood is too much? One drop more than you need.

Currien MacDonald, MD, CIP
Medical Director
Aspire IRB