The test is a haemaglobin and is the same whichever method is used. In The UK you can use a laboratory who require a tube full but many AMEs have bought machines that do it on a capillary sample from the finger. It is up to the AME not the CAA. I have posted here before about the different machines
I suspect therefore it simply depends on what equipment the doctor has to hand! But do tell us why it matters? It is the same test