It is really hard to get more than a single drop out of fingerprick. The reason why is the capillaries close up very quickly - if you are really lucky you can get 2 drops out, but anything more is a struggle.

The second problem with fingerprick blood is you pick up quite a bit of skin tissue in the process, but the amount collected is quite variable. What you end up measuring is blood + some unknown percentage of skin tissue derived material. None of these things are good for reproducibility.