Part of could be that it is easier to fuck up a finger prick blood draw and cause inaccurate results.  Improper cleaning [1], not getting the blood out in the right way, or even certain patient conditions.  Less likely to mess up (in a way that causes test inaccuracy) when you drain a whole vial out of their arm.<p>[1] <a href="http:&#x2F;&#x2F;www.ncbi.nlm.nih.gov&#x2F;pmc&#x2F;articles&#x2F;PMC1238762&#x2F;pdf&#x2F;brjgenprac00036-0046a.pdf" rel="nofollow">http:&#x2F;&#x2F;www.ncbi.nlm.nih.gov&#x2F;pmc&#x2F;articles&#x2F;PMC1238762&#x2F;pdf&#x2F;brjg...</a>