From experience, a blood sample which has been milked out of a finger is quite different to a venous blood sample.

(I am an anaestheologist and often perform or ask nurses to perform post-op point of care pinprick blood counts to look for anaemia in patients who have had big surgeries. We do 2 readings to check for error, and I often find that samples which have been squeezed out have a haemoglobin value 10-20% different to those which are drawn from a vein due to expression of tissue fluid.)