"The fingertips of these fingers are usually less calloused, have fleshier pads and cause less discomfort for the patient."

"thumb... has a pulse. The index tends to be more calloused..area is also more sensitive.. the pinky doesnt hv sufficient tissue depth to prevent injury." https://twitter.com/medic_cosity/status/1304630238664511488

"When performing a fingerstick, the phlebotomist should puncture either side of the fleshy pad of the middle or ring finger, but not the extreme side of the finger. The exact center of the fleshy pad or the tip of either finger should also be avoided."

"Avoid areas of the finger that are cold, swollen, inflamed, calloused" (=hardened) "or cyanotic" (=interesting one, a separate thread for the same). "The bottom image indicates the correct area to use for skin puncture."

Source: https://www.labce.com/spg296239_using_the_fingers_for_dermal_puncture.aspx