In phlebotomy, there is widespread avoidance of 25G and 27G butterflies based on belief that such small-bore needles hemolyze and/or clot blood samples and hence invalidate blood tests. Contrary to this belief, theoretical calculation and in vitro experiment both showed the exact opposite: namely, that shear stress and hence hemolysis decrease with decreasing needle bore (but the decrease can be clinically insignificant). In agreement with these results, a subsequent clinical trial found that 21G, 23G, and 25G butterflies connected directly to vacuum tubes caused the same amount of hemolysis and gave the same coagulation panel test results.
