I’ve been seeing several posts in Medlab-L about the need for a discard tube when collecting hemostasis specimens using butterfy needle sets. The air within butterfly set tubing occupies approximately 0.5 mL and reduces first tube collection by that volume. For a 3.0 mL total collection tube, this is a 16% reduction in total final volume. Most manufacturers assert that anything over a 10% reduction prolongs PT and PTT results as it causes a relative increase in the anticoagulant to blood ratio.
This problem is best resolved using a discard tube. We collect two blue-stopper tubes and throw away the first. We can also substitute a non-additive tube such as a glass red-stopper tube for the first blue-stopper.
We have to be careful to retain the correct order of draw, given in our latest “Hemostasis Blood Specimen” module, as we do not want to draw an additive tube just before the blue-stopper. This may cause additive carryover affecting hemostasis test results. Plastic red-stopper tubes have particulate activator added, and should not be used ahead of the hemostasis tube. Geo.