To obtain the best capillary sample for testing, care must be taken to avoid clotting. Because clotting starts immediately after the skin is punctured, clotting is the greatest challenge in collecting a good capillary sample. The first drop of blood contains interstitial fluid which in turn contains tissue thromboplastin. Tissue thromboplastin is released as soon as the skin is broken and tissue penetrated, as a first step in the coagulation cascade. Therefore, the first drop of blood should be wiped away with sterile gauze.
If platelet clumps are formed during collection, test results will be inaccurate or erroneous. Tests such as complete blood counts (CBC) will be affected. Also, small clots can block the instrument's aspiration probes or tubing, which in turn can cause delay in testing and unscheduled maintenance.
The healthcare provider performing the capillary puncture should work quickly to avoid the formation of microclots. Completion of capillary blood collection should be under two minutes.