Poor venipuncture will result in activation of coagulation due to contamination with tissue factor. The sample should be mixed with anticoagulant rapidly to prevent clotting occurring within the tube. Therefore, vacutainer collection is preferred, although in some instances vacutainers can underfill. The hematocrit of the patient should be considered. The converse is true in anemic animals, in which a sample may clot due to undercitration.
Coagulation factors are notoriously unstable.
Samples should be submitted as soon as possible after collection. As most of us do not have access to a clinical laboratory laboratory on site, this usually means shipping the sample.