The specimen of choice for coagulation testing is plasma.
Venous blood is drawn into a 3.2% buffered sodium citrate tube (light blue top tube), yielding a whole blood sample with a 9:1 blood-to-anticoagulant ratio. Inadequate filling of the collection tube will decrease this ratio and may affect test results.
A blue-top tube for coagulation testing should be drawn before other tubes containing additives. This includes tubes containing other anticoagulants and/or plastic serum tubes containing clot activators. A serum tube that does not contain an additive can be collected before the blue-top tube.
If a winged blood collection set is used to draw a specimen for coagulation testing, a discard tube should be drawn first. The discard tube must be used to fill the dead space in the blood collection tubing to maintain the proper anticoagulant/blood ratio, but it does not need to be filled. The discard tube should be a nonadditive or a coagulation tube.
If a blood specimen used for coagulation testing must be collected from an indwelling line containing heparin, the line should be flushed with 5 mL of saline. The first 5 mL of blood, or 6 times the line volume (dead space volume of the catheter), should be drawn off and discarded before the coagulation tube is filled.
