It is optimal to obtain coagulation
specimens by peripheral venipuncture. If multiple tubes are collected, the
coagulation samples should be drawn last. If only a coagulation specimen is
collected, draw 1-2 mL of blood into a separate tube and discard it before
collecting the test specimen. This procedure prevents contamination of the specimen with tissue thromboplastin.
Blood for coagulation testing is collected in blue top tubes containing 3.2% liquid sodium citrate as the anticoagulant (3.8% is no longer recommended).
It is critical that the tubes are adequately filled so that the ratio of blood to sodium citrate is 9:1. Under- filled tubes will yield falsely increased clotting times due to the excess of anticoagulant. The minimun fill volumes are as follows: 4.0 ml for the 4.5 tube; 2.4 ml for the 2.7 tube; 1.6 ml for the 1.8 ml tube.
When samples are obtained from indwelling arterial or venous
catheters, the heparinized fluid infusion should be stopped. Collect 15mL
of blood through the indwelling catheter and discard it. Collect the test
specimen. Resume the heparin infusion after the specimen is obtained. Write
"A-Line" or "Line Draw" on the requisition. It is important
all coagulation specimens are obtained without heparin contamination. Heparin
contamination can affect coagulation test results.
If testing of a coagulation
specimen is delayed, centrifuge the specimen and transfer the plasma to a
plastic vial and refrigerate.
Special rejection criteria for coagulation specimens:
- Specimen hemolyzed
- Specimen received inlaboratory more than 4 hours after collection
- Hct > 55%
- Under - filled tubes