It is optimal to obtain coagulation specimens by peripheral venipuncture. If multiple tubes are collected, refer to the Tube Guide - Order of Draw. 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:
- Moderate to marked hemolyzed sample
- Specimen received in laboratory more than 4 hours after collection
- Hct > 55%
- Under - filled tubes