LabNotes - Volume 14, No.1, 2004

Understanding Additives: Heparin

Heparin is an anticoagulant commonly used in chemistry and special chemistry testing. It is the recommended anticoagulant for many determinations using whole blood or plasma specimens because of its minimal chelating properties, minimal effects on water shifts, and relatively low cation concentration.

Heparin acts primarily through a complex that it forms with antithrombin III. This complex accelerates the inhibition of thrombin and activated Factor X to prevent clotting or activation of thrombin, which in turn prevents the formation of fibrin from fibrinogen. The source of heparin is usually either bovine or porcine lungs and intestines.

There are currently three salts of heparin that are commonly used in blood collection devices: ammonium, lithium and sodium. Lithium heparin is the recommended form of heparin to be used because it is least likely to interfere when performing tests for other ions. Lithium heparin is essentially free of extraneous ions. It should not be used for collection of blood for lithium levels.

Heparin is the only anticoagulant that should be used in a blood collection device for the determination of pH, blood gases, electrolytes and ionized calcium. Heparin should not be used for coagulation or hematology testing.

The heparin used in BD Vacutainer® Blood Collection Tubes and BD Microtainer® Tubes is preservative free. The recommended range of heparin in evacuated tubes is 10 to 30 USP units of heparin/mL of blood. Plastic microcollection tubes may contain less than 15 USP units/mL of blood. Tubes containing heparin should be inverted 8 to 10 times after collection to ensure thorough mixing of the additive with the blood and, therefore, complete anticoagulation of the sample.

References:

1. NCCLS, Tubes and Additives for Blood Specimen Collection - Fifth Edition; Approved Standard, H1-A5, Vol. 23 No.33, 2003.
2. Burtis, Carl and Edward Ashwood, Tietz Fundamentals of Clinical Chemistry, Fifth Edition, 2001.

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