Monitoring heparin therapy: APTT results from partial- vs full-draw tubes.

Results of the activated partial thromboplastin time (APTT) test can be influenced by biologic, preanalytic, and analytic variables. When a patient is being treated with unfractionated heparin, the correct interpretation of the APTT test result is essential. Laboratories should evaluate all variables influencing this result, particularly when determining the "therapeutic" range for heparin treatment. This study compared the APTT results assayed on specimens drawn into 2 different types of evacuated blood collection tubes. A statistically significant difference was seen in the results when the APTT was outside the reference interval, including results in the therapeutic range for unfractionated heparin. When the therapeutic range is determined by the laboratory, the evacuated blood collection tube system must be standardized.