aPTT in Heparinized Patients: Influence of the Interval Between Sampling and Testing

Abstract

Activated partial thromboplastin times (aPTTs) from 29 whole blood samples were drawn from patients receiv ing unfractionated heparin through a constant intravenous drip. Three aPTTs were determined for each whole blood sample. The first aPTT was performed on a separated portion of the original sample at approximately 30 min from the time of sam pling. A second was performed on the same separated sample at approximately 90 min from the time of sampling, and the third was performed on the original sample at 90 min. The mean aPTT of the set of 30-minute separated samples was 79.7 seconds. The mean aPTT of the 90-minute separated samples was 72.8 seconds. The mean aPTT of the 90-minute original sample was 60.8 seconds. These results demonstrate that an aPTT performed 30 min after sampling yields a value signifi cantly greater than an aPTT performed at 90 min. The lower aPTT values may lead to an unnecessary increase in heparin dosage. This situation underscores the need to further investi gate the aPTT as a foundation for the standardization of the protocol for the use of heparin.

Monitoring Unfractionated Heparin Therapy with Antifactor Xa Activity Results in Fewer Monitoring Tests and Dosage Changes than Monitoring with the Activated Partial Thromboplastin Time

“…In addition, the response of the aPTT to UFH varies markedly with the kind of equipment, thromboplastic reagents, and timing that a laboratory uses, making interlaboratory variation important. 20,[23][24][25][26][27][28][29][30][31] Heparin activity, measured as either antifactor IIa 32,33 or antifactor Xa34 activity, has been considered too expensive and cumbersome to be used for routine monitoring [6][7][8] despite the fact that the assay is available in over 4000 laboratories in France alone. 8 Technologic improvements have made the assays much easier to perform, and it might be a more accurate way to monitor therapy in at least some subgroups of patients.…”