The Optimum Number and Types of Plasma Samples Necessary for an Accurate Activated Partial Thromboplastin Time–Based Heparin Therapeutic Range

Abstract

Context.—Monitoring of unfractionated heparin therapy by activated partial thromboplastin time (aPTT) using the ex vivo method for determining the aPTT-based heparin therapeutic range (HTR) is the standard of practice. Many intrinsic and extrinsic factors influence its accuracy. Objective.—To investigate the optimum number and types of samples acceptable for an accurate ex vivo HTR determination. Design.—Values from patients receiving unfractionated heparin are used to determine the HTR by published guidelines. The number and types of samples are changed to investigate the effect on HTR parameters. Results.—Absolute minimum number of samples for an accurate HTR is 20, with fewer than 10% of the samples from the same patient or 50% of the samples with international normalized ratio of 1.3 to 1.5. Conclusions.—The ex vivo HTR method is the best protocol currently available; however, the number of samples used affects its accuracy. The optimum number of samples is 30 or more but the absolute minimum number is 20. In addition, limitation of specific sample types also affects the HTR parameters. An inaccurate HTR may be calculated if inappropriate sample number or types of samples are used.