Activated Clotting Time Versus Activated Partial Thromboplastin Time for Therapeutic Monitoring of Heparin

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Abstract
Objective: To compare and contrast the activated partial thromboplastin time (aPTT) and the activated clotting time (ACT) for the therapeutic monitoring of heparin therapy. Data Sources: Relevant articles were identified through an English-language MEDLINE search from 1966 to 1995. Additional sources were identified from the reference lists of these articles. Study Selection: Studies that investigated the use and limitations of the individual assays and those offering direct comparisons were chosen for review. Data Extraction: Features demonstrating clinical applications and limitations of the aPTT and the ACT were extracted. Where possible, data suggesting preferential application of either assay also were extracted. Data Synthesis: Both the aPTT and ACT are clinically useful for the monitoring of heparin therapy. The aPTT is used more frequently for routine monitoring; the ACT is used in specialized situations requiring large heparin doses. The ACT is typically performed at bedside and is capable of yielding results rapidly and perhaps at a lower cost than an aPTT performed by a central laboratory. Most practitioners are familiar with the central laboratory aPTT. A bedside aPTT device is available, but is not yet in widespread clinical use. Both assay techniques are subject to various limitations. Conclusions: The ACT is theoretically equally as useful as the aPTT for the routine monitoring of heparin therapy, but has not been well-studied. The ACT appears more useful in situations in which high serum concentrations of heparin are required. Further cost-effectiveness and clinical outcome studies directly comparing the ACT and the aPTT in specific clinical situations are needed.