The PT and APTT are important screening tests used in laboratory evaluation of patients suspected to have disorders of blood coagulation, including the presence of circulating coagulation inhibitors. Clinical and Laboratory Standards Institute (CLSI) recently published an updated document, One-Stage Prothrombin Time (PT) Test and Activated Partial Thromboplastin Time (APTT) Test; Approved Guideline”Second Edition (H47-A2), which provides guidelines for performing the PT and APTT tests in the clinical laboratory, for reporting results, and for identifying sources of error.

Richard A. Marlar, PhD, Professor of Pathology, Chief, Clinical Pathology, University of Oklahoma Health Sciences Center, and Chairholder of subcommittee that developed the document, says, The PT and PTT assays are two of the most utilized and perhaps versatile tests in the clinical laboratory. Performing them correctly and in the appropriate setting can be a difficult task. H47-A2 presents the state of the art using evidence-based methods and criteria for performing these assays. This document was designed to provide all sizes and complexities of laboratories to use the PT and PTT correctly for the diagnosis and treatment of coagulation disorders.

The objective of the guideline is to improve test reproducibility through standardization of technique and ensure clinical relevance by setting test performance goals. The document also highlights the international effort for standardization of the PT using the international normalized ratio (INR).

This document is a revision of H47-A, which was published in 1996 and includes the following updates:

  • validating and calibrating PT reagents;
  • local system calibration;
  • PT mixing studies;
  • APTT mixing studies;
  • monitoring direct thrombin inhibitors;
  • establishing heparin therapeutic ranges; and
  • factor sensitivity determination.

The document is written for laboratory and/or clinical personnel responsible for the performance, quality control, and reporting of the PT and APTT tests, as well as for manufacturers of coagulation instruments and reagents who are responsible for maintaining appropriate performance standards.

For additional information on CLSI or for further information regarding this release, visit our website at or call +610.688.0100.

CLSI, formerly NCCLS, is a global, nonprofit, membership-based organization dedicated to developing standards and guidelines for the health care and medical testing community. CLSI™s unique consensus process facilitates the creation of standards and guidelines that are reliable, practical, and achievable for an effective quality system.

Clinical and Laboratory Standards Institute (CLSI)
Amanda Holm, Marketing Manager
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