Clinical and Laboratory Standards Institute document H54-A—Procedures for Validation of INR and Local Calibration of PT/INR Systems; Approved Guideline is one in a series of guidelines that addresses methodology in blood coagulation testing. It is intended to provide guidance for both manufacturers and clinical laboratory personnel responsible for reporting patient INR results. H54 describes the use of certified plasmas to enhance performance of the prothrombin time (PT)/International Normalized Ratio (INR) system test; reviews limitations of the PT/INR system that may occur when a manufacturer-determined ISI is used without local verification or calibration; and provides a rationale for performing local ISI verification with recommendations as to when PT calibration may be indicated. This guideline is published in two parts. Part I provides an expanded account of the subject and Part II is an abbreviated version that may be useful in the clinical laboratory. Methods of calculating local ISI are provided and the procedure for creating a calibration line for direct INR determination is included. In the expanded guideline, the method of certified plasma preparation and method of INR value assignments are also described in detail. This guideline includes a recommended INR range that certified plasmas should cover and recommended number of certified plasmas required for local ISI calibration. A protocol for performing calibration of PT systems is provided. The objective of this guideline is to improve precision and trueness (accuracy) of PT/INR systems and enhance both laboratory standardization and patient care.

CLSI document H54 reviews limitations of the INR system and provides a rationale for performing local PT/INR verification with recommendations as to when calibration may be indicated. The method of certified plasma preparation and method of INR value assignments are described in detail. Also included are the recommended INR range that certified plasmas should cover and recommended number of certified plasmas required for local ISI calibration. A protocol for performing calibration of PT systems is provided. Methods of calculating local ISI are included, as well as the procedure for creating a calibration line in order to interpolate a direct INR. 

To facilitate improved precision and accuracy of PT assay results, enhance laboratory standardization, and thereby optimize patient results and care, the guideline has been divided into two parts. Though both may be educational for all users, Part I is primarily intended for manufacturers of the reagents and instruments used in the PT/INR system, and for manufacturers of material such as certified plasmas to standardize the PT assay; Part II is written for laboratory professionals responsible for the performance of prothrombin time (PT) assays.