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A program for thromboplastin calibration.

Throughout the world, oral anti-coagulant cntrol is based on the prothrombin time test using various thromboplastims. The prothrombin time (PT) is expressed directly as an absolute time or as a ratio of the patient's PT to the mean of the normal range.

Because of the inevitable confusion resulting from the use of many different systems (defined as an instrument coupled with thromboplastin) and the varying sensitivies of thromboplastin preparations, the World Health Organization in 1977 established a primary international thromboplastin reference preparation made from human brain. In 1979, secondary reference preparations of rabbit and bovine thromboplastins were prepared.

The International Committee for Standardization in Haematology (ICSH) and the International Committee on Thrombosis and Haemostasis on Thrombosis and Haemostasis (ICTH) have now made a joint recommendation for the adoption of this calibration system. It was recommended that results be expressed as international normalized ratios (INRs) which are, in fact, normalized prothrombin time ratios. It was also proposed that manufacturers of thromboplastins indicate the relationship of their material to the reference preparation by determining the comparative slope (or sensitivity). Finally, they should provide a table indicating the relationship between the conventional terms for expression of prothrombin time test results and the INRs.

Users of commercial thromboplastin preparations are in turn urged to follow the manufacturers' recommendations for use of the slope of the thromboplastin to calculate the INR. In patients receiving oral anticoagulant therapy, the report should include this measurement along with the more traditional measurements in order to educate the clincian. This is especially important for the safety of any patient who is likely to be referred to another laboratory that may employ a different prothrombin time system.

Recent studies reported by Hull et al indicate that American patients on the average are maintained at a level of coumadin anticoagulation somewhat higher than British or Dutch patients. This appears to result in a greater incidence of hemmorhagic complications, which fortunately seem to be mild, for the most part.

Until American manufacturers include these calibration data, it may be appropriate for labs monitoring anticoagulated patients to calibrate their particular thromboplastins against reference materials. In our lab, we calibrated our thromboplastin (Simplastin) against the certified reference rabbit thromboplastin (RBT/79) available from the Community Bureau of Reference in Bruseels.

The mathematical manipulations are sufficiently complex so that the interested parties probably will be deterred by the requirement to calculate orthogonal regression lines, slopes, standard deviations of logarithms of prothrombin times, and so forths.

The following program, written in BASIC on an IBM PC in our coagulation laboratory, allows one to analuze the comparative prothrombin times and generate a listing (Table 1) which gives measured prothombin times, prothrombin time ratios, and the proposed INRs. The program may be used on such other microcomputers as apple models and TRS-80s. For illustrative purposes, we have chose one-second intervals for PTs from 10 up to 30 seconds. To increase the intervals to, say, half-second intervals, modify program step 510 to read: 510 LET S = S + .5.

We hope this program will be of use in efforts toward appropriate calibration of this important laboratory test.
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Author:Koepke, John A.; Stewart, Charles E.
Publication:Medical Laboratory Observer
Date:Jul 1, 1984
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