A Simple Nomogram for the Derivation of International Normalised Ratios for the Standardisation of Prothrombin Times

Summary

The WHO international scale of reporting prothrombin time results is based on the calibration of local and commercial thromboplastin reagents used in the test against an international reference preparation to derive an International Sensitivity Index (ISI) for each batch of local reagent. This quantifies an individual reagent’s responsiveness to the coumarin induced defect.

Once the ISI has been assigned, the derivation of International Normalised Ratios (INR) for reporting results depends on the use of a scientific calculator by which the local ratio is raised to the power of the ISI of the local reagent or use of an alternative formula and logarithms.

A system for interpretation of INR, without the need for calculations, is presented for the known range of thromboplastins between 1.0 and 3.0 ISI in nomogram form. This chart may facilitate the general application of the INR system.

Key words

Prothrombin time - Standardisation - International Normalised Ratios - International Sensitivity Index

• References

• 1 Poller L, Thberner DA. Dosage and control of oral anticoagulants. An international survey Br J Haematol 1982; 51: 479-485
• 2 Taberner DA, Poller L, Burslem RW, Jones JB. Oral anticoagulants controlled by British Comparative Thromboplastin versus low dose heparin in prophylaxis of deep vein thrombosis. Br Med J 1978; i: 272-274
• 3 Hull R, Hirsh J, Jay R, Carter C, England C, Gent M, Turpie AG, McLaughlin D, Dodd P, Raskob G, Ockelford P. Different intensities of anticoagulants in the long-term treatment of proximal vein thrombosis. N Engl J Med 1982; 307: 1676-1681
• 4 WHO Expert Committee on Biological Standardisation. 31st Report. WHO Tech Rep Ser 1983; 687: 81-105
• 5 WHO Recommended Methodology for Using WHO International Reference Preparations of Thromboplastin. WHO; Geneva: 1985
• 6 Loeliger EA. International Committee for Standardisation in Haematology. Recommendations for reporting prothrombin time in oral anticoagulant control Thromb Haemostas 1985; 53: 155-156