The Dependence of the International Sensitivity Index on the Coagulometer Used to Perform the Prothrombin Time
Received 07. März 1989
Accepted after revision 05. Februar 1990
30. Juni 2018 (online)
This study was designed to detect any effect that different types of coagulation instrument may have on the International Sensitivity Index (ISI) of a thromboplastin.
Manufacturers of commercial thromboplastins now calibrate their reagents against the World Health Organization international reference preparation to assign them an IST. This enables the prothrombin time (PT) estimated with that reagent to be expressed as an International Normalised Ratio (INR).
One batch of Thromborel S was calibrated against the Australasian Reference Thromboplastin (ART). The Thromborel S was used on three photo-optical instruments, the Automated Coagulation Laboratory (ACL) (Instrumentation Laboratory), the Cobas Fibro (Roche), and the Coag-a-Pet (General Diagnostics). PTs using ART were performed manually using the reference method.
The ISIs calibrated in our laboratory when the ACL and Cobas Fibro were used were not significantly different at the 95% level, being 1.102 ± 0.018 and 1.134 ± 0.022 respectively. The ISI with the Coag-a-Pet of 1.223 ± 0.023 was significantly different to that of the ACL and the Cobas Fibro at the 95% level.
The flowcharts for a computer program to perform the necessary calculations are provided. The program allows for the entry and editing of data from the calibration procedure, and provides a mean normal PT and normal range, the ISI and 95% confidence limits of the calibration, and a chart for the conversion of the test PTs to INRs.
The authors have made available an IBM compatible program for the calibration of thromboplastins.
- 1 Ray MJ, Hawson GA T. (Letter) The Australian Reference Thromboplastin. Newsletter 03/1983
- 2 Kirkwood TB L. Calibiation of reference thromboplastins and standardisation of the prothrombin time ratio. Thromb Haemostas 1983; 49: 238-44
- 3 Lam Po-Tung PR L, Paull H. The Australian Reference Thromboplastin and INRs. In: Advances in Coagulation Testing: Interpretation and Application Triplett DA. (ed) College of American Pathologists; Illinois: 1986
- 4 ICSH/ICTH recommendations for reporting prothrombin time in oral anticoagulant control. J Clin Pathol 1985; 38: 133-4
- 5 Poller L, Taberner DA, Thomson JM, Darby KV. Survey of prothrombin time in National Quality Assessment Scheme exercises (1980-87). J Clin Pathol 1988; 41: 361-4
- 6 McKeman A, Thomson JM, Poller L. The reliability of international normalised ratios during short-term oral anticoagulant treatment. Clin Lab Haematol 1988; 10: 63-71
- 7 Evatt BL, Brogan D, Triplett D, Koepke J, Waters D. Effect of thromboplastin and instrumentation on the prothrombin time test. Clin Lab Haematol 1981; 3: 331-42
- 8 van den Besselaar A MH P, Evatt BL, Brogan DR, Triplett DA. Proficiency testing and standardization of prothrombin time: effect on thromboplastin, instrumentation, and plasma. Am J Clin Pathol 1984; 82: 688-99
- 9 Poller L. Laboratory of oral anticoagulants. Br Med J 1987; 294: 1184
- 10 Loeliger EA, van den Besselaar A MH P, Lewis SM. Reliability and clinical impact of the normalisation of the prothrombin times in oral anticoagulant control. Thromb Haemostas 1985; 53: 148-54
- 11 Lewis SM. Thromboplastin and oral anticoagulant control. Br J Haematol 1987; 66: 1-4
- 12 Thomson JM, Tomenson JA, Poller L. The calibration of the second primary international reference preparation for thromboplastin (Thromboplastin, human, plain, coded BCT/253). Thromb Haemostas 1984; 52: 336-42
- 13 Freund JE. Mathematical Statistics. Prentice-Hall; London: 1972
- 14 Poller L, Thomson JM, Taberner DA. Effect of automation on prothrombin time test in NEQAS surveys. J Clin Pathol 1989; 42: 97-100