The reliability of manufacturer-determined, instrument-specific international sensitivity index values for calculating the international normalized ratio

M. T. Cunningham, G. F. Johnson, B. J. Pennell, J. D. Olson

Research output: Contribution to journalArticle

33 Scopus citations


Some thromboplastin manufacturers are currently supplying the instrument- specific international sensitivity index (ISI) values of their reagents, allowing clinical laboratories to calculate instrument-specific international normalized ratio (INR) values on plasma samples from patients receiving coumarin therapy. However, the assumption that systematic interinstrument variability in the INR would be eliminated if manufacturer-determined ISI values were used remains unsubstantiated. This assumption was evaluated by comparing INR values obtained on one instrument that measures a mechanical endpoint (fibrometer) with one that measures a photo-optical endpoint (MLA- 700). Three thromboplastin reagents with instrument-specific ISI values supplied by the manufacturer (ISI range, 1.23-2.79) were used. For two of three reagents, the fibrometer INR values were significantly higher than the MLA-700 INR values (P < .01). Analysis of log prothrombin time ratio plots showed that this systematic variability was caused by inaccurate manufacturer ISI values. Of clinical significance is that the inaccurate ISI values produced a high number of discordant INR values between these two instruments (> 47% of plasma samples had one INR value within and one out of the recommended therapeutic range). The implication of these findings for laboratory monitoring of oral anticoagulation is discussed.

Original languageEnglish (US)
Pages (from-to)128-133
Number of pages6
JournalAmerican journal of clinical pathology
Issue number1
StatePublished - Jan 1 1994



  • International normalized ratio
  • International sensitivity index
  • Oral anticoagulation
  • Prothrombin time

ASJC Scopus subject areas

  • Pathology and Forensic Medicine

Cite this