Expression of rheumatoid factors in titres or units?

About 1% of the Dutch population is per year checked for the first time for the presence of rheumatoid factors (RF). Unfortunately, the interlaboratory variations of the titres of RF tests are enormous. We therefore (re)studied the effect of comparisons of the results with the WHO standard rheumatoid serum, leading to expression of the results in international units (IU), instead of in titres. Testing 5 sera in duplicate in 33 laboratories, a significant decrease of the interlaboratory variability was noted for the Waaler-Rose test, the Latex fixation test and the ELISA for the demonstration of RF's. Also the results of the antiperinuclear factor (APF) test improved significantly. The results of RF's demonstrated with the indirect immunofluorescence technique showed already a satisfactory comparability. Expression in IU's had no significant effect for this test. The introduction of the use of a reference serum met no opposition. Nearly all Dutch laboratories express now their results in units.