Quality control of rheumatoid factor (RF) testing in Finland.

RF has been on the national laboratory quality control program since 1981. A pilot study had showed that most participants would be small laboratories performing qualitative slide latex tests only. Since we nevertheless wanted to be able to extrapolate test sensitivity data for each laboratory, we have sent out sets of 5-6 coded samples that in fact constitute a double dilution titration series. RF positive sera were pooled and the material calibrated with the WHO RF standard. The samples have contained 5-160 IU RF/ml and have been distributed once a year. The number of participants has varied from 58 to 104. Interlaboratory variation has diminished considerably since 1981. Inversions in the estimates of RF contents in the sample series have been notably rare. Waaler-Rose returns have mostly been too small to permit statistical evaluation. The same so far goes for the turbido- and nephelometric methods, in use since 1985.