Immunoassay of rheumatoid factor by latex particle counting.

We assayed rheumatoid factor by instrumental latex particle counting. The calibration curve ranged from 12.5 to 500 int. units/mL. Maximal within- and between-assay CVs were 5 and 11%, respectively. Analytical recoveries ranged from 92.4 to 108%, and the relation between results and dilutions was linear in the range of 15 to 400 int. units/mL. Correlation with an enzyme-linked immunoassay (ELISA, Cordia kit) was r = 0.934 (n = 58), with turbidimetry r = 0.825 (n = 100), and with the Waaler-Rose test r = 0.834 (n = 73). Of 260 blood donors, 95% gave a value less than 10 kilo-int. units/L, which was taken as the upper normal limit. In a population of patients with rheumatoid arthritis (n = 47), 87.2% had rheumatoid factor greater than 10 kilo-int. units/L.