Performance of a new rate-nephelometric assay for rheumatoid factor, and its correlation with tube-titer results for human sera and synovial fluid.

We evaluated a rate-nephelometric assay for rheumatoid factor (RF), as developed for use with the Beckman Immunochemistry System (ICS). Within- run precision (CV) for low-, mid-, and high-dilution samples (60-400 kilo-int. units/L) was 3.5, 1.5, and 1.6%; between-run precision was 3.4%. Analytical linearity was excellent. Biological interference resulted in some degree of nonlinearity in more than 70% of the patients' samples tested. Sensitivities were 96.5 and 94.1% and specificities were 98.5 and 95.3% for the ICS RF and Wampole slide methods, respectively, for a clinically defined population of 170 patients. Results for 100 ICS RF-positive samples correlated well with concomitantly measured Calbiochem-Behring tube-titers. Weekly measurement of Calbiochem-Behring. Hyland Diagnostics, and ICL Scientific tube-titer values for RF along with the ICS RF values on samples from the same patients indicated stable ICS RF values but showed at least +/- 1 tube dilution variances both within and between the tube-titer methods. Therefore, it may not be appropriate to compare a precise new method with a relatively imprecise comparison method. Treatment of RF-positive samples with dithiothreitol, which disrupts the pentameric character of immunoglobulin M, rendered the samples negative by repeat ICS RF assay and confirmed the method's specificity for pentameric immunoglobulin M-RF. Serum RF concentration as determined by ICS paralleled changes in clinical symptoms in a patient treated with both effective and relatively ineffective regimens, which suggests a useful role for the assay in monitoring efficacy of clinical treatment.