Common tests for rheumatoid factors: poorly standardized but ubiquitous.

Rheumatoid factor (RF) test results reported in the College of American Pathologists' surveys for 1983–1985 lacked inter-laboratory reliability and mutual validity. Using the 4 most popular commercial kits for RF testing, participating laboratories consistently identified as “positive” or “negative” all but the weakly positive samples. A wide range of titers was reported on qualitative testing, however. One popular kit using a modified sheep red blood cell agglutination technique yielded results that differed markedly from those with other kits. Investigators apparently have paid little attention to these discrepancies. In Arthritis & Rheumatism, from 1983 to 1985, over 50% of the articles that referred directly or indirectly to RFs omitted details of RF methodology. Until a reliable RF test is adopted, it is essential that such methodologic information be specified.