A comparison of IgM rheumatoid factor by nephelometry and latex methods: clinical and laboratory significance

To study the comparative performance characteristics of nephelometric and IgM latex rheumatoid factor (RF) with regard to diagnosis, to determine the level of test result that most accurately classifies patients as having rheumatoid arthritis (RA) versus noninflammatory rheumatic disorders, to develop mathematic and graphic conversion tools, and to determine the usefulness of serial nephelometric RF factors test in assessing change in clinical status.

Sera and clinical measurements were obtained from 576 rheumatic disease patients during routine clinic visits and were analyzed by latex and nephelometric methods.

Nephelometric and latex methods were highly correlated (0.872) and generally equivalent in their ability to discriminate between RA and noninflammatory conditions. Serial measures of RF were correlated with changes in clinical activity, but were much less effective in this respect than erythrocyte sedimentation rate or C-reactive protein.

Both tests yield equivalent results. Mathematic and graphic models will allow conversion between tests so that clinicians can use either or both methods effectively. RF is modestly correlated with changes in clinical activity and therefore is not a useful measure to follow clinical improvement or worsening.