The comparative merits of agglutination techniques (differential agglutination titre (DAT)/latex slide test) and nephelometry for the routine measurements of IgM rheumatoid factors have been studied in this 2-part paper. The first part investigates the errors inherent in standard DAT measurements, and the second determines the rate of false positive results in an elderly nonrheumatoid population. It was found that the errors in agglutination techniques are due to differences in setting up the tests rather than in interpreting the results, and that the coefficient of variance was consistently around 20%. Approximately 1 in 5 of DAT results have such a high error as to be clinically valueless. The false positive rate in the elderly nonrheumatoid population was only 2% with nephelometry compared with 9% on the latex slide test. A previous study had demonstrated the advantages of nephelometry over DAT in routine rheumatological use, showing not only greater reproducibility but also a more accurate positive detection rate than the DAT in rheumatoid arthritis. The results also suggest that the traditional spectrum of rheumatoid factors of low titre detectable in the general population is probably an artefact inherent in agglutination techniques and that a definitive cut-off point is more likely. As nephelometers are generally available in biochemistry laboratories there is much to recommended their routine use for the measurement of IgM rheumatoid factors.
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