Rate nephelometric determination of rheumatoid factor: comparison between Kallestad QM-300 and Beckman ICS-II (RF) methods.

The Kallestad Corporation recently suggested that their new buffer system for the nephelometric detection of rheumatoid factor conferred advantages over existing systems. Two rate nephelometric procedures, the Kallestad QM-300 and the Beckman ICS-II (RF), were therefore compared. Sera (n = 157) were selected on the basis of a previous ICS-II value. The results on the QM-300 of sera with an initial rheumatoid factor value of less than 400 IU identified two groups. Group 1 (n = 109) showed a good correlation with the ICS-II method while group 2 (n = 13) was highly discordant with the QM-300, producing significantly higher values. The values of 35 sera with an initial rheumatoid factor of greater than 400 IU were likewise highly discordant, with the QM-300 producing significantly lower values. Dilution recovery experiments implied that the Beckman buffer was likely to be contributory. As the formulae of the buffers remain proprietary, the reasons for the differences are speculative. The findings could be taken to indicate that the Kallestad value is a more accurate indicator of the quantity of rheumatoid factor than the Beckman value.