Overestimation of serum levels of rheumatoid factor caused by the presence of an incorrect calibrator in the Dade Behring kit

The level of rheumatoid factor (RF) in sera is a widely used criterion that has been adopted by the American College of Rheumatology (ACR) for the diagnosis of rheumatoid arthritis (RA). Measurement of RF was done initially using the Waaler–Rose test. This was succeeded by the latex agglutination test, in which denatured human IgG is absorbed by latex granules. At present, moreover, automated analyzers are being used worldwide with quantitative methods such as turbidimetry or nephelometry to measure RF. For instance, the RF measurement kit developed by Dade Behring (DB) (Marburg, Germany) uses a latex agglutination method quantifi ed by nephelometry; its market share is 22% in Japan and 17 % worldwide. In June 2006, DB notifi ed us that they discovered a discrepancy between their reference material (RM) and the international RM for RF, and that the company would replace its RF standard with new lots whose consistency with the international RM had been confi rmed. According to the notifi cation, DB had re-evaluated the traceability of RF on the basis of ISO/EN17511 (quantifi cation of biological samples – traceability for the indicated values of standard preparations and control substances) and found that there was a 50% discrepancy between the company RM and the international RM for RF (NIBSC 64/002), i.e., levels determined based on the company RM were approximately twofold higher than those determined based on the international RM. Our laboratory had been using the DB kit since March 2000. Upon receiving this notifi cation, we realized that it would be necessary to warn our clinicians of the possibility that they may have misdiagnosed patients as having RA on the basis of falsely positive RF tests. Accordingly, we immediately investigated the time frame and the extent to which this problem had impacted our clinical laboratory. For this analysis, we fi rst assayed a set of samples for RF, comparing the results obtained using commercial standard materials (SMs) from old and new lots. We then reassayed stored specimens (collected from 2000 to date) with the new lot. Finally, we analyzed the changes in monthly RF positivity rates using past data imported from the laboratory information system. RF was measured using a combination of N-latex RF II (DB) and a Behring Nephelometer II (BNII, DB). In the fi rst step, the correlations relating RF readings made with old (No. 183836) and new (No. 183837) lots of N-rheumatology standard SL were analyzed using 60 clinical samples sent to our laboratory for RF measurement. The correlation coeffi cient for the RF levels obtained based on calibration curves prepared using the old and new lots was high (r = 0.983), but the regression line was defi ned by the formula Y = 2.03X + 1.96 (n = 44; Y, old; X, new), as shown in Fig. 1. Thus, the RF levels measured with the old lot appeared to be almost twice as high as those measured with the new lot, which was consistent with the information supplied by DB. Next, 141 samples collected from fi ve patients during the period extending from 2000 to 2006 (7 RF negative samples from one patient, 35 weakly positive samples from one patient, and 99 positive samples from three patients) and stored at −40°C were assayed for RF using commercial SM from the new lot for calibration. When we compared the new RF levels with those determined from past measurements, we found that RF levels measured after May 2003 tended to be higher than those measured with the new lot (Fig. 2A). All of the past samples from one RF-negative RA patient were also judged as RF-negative based on the N. Hayashi · S. Kawano Department of Clinical Laboratory, Kobe University Hospital, Kobe, Japan