Use of likelihood ratios can improve the clinical usefulness of enzyme immunoassays for the diagnosis of small-vessel vasculitis.

tested with 2 different lots of the Combo SP device (lot nos. 8080021 and 9020102). The results showed a lot-to-lot difference in the effect of hCG cf on the results for hCGpositive urine (Fig. 1). Lot 8080021 was unaffected by up to 1 10 pmol/L hCG cf. On the other hand, lot 9020102 was inhibited by in a dose-dependent manner and at an hCG cf concentration of 1 10 pmol/L produced a result that was nearly invisible (Fig. 1). These data demonstrate the same inhibition by hCG cf, in a single lot of the Combo SP Rapid Test device, that was previously reported for the OSOM and ICON devices. Why these devices exhibit such between-lot variation is unclear. The package insert states that the device uses 1 monoclonal antibody and 1 polyclonal antibody. It is possible that one lot used a new batch of polyclonal antibody with a more avid specificity for hCG cf or that it used polyclonal antibody at a concentration that yielded a lower binding capacity for hCG. We previously provided data showing that caution is warranted when hCG devices in which hCG cf causes negative interference are used to test women who are pregnant beyond 5–8 weeks gestation, because false-negative results may occur (1). Our new data suggest that the same caution should be used when interpreting a negative result from any qualitative hCG-testing device. At this time, we know of no device that is free from the inhibition effect by hCG cf. This observation has been reported to the manufacturer and to the US Food and Drug Administration.