A new sensitive and specific enzyme-linked immunosorbent assay for IgD.

We have developed a new highly specific ELISA for IgD, and then used it to measure levels of circulating IgD in the serum of 480 un-selected patients from the East Anglia region of UK. The assay is both extremely sensitive and specific, with a minimum detected IgD concentration of 30 pg/ml and more than 10,000-fold specificity for IgD over all other human immunoglobulins. The assay shows linear dilution characteristics with both purified IgD and human serum, and spiking of purified IgD into either purified immunoglobulins or human serum shows c. 100% recovery. Furthermore, intra-assay and inter-assay coefficients of variation for repeated measurements of the same samples are below 10% and 15% respectively. Measurement of IgD levels on the un-selected patient population showed levels to range from <300 pg/ml to over 100 microg/ml, with a geometric mean of 8 microg/ml. The distribution is approximately normal after log transformation. Levels of circulating IgD were higher in men than in women. There was a significant negative correlation between levels of IgD and age in women, but not in men. Moreover, after adjustment for age and sex, there were statistically significantly higher levels of circulating IgD in male (but not female) smokers, compared to their non-smoking counterparts. These results highlight the care that needs to be taken to control for age, sex and cigarette smoking when examining levels of circulating IgD in future studies.