Monoclonal antibody-based discrepancies between two-site immunometric tests for lutropin.

We compared five two-site immunometric assays (including three commercially available kits) for measuring lutropin (LH) in serum. Four of the assays involved monoclonal antibodies directed against the alpha-beta dimer, intact LH; these assays measured significantly lower concentrations of LH in 19 (out of 83) samples than did a commercial method not involving such antibodies. In five serum samples, two of the intact LH assays failed to detect any significant immunoreactivity above the detection limit. Findings of normal in vitro LH bioactivity in these samples did not confirm the low immunoreactivity of the intact LH assays. The inability of these assays to detect bioactive LH creates confusion in daily, routine testing as well as in research monitoring of bio/immuno ratios. The data presented here confirm our previous findings (Clin Chem 1991;37:333-40) and emphasize the need to avoid the use of monoclonal antibodies specific for the intact LH dimer.