Undetectable luteinizing hormone levels using a monoclonal immunometric assay. 
Previous studies have shown wide discrepancies among the results obtained with different immunometric assays.  We present five cases (out of 4000 women) whose plasma luteinizing hormone was not detected using a LH immunometric assay (LH Stratus Baxter) but was recognized by other kits.  These cases concerned one 28-year-old woman presenting with infertility and four postmenopausal women.  The LH Amerlite kit gave detectable but low results.  The results obtained with the other kits were > 7 IU/l. FSH levels were > 7 IU/l. In one case, sera were taken before and after the menopause; differences between the LH results increased.  Discrepancies among LH assay kits have been attributed to variation both in standard curve calibration and in epitope specificity of the kit monoclonal antibodies.  The Baxter kit might misrecognize some isoforms present in postmenopausal women.  The present data illustrate the potential false results with such immunoassays in routine clinical laboratory testing.  When undetectable LH results are not clinically explained or when disparities between LH and FSH are observed, we suggest using a second methodology or a bioassay if necessary.  Improvement in LH assays and standardization might resolve the problem of discrepancies between the LH results.