EVALUATION OF MATERNAL SERUM IMMUNOREACTIVE INHIBIN AS A FIRST TRIMESTER MARKER OF DOWNS-SYNDROME

BACKGROUND AND OBJECTIVE Maternal serum immunoreactive inhibin has bee n shown to be significantly elevated in Down's affected pregnancies in the second trimester, suggesting that it may be useful in prenatal di agnosis. We have investigated whether it is similarly elevated in the first trimester. DESIGN Stored maternal sera from women with Down's af fected pregnancies and chromosomally normal control pregnancies were r etrieved for analysis. These sera had been collected prospectively at either 11 or 12 weeks gestation as a routine antenatal booking procedu re. SUBJECTS From records, 11 women were identified as having had a Do wn's pregnancy. For each of these, 4 controls matched for gestation an d duration-of-storage were also identified. MEASUREMENTS Two different inhibin immunoassays were evaluated, one using an antibody raised aga inst 31 kDa bovine inhibin and the other, a commercial two-site assay, using two antibodies directed against two distinct alpha-subunit epit opes. RESULTS Neither assay detected a significant effect of gestation on serum inhibin levels. After combining the data from both gestation s, no significant difference between the Down's samples and controls f or either assay was detected. However, analysis of the data for each g estation separately revealed that one assay detected a significant dif ference in inhibin levels between Down's affected and unaffected pregn ancies at 11 weeks gestation (mean +/- SEM 3186 +/- 195 vs 2020 +/- 17 2 ng/l, P < 0.01) but not at 12 weeks. The other, commercial, assay di d not detect a significant difference at either gestation. In addition , there was poor association between the results of the two assays. CO NCLUSIONS These data suggest that immunoreactive inhibin, as detected by these assays, will not be useful as a late first trimester marker f or Down's syndrome and also that these two assays detect different inh ibin species in pregnancy serum.