Evaluation of a dimeric inhibin-A assay for assessing fetal Down syndrome: establishment, comparison, and monitoring of median concentrations for normal pregnancies

Abstract

Context: Several studies report the role of dimeric inhibin-A in assessing risk for fetal Down syndrome. The majority, however, use the Serotec inhibin-A assay and not the newer Diagnostic Systems Laboratories inhibin-A enzyme-linked immunosorbent assay (ELISA).

Objectives: To establish normal gestational age day-specific medians, to compare our results against previous studies pertaining to the inhibin-A ELISA, and to evaluate long-term assay performance.

Design: Using the inhibin-A ELISA, 100 specimens were assayed for each completed week of gestation for weeks 15 to 20, 50 specimens for 14 weeks, and 54 specimens for 21 weeks or older. Regressed inhibin-A medians were calculated employing a second-degree polynomial fit of the arithmetic medians. Thereafter, inhibin-A ELISA lot comparisons were performed to evaluate consistency.

Results: Regressed values of 182, 174, 175, 184, 201, and 226 pg/mL resulted for weeks 15 to 20, respectively [pg/mL inhibin-A = 4.1528(gestational age)2 - 136.49(gestational age) + 1294.9]. A comparison with 2 other studies shows our values to be lower overall by 15 +/- 11.4% and 16 +/- 2.6%. However, variability between kit lots was as high as 30%.

Conclusions: The equation derived provides for the calculation of gestational age day-specific inhibin-A medians for integration into maternal serum screening programs with a subsequent decrease in false-positives expected and observed. Our medians differ considerably from those of other studies, with limited data, using the Diagnostic Systems assay. However, lot changes since the initial analysis have exhibited similar inconsistencies. Therefore, we recommend that others incorporating the assay into their screening programs carefully establish, monitor, and adjust their medians accordingly as a result of potential variations.