Comparison of Inhibin A Immunoassays: Recommendation for Adoption of Standardized Reporting

Serum inhibin A measurements have become increasingly common in clinical use, and their inclusion in prenatal screening for Down syndrome has been recently advocated. Two methods are commercially available for the specific measurement of dimeric inhibin A in human serum or plasma (1). Given the absence of a gold standard and the evolving clinical utility of inhibin A, a detailed comparison of the two inhibin A ELISAs is warranted. In the studies reported here, we undertook the evaluation of these methods and a direct comparison of their performance. The results obtained support the use of both methods and provide a basis for comparing values across these methods. Furthermore, we recommend that laboratories adopt a reporting unit of IU/mL based on the recombinant human Inhibin A International Reference Preparation (IRP) distributed by the National Institute for Biological Standards and Control on behalf of the WHO (91/624). Inhibin A is a dimeric glycoprotein hormone belonging to the transforming growth factor-β superfamily of cytokines. In addition to its numerous local, tissue-specific regulatory or “cytokine-type” actions (2)(3)(4), ovary-produced inhibin A is an important negative feedback hormone that suppresses pituitary secretion of follicle-stimulating hormone during the late follicular and luteal phases of the menstrual cycle (5)(6). Furthermore, circulating concentrations of inhibin A appear to reflect tumor mass for certain forms of ovarian cancer (7), particularly granulosa cell adenocarcinomas, and measurement of inhibin A also may be useful in assessment of gestational trophoblastic disease (8)(9). In addition, the measurement of serum inhibin A has recently been demonstrated as useful in the clinical setting as part of the “quadruple” antenatal screen for Down syndrome (10)(11)(12). The only analytical methods developed to date for measuring the picomolar concentrations of inhibin A found in peripheral blood …