Evolution of an inhibin A ELISA method: implications for Down's syndrome screening

A technical and clinical evaluation of a new assay for inhibin A and its use in second trimester Down syndrome screening

BACKGROUND

The objective of the study was to compare a new AutoDELFIA® Inhibin A kit (B064-102) with the Access Inhibin A kit (A36097) using clinical specimens and to evaluate the AutoDELFIA® Inhibin A assay performance in screening for Down syndrome in the second trimester of pregnancy.

METHODS

Using clinical samples, we performed a method comparison between new and existing inhibin A kits and assessed AutoDELFIA® Inhibin A kit precision performance. Normal median values for the second trimester of pregnancy were also determined. Finally, we evaluated the screening performance of the AutoDELFIA® Inhibin A kit together with other second trimester biomarkers for the detection of Down syndrome.

RESULTS

The two methods showed a high degree of correlation (r=0.99, Pearson and Spearman correlation), and the average relative level difference between the methods at a concentration range of 41.7-1925 pg/mL was 19.6% [95% confidence interval (CI) from 17.6% to 21.5%]. The acceptable precision of the AutoDELFIA® Inhibin A kit was demonstrated: the within-lot CV% varied from 1.9% to 3.9%. The screening performance results show that AutoDELFIA® Inhibin A when added to a combination of other second trimester serum markers [human alpha foetoprotein (hAFP), free beta subunit of human chorionic gonadotropin (free hCGβ) and unconjugated estriol (uE3) or hAFP and free hCGβ] improves the detection rate of screening in both combinations.

CONCLUSIONS

The performance of the AutoDELFIA® Inhibin A assay is highly acceptable for routine laboratory use for screening Down syndrome in the second trimester of pregnancy.

Use of detergent-based buffers allows detection of precursor inhibin forms in an immunoassay format

Inhibin ELISAs are used in monitoring aspects of reproductive function, however these assays are based on the measurements of the mature 30kDa inhibin forms and not precursor forms. In conventional ELISA formats, the 105kDa unprocessed 'Pro-inhibin' forms are immunologically inactive, but the immunoactivity can be recovered in the presence of detergents. The immunoactivity of Pro-inhibin forms was assessed in the presence of a range of detergents utilising antibodies to the α-, βA- and βB-subunits of inhibin. In contrast to mature forms, unprocessed inhibin forms showed a 10-40 fold increase in inhibin A and total inhibin immunoactivities under optimised detergent (0.5% SDS/2% Triton X-100) conditions. The suppressed immunoactivity of the Pro-inhibin forms in these immunoassays was attributed to steric hindrance by the respective βA- and α-subunit prodomains. This study details a detergent-based immunoassay that allows detection of previously undetectable precursor inhibin forms.

Aneuploidy screening in the first trimester

This article reviews the performance of first trimester screening for chromosomal anomalies using various combinations of ultrasound and maternal serum biochemical modalities. Detection rates in excess of 90% can be routinely achieved for Trisomy 21, Trisomy 13, Trisomy 18 using a combination of fetal nuchal translucency (NT) thickness and maternal serum free ss-hCG and PAPP-A at 11 + 0 to 13 + 6 weeks of gestation.

Use of serum inhibin A and human chorionic gonadotropin measurements to predict the outcome of in vitro fertilization pregnancies

OBJECTIVE

To assess the clinical value of maternal serum inhibin A measurements in early pregnancy in the prediction of outcome of IVF pregnancies and to compare the predictive accuracy of inhibin A concentrations with serum hCG concentrations.

DESIGN

Retrospective study.University-based IVF program.

PATIENT(S)

One hundred fifty IVF pregnancies of 150 couples were studied during a 4-year period.

INTERVENTION(S)

None.

MAIN OUTCOME MEASURE(S)

The association between pregnancy outcome and age, number of collected oocytes, number of transferred embryos, and serum hCG and inhibin A concentrations in early pregnancy was studied with logistic regression. Predictive accuracy of inhibin A and hCG concentrations was calculated by receiver-operating characteristic (ROC) analysis.

RESULT(S)

Lower serum concentrations of inhibin A and hCG were associated with increased odds for preclinical abortion and early pregnancy loss, whereas higher inhibin A and hCG concentrations were observed in multiple ongoing pregnancies. Inhibin A measurements were superior to hCG in the prediction of preclinical abortions; no significant difference was observed between the predictive value of hCG, inhibin A, or their combination in differentiating between ongoing pregnancies and early pregnancy losses. The discriminative potential of inhibin A for prognosticating multiple ongoing pregnancies was lower than that of hCG.

CONCLUSION(S)

Although serum inhibin A concentrations are more accurate than hCG levels for predicting preclinical abortion after IVF, they had no advantage in forecasting ongoing or multiple ongoing pregnancies, suggesting that routine assessment of serum inhibin A concentrations during follow-up of IVF pregnancies is unjustified.

Molecular weight forms of inhibin A, inhibin B and pro-alphaC in maternal serum, amniotic fluid and placental extracts of normal and Down syndrome pregnancies

BACKGROUND

Inhibin A, an established prenatal marker of Down syndrome (DS), exists in the maternal circulation in a number of isoforms. The present study explored whether specific inhibin A isoforms may be selectively increased in DS, offering the prospect of improved marker performance.

METHODS

Second trimester maternal serum, placental extracts and amniotic fluid (AF) pools from both normal and DS pregnancies were fractionated by a combined immunoaffinity (IA) chromatography, preparative polyacrylamide gel electrophoresis (Prep-PAGE) and electroelution procedure. Inhibins A, B and pro-alphaC were determined in the eluted fractions by specific enzyme-linked immunosorbent assays (ELISAs) and the profiles of immunoactivity (IA) characterized in terms of molecular weight (MW) and percentage recovery.

RESULTS

The MW patterns of inhibin A and pro-alphaC in maternal serum and AF were similar between DS and control pregnancies, both showing peaks between 25-40 k and approximately 65 k. AF contained, in addition, a higher proportion of <30 k inhibins A and B, and <25 k pro-alphaC forms. There were large differences in the inhibin forms present in DS placentae, with more 70 k and less 30-40 k inhibin A than in controls.

CONCLUSIONS

The present data suggest that the processing, cleavage or secretion of inhibin MW forms by the DS placenta differs from normal. However, these differences are not reflected in maternal serum and so improvements in serum screening will not be afforded by measuring specific inhibin A isoforms.

Advances in prenatal screening for Down syndrome: I. general principles and second trimester testing

BACKGROUND

Down syndrome is one of the most important causes of mental retardation in the population. In the absence of prenatal screening and diagnosis, prevalence at birth in the United States would currently exceed 1:600. The purpose of prenatal screening is to identify those women at the increased risk for an affected pregnancy and to maximize the options available to these women.

TESTS AVAILABLE

Second trimester serum screening involves combining the maternal age-specific risk for an affected pregnancy with the risks associated with the concentrations of maternal serum alpha-fetoprotein (MSAFP), unconjugated estriol (uE3), and human chorionic gonadotropin (hCG) (triple testing). A forth analyte, inhibin-A (INH-A), is increasingly being utilized (quadruple testing). Optimal second trimester screening requires the integration of a number of clinical variables, the most important of which is an accurate assessment of gestational age. In addition to Down syndrome, the triple and quadruple tests preferentially identify fetal trisomy 18, Turner syndrome, triploidy, trisomy 16 mosaicism, fetal death, Smith-Lemli-Opitz syndrome, and steroid sulfatase deficiency. Some programs modify the Down syndrome risks generated through maternal serum screening tests with fetal biometric data obtained by ultrasound. Other second trimester tests have shown promise, including the analysis of maternal urine and fetal cells in the maternal circulation, but none are in routine clinical use.

CONCLUSION

The second trimester triple and quadruple tests provide benchmarks for evaluating new screening protocols. The combination of fetal biometry, new test development as well as clarification of the role of co-factors that affect the concentrations of analytes in existing tests should lead to greater efficacy in second trimester screening for Down syndrome.

Alterations in peripheral concentrations of inhibin A in cattle studied using a time-resolved immunofluorometric assay: relationship with estradiol and follicle-stimulating hormone in various reproductive conditions

The aims of this study were to develop a sensitive and specific assay for bovine inhibin A using europium and to investigate the endocrine role of inhibin A in various reproductive conditions by characterizing the relationship between profiles of inhibin A, FSH, and estradiol and follicle growth during the postpartum period, during the intact estrous cycle, and in cows with follicular cysts. The time-resolved immunofluorometric assay (Tr-IFMA) for bovine inhibin A, using purified polyclonal antibodies to alpha and beta(A) subunits, was specific for bovine inhibin A and did not cross-react with bovine activin A, activin AB, activin B, pro-alphaC or human recombinant inhibin B. The detection limit of the IFMA was 3.3 pg/ml expressed in terms of bovine 32-kDa inhibin A. Dose-response curves of plasma samples obtained from intact and FSH-stimulated cows and cystic cows were parallel to the standard without any preassay processing of samples. Plasma inhibin A levels increased (P < 0.01) concomitant with emergence of nonovulatory or ovulatory follicular waves during the postpartum period. In cystic cows, plasma inhibin A was sustained at high levels for a longer period, associated with growth of persistent dominant follicles. The highest levels of inhibin A were noted during the growth phase of normal and persistent dominant follicles; however, inhibin A levels declined (P < 0.01) as these dominant follicles ceased to grow or ovulated. An inverse relationship between patterns of plasma inhibin A and FSH existed during each follicular wave in the three physiologic conditions. Increases in plasma inhibin A levels were associated with increases in plasma estradiol levels during most follicular waves; however, there was no increase in plasma estradiol level and no relationship between patterns of estradiol and FSH during follicular waves observed during the early postpartum period or midluteal phase of the estrous cycle. In conclusion, the Tr-IFMA does not require pretreatment of samples and can be used for precise measurement of bovine inhibin A without interference with free inhibin alpha subunits. Inhibin A, produced primarily during growth of the dominant follicle, functions as a negative feedback regulator for FSH secretion throughout the postpartum period and the estrous cycle, whereas estradiol appears to have a minor role in regulation of FSH compared with inhibin A, especially during the early postpartum period and midluteal phase of the estrous cycle. The results also indicate that a persistent dominant follicle sustains inhibin A production for a longer period than the dominant follicle emerging in the estrous cycle and establishes long-term dominance by suppressing emergence of a new follicular wave.

Measurement of inhibin A: a modification to an enzyme-linked immunosorbent assay

Inhibin A is a useful prenatal marker of Down syndrome. Currently, the available enzyme-linked immunosorbent assays (ELISAs) for inhibin A are based upon the same paired monoclonal antibodies. In the present study we have confirmed for one of those ELISAs that short-term sample storage as whole blood leads to a significant decline in detectable inhibin A and that this is most likely due to erythrocyte catalase interference with a critical oxidation step in the assay. While this interference can be eliminated by heating the samples pre-assay, this process is labour intensive. In the present study we have demonstrated that the addition of 3-amino-1,2,4-triazole (AT), a catalase 'suicide' inhibitor, also prevents the decline of inhibin A levels in samples stored as whole blood. We suggest that the addition of AT to samples prior to assay is a simple modification to the inhibin A ELISA that affords optimum performance.

Enzyme immunoassays for inhibins, activins and follistatins

In this short review, the authors summarise the inhibin, activin and follistatin assays developed by the Oxford group and collaborators, and some of the main purposes for which they have been applied. Over 500 research publications have used these assays. We also discuss new assays recently developed at the request of our collaborators for particular applications, and comment on outstanding assay problems.