1 Research Institute for Endocrinology, Reproduction and Metabolism, Department of Obstetrics and Gynaecology, Vrije Universiteit Medical Centre, Amsterdam and 2 Division of Prenatal Diagnosis, Department of Obstetrics and Gynaecology, Vrije Universiteit Medical Centre, Amsterdam, The Netherlands
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Abstract |
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Key words: Down's syndrome/FSH/predictive value
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Introduction |
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In this study, we evaluated whether the relation between elevated basal FSH concentrations and Down's syndrome pregnancies could be useful in screening programmes identifying women at risk for a Down's syndrome pregnancy.
Materials and methods
We estimated test characteristics of pre-conceptional screening for elevated basal FSH concentrations to identify women at risk for a Down's syndrome pregnancy, using the results of a previously published casecontrol study in 118 women with a history of a Down's syndrome pregnancy and 102 controls (van Montfrans et al., 1999). In short, cases were recruited by advertisements in the magazine of the Dutch Down's Syndrome Foundation and every woman coming forward who had a history of a Down's syndrome pregnancy was asked to recruit one control participant from the same geographical area. Cases and controls were all <41 years of age. Participants provided serum samples on menstrual cycle day 2, 3 or 4 in three consecutive menstrual cycles after their pregnancies for measurement of FSH, oestradiol and inhibin-B concentrations. Written informed consent was signed by all participants and the study was approved by the hospital ethics committee.
Laboratory assays
FSH concentrations were measured with an MEIA (micro particle enzyme immuno assay; Abbott Laboratories, Abbott Park, IL, USA), calibrated against the second International Reference Preparation for FSH (78/549). The intra- and interassay coefficients of variation were 3.77.6% and 05.9% respectively. Oestradiol concentrations were measured by a competitive immuno-assay (Amerlite, Kodak Clinical diagnostics, Amersham, UK), with intra- and interassay coefficients of variation of 13 and 11% respectively for oestradiol concentrations below 500 pmol/l. Inhibin B concentrations were measured immunometrically by a commercially available assay (Serotec Ltd, Oxford, UK). The interassay variability for the inhibin B assay was 17% at 25 ng/l, 14% at 55 ng/l and 9% at 120 ng/l, while the lower limit of detection was 30 ng/l.
Statistical analysis
Age and tobacco use (possible confounders of basal FSH concentrations) (Cramer et al., 1994) and parity were compared used Student's t-tests and
2-tests, when appropriate. Mean hormone concentrations for FSH and oestradiol over three menstrual cycles were compared using a Student's t-test, and the number of women in each group with an FSH concentration >11.5 IU/l was compared with a
2-test. A multivariate logistic regression analysis was performed to evaluate the effect of basal FSH concentrations, age, smoking habits and parity towards the occurrence of Down's syndrome pregnancies.
Inhibin B concentrations were measured in the cycle with the highest basal FSH concentration, and mean Inhibin B concentrations were compared with a Student's t-test. Since there was no significant difference in oestradiol and inhibin B concentrations between Down's syndrome mothers and controls, only the predictive value of basal FSH concentrations was further evaluated. tection rates (sensitivity) and false positive rates (1-specificity) for increasing basal FSH threshold concentrations to identify Down's syndrome pregnancies were calculated and plotted in a receiver operator curve (ROC). Using this curve, the optimal basal FSH threshold value was identified by the point with optimal detection rate and a false positive rate <10%. Several types of bias are possible in this type of study possibly leading to optimistic estimates of the detection rates. Methods such as Lachenbruchs `leaving one out' method, to correct for such bias (Lachenbruch, 1975), were not used in this study because the effect could only be limited since the patient numbers were large. The positive and negative likelihood ratios corresponding with the optimal basal FSH threshold value were calculated. We then calculated the odds of a live born child with Down's syndrome, given an elevated basal FSH concentration for maternal ages between 2440 years, by multiplying the age related risk (Bray et al., 1998
) with the positive likelihood ratio (Cuckle and Sehmi, 1999
). Accordingly, the odds for delivering a live born child with Down's syndrome given a negative test result were calculated by multiplying the negative likelihood ratio with the age-related odds. The odds ratios would not be influenced by corrections for bias since only one variable was entered into the model in search of the best predictor.
We further calculated the detection rate of a hypothetical screening programme consisting of pre-conception measurement of basal FSH values, followed by first trimester serum screening for pregnancy associated para-protein A (PAPP-A) and free ß-HCG and by nuchal translucency (NT) measurement in all women (Wald et al., 1999). The detection rate of this regimen was calculated by adding the detection rate of basal FSH screening (14 at a false positive rate of 5%) to the product of the detection rate of combined first trimester screening (85 at a false positive rate of 5%) (Wald et al., 1999
) multiplied by the false negative rate (1-detection rate, 86%) of basal FSH screening. It was assumed that pre-conceptional basal FSH concentrations were not correlated with first trimester markers.
Furthermore, we calculated the number of procedure-related losses of unaffected pregnancies per identified Down's syndrome pregnancy and per 100 000 women screened, both for pre-conceptional basal FSH screening alone and in combination with first trimester screening. The rates of procedure-related losses were calculated by multiplying the number of false positive test results by the rate of pregnancy loss due to amniocentesis or chorionic villus sampling. We estimated the rate of unaffected fetuses lost due to amniocentesis or chorionic villus sampling at 0.9 and 1.4% respectively, based on a review of randomized trials (Wald et al., 1997). For this calculation, we used a threshold value for basal FSH values of 11.5 IU/l, associated with a detection rate of 14%, a false positive rate of 5% and a positive likelihood ratio of 2.8, and we assumed an a priori risk for a Down's syndrome pregnancy of 1:500 (corresponding with the age related risk in a 33 year old woman, the mean age of participants in our study).
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Results |
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Discussion |
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In subfertile patients however, signs of depletion of the primordial follicle pool may be identified by measurements of FSH during routine subfertility evaluation. Especially in artificial reproduction technique programmes, basal FSH concentrations have proven prognostic value for fecundity rates (Muasher et al., 1988; Scott et al., 1989
; Toner et al., 1991
; Magarelli et al., 1996
; Martin et al., 1996
; Buyalos et al., 1997
; Gurgan et al., 1997
; Sharif et al., 1998
). Many clinics therefore screen for elevated basal FSH concentrations in patients undergoing assisted reproductive technology regimens. On the basis of current knowledge, women known to have elevated basal FSH concentrations, who achieve pregnancy, are likely to have an increased risk for a Down's syndrome pregnancy (estimated increase 2.8-fold). These women could be informed of their increased risk. In our opinion however, non invasive screening tests such as first trimester serum screening and NT measurement should be offered first to these women, instead of chorionic villus sampling or trans-abdominal amniocentesis. Clinics offering invasive prenatal diagnostics using a risk threhold value of 1:250, could inform their patients aged >32 years of the increased risk in the presence of elevated basal FSH concentrations.
Although the detection rate of basal FSH screening proved insufficient for routine screening for Down's syndrome pregnancies, other markers for early depletion of the primordial follicle pool might provide a better screening test. Several other methods for measuring depletion of the primordial follicle pool are available, one of which is the clomiphene citrate challenge test, which is performed by measuring FSH concentrations before and after the administration of a fixed dosage of clomiphene citrate. The sensitivity of the clomiphene citrate challenge test to identify women with depletion of the primordial follicle pool is reported to be higher than that of screening for elevated basal FSH concentrations (26 versus 8%) (Barnhart and Osheroff, 1998). One may therefore speculate that a clomiphene citrate challenge test would have a higher sensitivity to identify women at risk for a Down's syndrome pregnancy. Other authors have reported that ultrasonographic assessment of the number of antral follicles early in the follicular phase of the menstrual cycle can be used to identify women at risk for a depleted primordial follicle pool (Scheffer et al., 1999
). There are however currently no data describing the relation between Down's syndrome pregnancies and abnormal results of the clomiphene citrate challenge test or with a low antral follicle count.
This study has several limitations, some of which may be attributed to the study design. First, our data were derived from a casecontrol study, which may have biased the results. For example recruitment bias may have occurred in patient and control selection, either causing overestimation or underestimation of our findings. Second, we assumed that elevated basal FSH values in women with a history of a Down's syndrome pregnancy and in controls were already present before pregnancy and not due to the Down's syndrome pregnancy. Although in our opinion it would be unlikely that FSH concentrations would rise specifically due to a chromosomally abnormal pregnancy, this study did not investigate basal FSH concentrations prior to pregnancy. Our data would have to be confirmed in a prospective study, measuring FSH concentrations prior to pregnancy, before definitive conclusions can be drawn.
In conclusion, we report that routine pre-conceptional basal FSH screening would not be useful to identify women at risk for a Down's syndrome pregnancy. However when elevated basal FSH concentrations are identified during subfertility evaluation, subfertile patients who achieve pregnancy theoretically are exposed to a 2.8 increased risk for a Down's syndrome pregnancy.
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Acknowledgements |
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Notes |
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References |
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Submitted on August 17, 2000; accepted on February 12, 2001.