Measurement of Unconjugated Estriol in Serum by Liquid Chromatography–Tandem Mass Spectrometry and Assessment of the Accuracy of Chemiluminescent Immunoassays

BACKGROUND

Unconjugated estriol (uE3) is routinely analyzed in clinical laboratories as risk assessment for Down syndrome. Immunoassays of various types are the most commonly used methods. The accuracies of RIAs and ELISAs for uE3 have been questioned, and to date there have been no independent studies investigating the accuracy of the relatively new chemiluminescent immunoassays. We developed and validated a liquid chromatography–tandem mass spectrometry (LC-MS/MS) method for uE3 measurements in serum.

METHODS

Serum samples from patients in the second trimester of pregnancy were used, and uE3 concentrations were measured by LC-MS/MS and the Beckman Coulter Access® 2 and Siemens IMMULITE 2000 automatic chemiluminescent immunoassay analyzers.

RESULTS

The LC-MS/MS method was validated and showed limit of detection 0.05 ng/mL; limit of quantification 0.2 ng/mL; linearity of response to 32 ng/mL; total imprecision of 16.2%, 10.4%, and 8.2% for uE3 at 1.10, 4.18, and 8.32 ng/mL, respectively; and analytical recoveries of 95.9%–104.2%. ANOVA of the correlation for LC-MS/MS results vs chemiluminescent immunoassays results showed R2 = 0.9678 (Access 2 = 0.9305 LC-MS/MS + 0.2177, Sy|x = 0.1786, P < 0.0001), and R2 = 0.9663 (IMMULITE 2000 = 0.8849 LC-MS/MS − 0.0403, Sy|x = 0.1738, P < 0.0001). Bland–Altman plots of uE3 results revealed concentration-dependent immunoassay biases. Mock risk analysis for Down syndrome showed no apparent difference in the risk assessment outcomes if the adjusted method-specific multiples of the median were used, and the assay imprecision was <10% CV.

CONCLUSIONS

Standardization of immunoassay methods for uE3 analysis is needed to improve the accuracy of the measurements.

Biochemical screening for Down’s syndrome in the first trimester of pregnancy

Down’s syndrome (DS) is the commonest single cause of severe mental retardation and accounts for just under a third of all cases. Until the late 1980’s population screening was based on offering amniocentensis to older women. This made little impact on the prevalence of DS because the majority (approximately 70%) of babies with DS are born to women under the age of 35 (Figure 1).

Evaluation of Down's syndrome screening population data sets by simulation: analyser-specific parameters may be superior to meta-analysis-derived parameters

OBJECTIVE

Choice of parameter sets used to calculate Down's syndrome risks is complicated. Published population statistics were compared with assay-specific parameters to optimise screening efficiency.

DESIGN

Weight-corrected Gaussian population statistics for alpha-fetoprotein (AFP), human chorionic gonadotropin (HCG) and unconjugated oestriol (uE(3)), expressed as log(10) multiples of median (MoM) were established for a Belgian cohort of 748 unaffected pregnancies. Using Cuckle's method and Access-specific data, Down's syndrome parameters were tailored to the Belgian cohort. Correlated marker triplets for affected and unaffected pregnancies were modelled and combined with maternal age to calculate term risks for Trisomy 21. Receiver-Operator-Curve (ROC) analysis was performed to identify the optimally-performing population set.

RESULTS

Log-normal distributions for the Access markers had geometric mean MoM values close to zero and standard deviation values equal to 0.1460 (AFP), 0.2185 (HCG) and 0.1317 (uE(3)). Correlation between AFP and other markers was significant (p < 0.001). Correlation between HCG and uE(3) was not significant (p = 0.4818). The median ratio between the lowest and highest risk outcomes for the test MoM set was 4.3. Areas under ROC curves differed significantly (p < 0.001) between the models and the analyser-assay specific parameters resulted in the largest area. At a 1 in 250 threshold, sensitivity and specificity were 69% and 96%. At false-positive rates (1-specificity) = 5%, sensitivity was 72.5%.

CONCLUSION

Population parameters significantly affect risk outcome and hence screening performance. Highest efficiency may be obtained with parameters tailored to an assay-specific population model. Consequently models from literature, without knowledge of the assay/analyser combination may lead to suboptimal performance.

Down's syndrome screening: a controversial test, with more controversy to come!

By 1998, most health authorities offered antenatal screening for Down's syndrome, usually by biochemical methods. To date, the development of this form of screening has not been coordinated by a national body and, consequently, there are wide variations in practice between localities. Fortunately, many of these variations have not led to any noticeable inequality of health provision, but the wide variation in risk cut offs used by different centres does. Other variations merely lead to potentially unnecessary expenditure; whereas it is believed that adding extra tests to the screening procedure is beneficial (such as double test to triple test), statistical evaluation of the confidence intervals for the detection rates quoted indicates that there is no evidence that the extra test provides an increase in detection. The cervical screening programme has progressively improved, partly through the auspices of a national framework. A similar national approach would benefit Down's screening and is only now being considered: the national screening committee (NSC) is currently drafting recommendations. To ensure optimum screening performance, the NSC should specify the risk thresholds applied, the screening protocols to be used--that is, an opt-in programme with a minimum (possibly even a maximum) of two biochemical analytes or a nuchal fold evaluation--and perhaps should even recommend national population parameters to be used for risk calculation. It might even be advisable for statistical work to be carried out to determine whether local derivation of medians is truly necessary. Furthermore, defined options for older women could be specified--for example, should all older patients have the option to proceed directly to amniocentesis if they wish or should National Health Service amniocentesis only be available for those with a "high risk" screening result. The difficulties that will face the NSC in deciding which screening policy to adopt are also considered; specifically, the lack of evidence to suggest that triple testing is superior to double testing, and the lack of evidence to prove the superiority of one analyte over another. This inadequacy of evidence is not from want of trying, but is caused by the problems of collecting enough data to provide statistical significance. Finally, there is one important difference between cervical and Down's syndrome screening that has a major impact on the advice given by any "expert"; namely, patents. Many aspects of Down's screening are subject to patents and, therefore, there is more potential for apparently uncontroversial decisions to rebound with future retrospective patent infringement claims. Thus, it would be sensible to insist that any member of a national body deciding upon Down's screening policy must fully disclose all potential conflicts of interest, both personal and family, before they are allowed to sit on the committee. Furthermore, if a national policy is decided upon, worldwide patent searches should be carried out to determine whether there are any possible unforeseen legal consequences of any recommendation.

Screening for Down syndrome during the first and second trimesters: impact of risk estimation parameters

OBJECTIVES

To evaluate impact of risk estimation parameters for screening for Down Syndrome during the first and second trimesters.

METHODS

We prospectively examined for their performance in the prenatal prediction of trisomy 21, alphafetoprotein (AFP), unconjugated estriol (uE3), total human chorionic gonadotropin (hCG), and its free subunits (free alpha-hCG, free beta-hCG) at both the first and second trimesters, and the impact of three sets of published risk estimation parameters. A total of 14,612 pregnancies were studied. All Down syndrome specimens (12 and 11 cases for first and second trimesters, respectively) and a sample of the unaffected pregnancies were analyzed.

RESULTS

The median multiple of median (MoM) for total hCG was lower in the first trimester (1.83 vs. 2.01 in the second trimester) but no loss in discriminative power was observed if the lower variability of the results in the first trimester is taken into account (interquartile range of 0.251 vs. 0.338). The choice of distribution parameters did not alter significantly the detection rates for the various combinations of markers (p > 0.05). False positive rates were affected significantly however and for the combination AFP-uE3-free beta-hCG they varied from 14.6% to 22.6% (p < 0.001).

CONCLUSIONS

Our results suggest that specific distribution parameters would be necessary to account for the lower variability of the markers in the first trimester and the peculiarity of the total hCG assay we used.

Advent of maternal serum markers for Down syndrome screening

Trisomy 21, or Down syndrome, is the most common serious autosomal chromosome aberration in which affected individuals survive beyond infancy. The association between advancing maternal age and increased risk of trisomy 21 is well known, and pregnant women older than 35 years at delivery are routinely offered invasive prenatal diagnostic testing. More recently, the use of maternal serum markers in the second trimester of pregnancy to predict the risk of trisomy 21 for women under the age of 35 has received intensive analysis. Maternal serum alpha-fetoprotein (MSAFP) was the first of these markers to be identified, and an inverse correlation between MSAFP level and risk of trisomy 21 was noted. A second marker, unconjugated estriol (uE3), has also been studied, and a correlation between low uE3 and trisomy 21 has been demonstrated, with a high level of correlation between AFP and uE3. The addition of uE3 to the screening protocol has not consistently improved detection rates, possibly because of its high correlation with AFP. A strong association of human chorionic gonadotropin (hCG) and Down syndrome was reported. This analyte is the most sensitive one in use today, although controversy exists regarding the best form of the analyte to use for trisomy 21 prediction. Several groups of investigators advocate measurement of total hCG, while others feel that measurement of the free-beta subunit of the molecule offers greater detection ability. The maximum detection rate that has been reported is 80 percent with a 5 percent false-positive rate using a combination of MSAFP, free-beta hCG, and maternal age.(ABSTRACT TRUNCATED AT 250 WORDS)

Screening for Down's syndrome

Down's syndrome (DS) is the commonest cause of severe mental retardation in children. It is the result of trisomy of chromosome 21 which is usually a random event though it is commoner in older mothers. DS can be diagnosed by chorionic villus sampling (CVS) and amniocentesis followed by karyotyping. Because of the risks associated with these invasive procedures, they can only be offered to a high-risk group. At one time the sole basis for identifying this increased risk was maternal age, but within the past ten years a series of biochemical and ultrasound abnormalities have been shown in DS pregnancies. The biochemical abnormalities include changes in the levels of most fetal and placental products in the maternal circulation. The best-known of these changes are the reduced levels of alphafetoprotein (AFP) and oestriol (E3) and increased levels of human chorionic gonadotrophin (hCG). The mechanism underlying these biochemical phenomena is unknown. Screening programmes involving the measurement of hCG and AFP, with or without additional parameters such as E3, at 15-18 weeks of pregnancy can typically identify 60% or more of cases of DS with a screen-positive rate of 5%. The combined risk derived from the various biochemical parameters, together with maternal age, is calculated by one of a number of computer programmes which have been developed for this purpose. There has been considerable discussion as to the exact biochemical tests which should be used for DS screening. This had led to controversy as to whether measurement of E3 has a place, and whether or not measurement of the free beta-subunit of hCG should replace measurement of the intact molecule. A notable recent development is the suggestion that measurement of the urinary beta-core of the hCG could be a highly discriminatory marker. A number of factors can affect the results of biochemical screening for DS. These include maternal weight, gestational age, ethnic origin, smoking, and diabetes. In addition, abnormal levels of the biochemical products may be found in other chromosome abnormalities.

First trimester biochemical screening for trisomy 21: the role of free beta hCG, alpha fetoprotein and pregnancy associated plasma protein A

The potential efficacy of screening for trisomy 21 in the first trimester, using maternal serum markers alpha fetoprotein, free beta human chorionic gonadotropin, unconjugated oestriol and pregnancy associated plasma protein A, was studied in an unselected population of women between the seventh and fourteenth week of gestation. Using a combination of alpha fetoprotein and free beta human chorionic gonadotropin, 53% of affected pregnancies could be identified at a false positive rate of 5%. Unconjugated oestriol and pregnancy associated plasma protein A levels were lower in cases of trisomy 21, but their inclusion with other markers did not significantly improve detection rate. Monitoring the same pregnancies also in the second trimester showed that screening in the first trimester identified the same cases as in the second. We conclude that first trimester screening using free beta human chorionic gonadotropin and alpha fetoprotein, is a viable possibility and will lead to detection rates in excess of 50%. Prospective studies are needed to confirm these observations.

Second-trimester maternal serum screening using alpha-fetoprotein, human chorionic gonadotrophin, and unconjugated oestriol: experience of a regional programme

Over a 2-year period from January 1991 to December 1992, second-trimester maternal serum screening for Down's syndrome using alpha-fetoprotein (alpha FP), human chorionic gonadotrophin (hCG), and unconjugated oestriol (uE3) was made available to five health districts in East Anglia, with a total population of 1.2 million. Amniocentesis was offered when the risk of Down's syndrome at term was 1:200 or greater. 25,359 singleton pregnancies were screened, representing an uptake of 77 per cent. The recall rate for the 24 per cent of women who had not had a dating scan prior to the test was 9.4 per cent compared with 3.9 per cent for those who had been scanned (P < 0.0005). Seventy-five per cent (36/48) of Down's syndrome pregnancies were detected for a false-positive rate of 4.0 per cent. Twenty-five out of 36 of detected Down's syndrome pregnancies were dated by scan prior to sampling, and in the 11 remaining cases, the dates were confirmed by scan after a high-risk result was obtained. The exclusion of uE3 from the screening protocol would have reduced the detection rate to 52 per cent (25/48) for the same false-positive rate. Eighty-five per cent of women identified at high risk accepted the offer of an amniocentesis. Other fetal abnormalities detected were trisomy 18 (3), trisomy 13 (2), 45,X (6), 69,XXX (5), other chromosome abnormalities (9), open neural tube defects (26), hydrocephalus (7), abdominal wall defects (4), and steroid sulphatase deficiency (6).

Prospective study of prenatal screening for Down's syndrome with free beta human chorionic gonadotrophin

OBJECTIVE

To assess the value and impact of a screening programme for Down's syndrome that uses the two maternal serum markers: alpha fetoprotein and free beta human chorionic gonadotrophin.

DESIGN

All women booked into clinics were screened. Further tests were offered to women with a risk of one in 300 or greater of having an affected baby. Follow up of outcome of all pregnancies.

SETTING

Biochemical screening laboratory serving two health districts.

SUBJECTS

8179 women of all ages with singleton pregnancies screened between 15 and 22 weeks' gestation from 1 April 1991 to 31 March 1992.

MAIN OUTCOME MEASURES

Detection rate of Down's syndrome, false positive rate, uptake of screening, uptake of amniocentesis in women identified as at increased risk, prevalence of Down's syndrome at birth.

RESULTS

Overall 89% (8317/9345) of women underwent screening. The rate of detection of Down's syndrome was 69% (11/16; 95% confidence interval 41 to 89%) with a 5.2% false positive rate (426/179; 4.7 to 5.7%). In women under 30 the detection rate was 50% (four out of eight; 32 to 86%) Uptake of amniocentesis was 89% (389/437), resulting in a reduction of prevalence of Down's syndrome at birth from 1.1 per 1000 in previous years (66/59,696) to 0.4 per 1000 during the screening year (4/9345). Additionally, several other abnormalities were identified.

CONCLUSION

The benefit of a high detection rate with this approach and the additional anomalies identified should encourage others to introduce screening programmes for Down's syndrome that use free beta human chorionic gonadotrophin and alpha fetoprotein.

Second-trimester unconjugated oestriol levels in maternal serum from chromosomally abnormal pregnancies using an optimized assay

Second-trimester unconjugated oestriol (UE3) levels were measured retrospectively in maternal serum from 78 chromosomally abnormal pregnancies and 390 matched controls using a radioimmunoassay kit (Amersham AMERLEX-M) optimized for use in the second trimester. Reduced levels of UE3 were found in a group of 49 Down's syndrome pregnancies with a median UE3 level of 0.79 multiples of the median (MOM) of the controls. Four trisomy 18 pregnancies had UE3 levels less than 0.7 MOM. There was a highly significant level of correlation between alpha-fetoprotein (AFP) and UE3 levels in the controls (r = 0.25, p < 0.01), the Down's syndrome pregnancies (r = 0.44, p < 0.01), and the other chromosome abnormalities (r = 0.61, p < 0.01). When used as an additional marker to AFP and human chorionic gonadotrophin in screening for Down's syndrome, UE3 does not appear to add to the sensitivity of such screening.