Critical issues in prenatal maternal serum alpha-fetoprotein screening for genetic anomalies

Biochemistry testing associated with pregnancy and the newborn period -- a lot has changed since you were a baby!

Everyone has been a newborn, and everyone's mother has been pregnant. Despite the commonality of these events, medical care and the clinical chemistry laboratory's role in it have changed remarkably over the last 50 years. This review is a historical overview of clinical chemistry testing that is related to pregnancy and the newborn period.

Maternal weight correction for alpha-fetoprotein: mathematical truncations revisited

The objective of this study was to determine if maternal weight correction as implemented 20 years ago for alpha-fetoprotein (AFP) testing is still appropriate, and what should be the weight cutoff for correction. AFP measurements from 44,951 biochemical screening cases from a single laboratory were divided by maternal weight, and separated into high >/=2.5 multiples of the median (MOM), and low <0.4 MOM. Without weight correction, AFP values are still skewed to lower levels of higher maternal weight (1.1% >/= 2.5 MOM). Using a truncated range to 200 lbs. (for women greater than 200), produces more uniform results than using an upper cutoff of 250 for truncation (2.7% vs 5.0% >/= 2.5 MOM). The same relationship is seen for "lows." Without correction, higher weight lowers AFP, correction to 250 lbs. overcompensates, and 200 lbs. gives the most even distribution, and, therefore, optimizes the distribution of highs.

Prenatal maternal dried blood screening with alpha-fetoprotein and free beta-human chorionic gonadotropin for open neural tube defect and Down syndrome

OBJECTIVE

Our purpose was to evaluate second-trimester prenatal screening for open neural tube defects and Down syndrome by use of dried blood specimen collection and transport.

STUDY DESIGN

A prospective study of 7497 dried blood specimens from patients <35 years old was performed. Specimens were assayed for maternal blood alpha-fetoprotein and free beta-human chorionic gonadotropin. Patient-specific risks for both disorders were calculated and used to determine whether further evaluation was indicated. The study included an evaluation of the median and SD of analyte multiple of the median levels.

RESULTS

The initial positive rate for open neural tube defect was 4.4% adjusted to 2.7% after ultrasonographic revision and collection of a second sample. The initial positive rate for Down syndrome was 3.6% adjusted to 2.8% after ultrasonographic revision. All seven cases of open neural tube defect were detected within the increased risk group. Six of 8 (75%) cases of Down syndrome were detected. The median alpha-fetoprotein multiple of the median was 3.5 in open neural tube defect cases and 0.6 in Down syndrome cases. The median free beta-human chorionic gonadotropin multiple of the median was 2.4 in Down syndrome cases. The SD (log e) of alpha- fetoprotein and free beta-human chorionic gonadotropin in 5868 unaffected white patients was 0.4022 and 0.5635, respectively.

CONCLUSION

Second-trimester dried blood screening for open neural tube defects and Down syndrome can achieve screening efficiency comparable to serum-based protocols with distinct advantages over the conventional method of blood collection.

Maternal serum free beta hCG screening: results of studies including 480 cases of Down syndrome

The median maternal serum free beta human chorionic gonadotropin (hCG) multiple of the median (MOM) of 480 Down syndrome cases in the second trimester was 2.64, significantly greater than the reported median MOM of intact hCG (p < 0.0001). In 234 of these cases from retrospective and prospective studies, the effectiveness of maternal serum free beta hCG was evaluated in combination with alpha-fetoprotein (AFP) and maternal age in second-trimester Down syndrome screening. Down syndrome detection in the gestational age range of 14-16 weeks was 82 per cent. In all gestational weeks (14-22), a 77.7 per cent Down syndrome detection rate was achieved. In prospective screening of 44,272 patients under the age of 35 years, 69 per cent of Down syndrome cases were detected (73 per cent in gestational weeks 14-16). The false-positive rate for the prospective study was 3.8 per cent. The use of free beta hCG combined with maternal serum AFP and maternal age-related risk for Down syndrome in a screening population (i.e., women under 35 years) yields an improved detection efficiency over other protocols.

Cardiac histologic pathology characteristic of trisomies 13 and 21

To identify fetal histologic features characteristic of specific chromosomal anomalies, we reviewed histologic slides of 415 cases, including therapeutic and spontaneous abortuses, stillbirths, and perinatal deaths. These included 126 cases (30%) with karyotypically confirmed trisomy 21 and 23 cases (5.5%) with trisomy 13. Two histologic abnormalities of the fetal heart were identified that correlated with specific karyotypic abnormalities: (1) a discrete central papillary muscle calcification was present in 14 of 85 (16%) cases with trisomy 21, in seven of 18 (39%) cases with trisomy 13, and in six of 255 (2%) controls (P less than .001); and (2) a focal ventricular epicardial lymphocytic infiltrate was present in 22 of 93 (24%) cases with trisomy 21 versus nine of 284 (3%) controls (P less than .001). When both histologic abnormalities coexisted, trisomy 21 was present in five of six cases (83%). Neither histologic finding was significantly associated with fetal or maternal infection or congenital heart defects. In a restricted prospective study of the hearts of fetuses with trisomy 21, papillary muscle calcification was demonstrated by specimen radiographs in four of six (67%) cases; one case was studied by specimen ultrasonogram, which identified a papillary muscle echodensity. We conclude that (1) a focal ventricular epicardial lymphocytic infiltrate is characteristic of trisomy 21, (2) papillary muscle microcalcifications are characteristic of trisomies 13 and 21, and (3) further studies are needed to determine whether papillary muscle calcification might be useful in antenatal ultrasonographic screening for chromosomal anomalies.

Maternal serum Down syndrome screening: unconjugated estriol is not useful

Study of 41 known Down syndrome cases and 441 matched controls did not confirm earlier reports that low unconjugated estriol levels can be used to detect fetal Down syndrome. Hence the obstetric community should exercise caution in using unconjugated estriol levels as a marker in prenatal Down syndrome screening.

Maternal serum alpha-fetoprotein screening. III. Pitfalls in evaluating black gravid women

Maternal serum alpha-fetoprotein levels are higher in black women. Misinterpretation of maternal serum alpha-fetoprotein screening results can subject black gravid women to unnecessary invasive diagnostic procedures and their calculable risks. Screening errors for black women can result from the use of normative data bases established with maternal serum samples drawn from other racial groups or the use of such data bases in conjunction with a published correction factor. Because the incidence of open neural tube defects is lower for blacks than for others, excessive false positive results for blacks (estimated to be 8817 to 28,215 cases annually) would be a pernicious misapplication of maternal serum alpha-fetoprotein screening. We address the problem outlined above and recommend independently developed, valid, normative data bases.

Maternal serum alpha-fetoprotein screening, maternal weight, and detection efficiency

Various methods of adjusting maternal serum alpha-fetoprotein levels based on maternal weight have been recommended in order to more appropriately assign risk in maternal serum alpha-fetoprotein screening. These adjustments may, however, result in a larger proportion of pregnancies designated as having increased risk and reduction in detection accuracy.