Preventing and detecting neural tube defects.

"Attention young women ... a modern miracle ... for only pennies a day, you can achieve better health throughout your lifetime and help protect the health of future generations!"

Such a statement may sound too good to be true, but it also may not be that exaggerated. A decade ago, the Department of Health and Human Services developed national health promotion and disease prevention objectives for the year 2000; and 22 priority areas were identified in the resulting document, Healthy People 2000.[1] In the area of prevention, improved maternal and infant health is the number one objective to be achieved by the start of the next millennium. In 1995, each priority area was reviewed, and additional objectives were developed, including a 50% reduction by the year 2000 in the incidence of spina bifida and other neural tube defects (NTDs).[2] One vital component in achieving this objective is to optimize folate levels.

Background

Along with 2 other B vitamins, folate prevents elevated plasma levels of the atherogenic amino acid, homocysteine. Folate is essential for normal development of the fetal nervous system, perhaps because of its ability to prevent elevated levels of homocysteine in amniotic fluid.[3] Folate also seems to protect the body in later life, as the risks increase for chronic diseases, including heart disease, stroke, and cancer, especially in the colorectum and uterine cervix.[4] Elevated levels of plasma homocysteine are common in the general population, especially among the elderly.[5] Independent of hypercholesterolemia, hyperhomocysteinemia is associated with atherosclerotic vascular disease[6,7] and may also increase the risk of thrombosis by altering platelet function and clotting-factor activity.[8]

In the U.S., the infant mortality rate is higher than that of 24 other industrialized nations, and reduction of this rate by approximately one third is a main priority for the Healthy People 2000 initiative.[2] Twenty percent of infant deaths are caused by birth defects, which makes these abnormalities the primary cause of infant mortality in the U.S.[9] Every 3.5 minutes, a baby is born with a birth defect[10]; and from 1980 to 1995, NTDs and other birth defects of the central nervous system were the second largest contributors to infant deaths attributable to birth defects.[11]

Adverse pregnancy outcomes are less likely if a woman's health is optimized before pregnancy. To reduce the risk of fetal NTDs, both preconception and prenatal care recommendations call for consumption of more folic acid than was established in 1989 as the recommended daily allowance (RDA) for nonpregnant women.[12] The March of Dimes Birth Defects Foundation recently announced that it will spend $10 million over the next 3 years to spread the word that folic acid can greatly reduce the risk of some birth defects, especially NTDs.[13]

Though not a miraculous cure-all, adequate consumption of folic acid offers a convenient, cost-effective strategy to help protect women and men against certain cancers and cardiovascular disease. By improving her own folate status now, a woman can also help protect her future children. Prevention is the ultimate goal, but assessment and reliable detection of fetal NTDs remain challenges for clinical laboratorians. This article will examine:

* the formation of NTDs

* the importance of folate for women of childbearing age

* a possible homocysteine connection to abnormal development of the central nervous system

* the importance of enzymes whose defects may contribute to NTDs

* some of the biochemical markers for diagnosing NTDs.

Types and formation of NTDs

The fetal nervous system originates early in pregnancy, usually 3-4 weeks after conception. As the embryo develops, the ectoderm-derived neural plate indents to produce a neural groove, which is essentially an open depression bordered by neural folds [ILLUSTRATION FOR FIGURE 1 OMITTED]. The neural groove is the first morphologic evidence of the nervous system. The 2 neural folds join and create the neural tube with a central canal. This neural tube becomes the nervous system that includes the brain, spinal cord and nerves. These first critical events begin approximately 18-20 days after fertilization occurs,[14] which is when a woman may note that her period is a week late, but may not even suspect that she is pregnant.

As fetal development progresses, neural groove closure begins and proceeds at multiple sites along the length of the tube. Closure of the head (cranial) end of the tube occurs by approximately 24 days; closure of the tail (caudal) end is completed by approximately 26 days.[15] Assuming that all goes well, the neural tube is completely closed after 4 weeks of gestation.

Abnormalities of the brain and spinal cord - such as anencephaly and spina bifida, respectively - arise if the neural tube doesn't close completely. Spina bifida is a general term indicating incomplete closure of the vertebral arches and exposure of the spinal cord. Of all infants born each year in the U.S., 1 in 2,000 have spina bifida and 1 in 8,000 have anencephaly.[16] In the latter, a major portion of the brain, skull, and scalp are missing. Most of these infants are stillborn or die a few days after birth.

In "open" spina bifida, which accounts for most cases of this type of malformation, neural tissue is exposed or protected only by a thin membrane. Approximately 10% of spina bifida defects are "closed" or covered by skin. Most babies with spina bifida survive but live with varying degrees of disability, including lower body paralysis, problems with bladder and bowel control, skeletal deformities, and some mental impairment. For these infants, related medical costs can easily exceed $250,000 in the first 5 years of life.[17]

NTD prevention

Folate's importance. Genetic and environmental factors, including a nutritional component, are associated with the occurrence of NTDs. The possibility that folic acid is essential for normal embryonic development of the human nervous system was raised 35 years ago.[18] A year later, a report noted that urinary excretion of formiminoglutamic acid (FIGLU - a histidine metabolite that accumulates in the absence of folate) was more common in pregnancies associated with congenital abnormalities, including NTDs, than in unaffected pregnancies.[19] Unfortunately, this intriguing observation was not pursued at the time of the report.

In the early 1980s, dietary intervention studies were conducted among pregnant women who had previously experienced a pregnancy that was complicated by an NTD.[20] These studies rekindled interest in the possibility of a connection between diet and NTDs. By the early 1990s, the international scientific and medical communities recognized that if pregnant women consumed folic acid-containing vitamin supplements for at least 1 month before conception through the first trimester, the occurrence of NTDs was substantially lowered.[21]

Folic acid is the oxidized monoglutamate form of folate. Because of its stability, this oxidized form is used in nutritional supplements and enriched foods. It is also better absorbed in the gastrointestinal tract than the naturally occurring food folates, which are reduced polyglutamate derivatives.[11] When food is heated, it also loses large amounts of these naturally occurring folates.

In 1992, the U.S. Public Health Service adopted a public health recommendation developed by the Centers for Disease Control and Prevention,

which stated that all women of childbearing age who can become pregnant should consume 0.4 mg (400 [[micro]gram]) of folic acid daily to reduce the risk of a pregnancy affected by an NTD.[22] This recommendation is easy to follow, because most over-the-counter multivitamin preparations contain 0.4 mg of folic acid.

Every year, based on a CDC estimate, 150,000 babies are born with birth defects.[23] An estimated 2,5003,000 infants are born with NTDs, and an additional 1,500 NTD-affected pregnancies are terminated.[24] The CDC estimates that 50-70% of these birth defects could be prevented if folate consumption were optimized.[24]

Oral contraceptives and a folate deficiency. During pregnancy, there is a substantial increase in cell division and the metabolic processes involving folate-dependent, enzyme-catalyzed, one-carbon transfer reactions. Evaluation of the erythrocyte folate level is considered to be a better indicator of long-term folate status than serum or plasma folate levels. However, serious questions have been raised about the methods used for erythrocyte folate assessment and the possible sources of variability. Consequently, concerns have been expressed about the accuracy of all previously published values for erythrocyte folate.[25]

Oral contraceptive use is associated with lowered folate levels in both serum and erythrocytes,[26] and a woman who stops taking oral contraceptives in hopes of becoming pregnant may not be able to optimize her folate status before conception. One study found that folic acid levels did not normalize until 4 months after cessation of oral contraceptives.[27] In the U.S., at least half of all pregnancies are reportedly unplanned.[28,29] Therefore, all women, especially those in the "reproductive years" (15-45 years of age), should always consume adequate amounts of folic acid.[3] Without fortified foods or supplementation, body folate levels depend on consumption of foods rich in folate. Most naturally occurring folate is obtained from citrus fruits and juices, legumes (dried beans and peas), and dark green, leafy vegetables.[30] Orange juice is a major source of folate in the U.S. diet because it is regularly consumed.[31]

Mandates and recommendations. In 1997, the Gallup Organization conducted a random phone survey for the March of Dimes Birth Defects Foundation.[32] Of the nationally representative sample of 2,001 women of child-bearing age, only 22% indicated an awareness of the U.S. Public Health Service recommendation on folic acid intake for all women who could bear children.[32] Overall, only one-third of all nonpregnant women reported taking a daily multivitamin containing folic acid. Every minute a baby is born to a teenage mother.[10] Yet, of those women under 25 years of age, only 19% reported daily consumption of a multivitamin.[32]

As of January 1, 1998, the Food and Drug Administration mandated that cereal-grain product foods, such as flour, cornmeal, and rice, would be supplemented with 140 [[micro]gram] of folic acid per 100 grams of each of these foods. This supplementation mandate acknowledges the importance of folic acid in the diet, especially in relation to vitamin-preventable NTDs. The current level of fortification will increase folate intake by 80-100 [[micro]gram] per day, but the average daily intake of folic acid from food may still be too low to reduce the risk of NTDs.[33] In April 1998, the Food and Nutrition Board of the Institute of Medicine recommended that all women capable of becoming pregnant consume 400 [[micro]gram]/day of synthetic folic acid from fortified food and/or supplements in addition to eating a diet rich in citrus fruits, legumes, and dark green, leafy vegetables. Pregnant and lactating women are advised to consume 600 [[micro]gram] of folate daily.

To improve the health of all adults, the Board also recommended that a dietary reference intake (DRI) for folate be 400 [[micro]gram] per day, which is approximately double the RDA set in 1989.[12] Adverse effects have been reported with daily folate intakes of 5 mg or greater among individuals with pernicious anemia, which involves a deficiency of vitamin [B.sub.12],[30] because the high folic acid intake may mask the [B.sub.12] deficiency. Therefore, an upper intake limit of 1,000 [[micro]gram] of folic acid daily has been recommended.[12] Other effects of high folate intake are not well known.

Not all NTDs can be prevented by folic acid supplementation. Thus, the American College of Obstetricians and Gynecologists recommends that all pregnant women should be offered prenatal screening or diagnostic testing to identify fetal abnormalities.[28]

Other risk factors for NTDs. Factors other than poor maternal folate nutrition are associated with increased risk of NTDs. Incidence varies with race, ethnicity, diabetic status, and the use of certain medications. Population studies show that NTD rates are higher among whites, especially those of Celtic (Irish, Scottish, Welsh) descent, than among African Americans.[34] The rate of NTDs among black infants has been reported to be one-third the rate for the U.S. white population.[35] Changes in folate metabolism induced by in-utero exposure to maternal anticonvulsant drugs, such as valproic acid, have been suggested as a possible explanation for the increased incidence of NTDs among infants of mothers with epilepsy.[36] Increased risk of NTDs has also been reported in women with type 1 (insulin-dependent) diabetes.[37] The risk of NTDs is not associated with maternal age, as it is in Down Syndrome (trisomy 21), where the risk increases beginning at approximately 35 years of age and rises sharply with advancing maternal age.[38]

Folate's specific functions and the homocysteine connection

The precise mechanism by which suboptimal folate levels increase the risk of NTDs is unclear. Two different causal mechanisms may be responsible: (1) simple folate deficiency or (2) a metabolic block or insufficiency arising from the genetic mutation of an enzyme that either requires or metabolizes folate.

Acting as a cofactor for enzymes involved in DNA and RNA synthesis, folate is essential for the normal biochemistry of cell division. Essentially, folate derivatives carry one-carbon fragments from donor to recipient molecules in nucleotide and amino acid syntheses. For example, folate is involved in supplying methyl groups to homocysteine for methionine synthesis. Researchers are now focusing on metabolic pathways for homocysteine metabolism [ILLUSTRATION FOR FIGURE 2 OMITTED]. Derangement of homocysteine metabolism has been reported in at least 22% of mothers who had children with NTDs.[39] Homocysteine is an intermediary metabolite of the essential amino acid, methionine. Approximately 50% of homocysteine formed in the methionine cycle is remethylated to form methionine. The remainder is irreversibly converted to cysteine through the vitamin [B.sub.6]-requiring enzymes of the transsulfuration pathway [ILLUSTRATION FOR FIGURE 2 OMITTED]. Alterations in homocysteine levels can occur because of both inherited and acquired defects (see Table). Inherited defects include enzyme deficiencies and transport defects. The origin of an acquired defect could be nutritional or drug-induced.

Maternal hyperhomocysteinemia caused by folate deficiency produces homocysteine-induced injury that may damage the mother's vascular endothelium and platelets and also the developing fetal nervous system. An association has been reported between a history of children born with an NTD and maternal mild hyperhomocysteinemia in the fasting state and after methionine loading.[3] Experimental evidence suggests that during gestation, hyperhomocysteinemia produces increased homocysteine levels in amniotic fluid, and these elevated levels cause chemical trauma by progressively damaging the exposed, unprotected neural tissue.[40]

Defective enzyme systems as causes of NTDs

Methyltetrahydrofolate reductase deficiency. Methionine deficiency has also been suggested to have a role in the pathogenesis of NTDs. A current theory proposes that maternal supplementation with folic acid prevents hyperhomocysteinemia and NTDs by at least partially correcting reduced activity of a genetic variant of the enzyme 5,10-methylenetetrahydrofolate reductase (MTHFR) ([ILLUSTRATION FOR FIGURE 2 OMITTED], "Remethylation"),[41] which catalyzes the conversion of 5,10-methylenetetrahydrofolate to 5-methyltetrahydrofolate (5-methyl THF). The latter is the major circulatory form of folate and is the primary methyl donor for the remethylation of homocysteine to methionine. Methylation of DNA is important in embryonic development. By first serving as a source of methyl groups and then impacting gene expression and regulation, methionine contributes to normal closure of the neural tube.[41-44]

A mutant gene for MTHFR leads to decreased methionine levels, and the presence of this gene has been reported to be increased in Irish and Dutch children with spina bifida.[45] Maternal and/or infant MTHFR genotype along with optimal maternal diet seem to interact in the mechanism by which the risk of NTD is reduced in this at-risk population.

Methionine synthase and vitamin [B.sub.12]. Vitamin [B.sub.12] deficiency has also been suggested as an independent risk factor for NTDs.[34,43,46] Methionine synthase (MS) is essential in the metabolism of both homocysteine and folate [ILLUSTRATION FOR FIGURE 2 OMITTED] and is also a key enzyme in methylation reactions, including those involved in the synthesis of myelin basic protein, a component of the myelin nerve sheath. To synthesize methionine, MS uses vitamin [B.sub.12] as its active cofactor, and 5-methyl THF is a substrate.

Methyl groups are transferred by MS from 5-methyl THF to vitamin [B.sub.12] bound to the enzyme. The addition of the methyl group to the vitamin generates methylcobalamin, which then serves as a methyl donor to generate methionine from homocysteine. Without vitamin [B.sub.12] to accept the methyl group from 5-methyl THF, THF cannot be regenerated ([ILLUSTRATION FOR FIGURE 2 OMITTED], "Remethylation"). This relationship is called the "methyl-folate trap", because without [B.sub.12], the methyl group cannot be removed from 5-methyl THF; and although THF is present, it is "trapped" and unavailable for subsequent reactions such as the biosynthesis of DNA. Thus, by impairing remethylation, a [B.sub.12] deficiency can secondarily create a folate deficit.

Although both methionine and [B.sub.12] are concentrated across the placenta, a recent study reported a relative [B.sub.12] decrease in the amniotic fluid of fetuses with NTDs.[43] The researchers also noted that in the amniotic fluid of fetuses with NTDs, ratios of product to substrate(s) for homocysteine remethylation suggest impaired MS. Elevated serum concentrations of methylmalonic acid (MMA) have been reported for women with NTD-affected pregnancies,[47] and increased levels of both homocysteine and MMA indicate a [B.sub.12] deficiency.

Whatever the specific metabolic disturbances that lead to increased risk of NTD, evidence continues to accumulate that genetic mutations are important, including those that affect uptake, distribution, or utilization of folate by maternal, placental, or fetal cells. Enzymatic abnormalities and functional availability of at least 2 B vitamins - folic acid and [B.sub.12] - whose metabolisms are intimately connected, are key factors in solving the puzzle of NTDs.[44] Appropriate levels of homocysteine also seem essential to normal fetal development.

Biochemical markers as predictors for NTDs

Clinicians use results from various biochemical markers when assessing the risk of NTDs. However, as a pregnancy progresses through the 3 trimesters, marker usefulness varies because their concentrations change. Thus, accurate interpretation of the pertinent laboratory data for NTD screening requires that the gestational age of the fetus be known with as much certainty as possible. Typically, the first trimester of pregnancy, during which embryogenesis occurs, includes the time from the first day of the last menstrual period to the end of 12 weeks' gestation. The fetus grows rapidly during the second trimester, which extends from the 13th through the 26th week of gestation. The third trimester, during which fetal maturation occurs, begins at the 27th week and extends to delivery, which normally occurs at 37-41 weeks' gestation. When performing prenatal screening for NTDs, current markers are not suitable for first trimester evaluation. Therefore, this prenatal screening is usually conducted during the second trimester.

Test results for NTD markers are reported as actual values and also as multiples of the median (MoMs) for a given week of gestation. The use of MoMs facilitates the comparison of results among laboratories using different reference standards and assay methodologies. An MoM is defined as the ratio of an individual marker concentration to the median concentration expected for a population of unaffected women at the same gestational age. The median value is defined as the "midpoint" where as many patients have values above the median as have values below the median. To reliably calculate median values, most authorities recommend obtaining 100 data points for each week of gestation. Log-linear regression analysis of available data and extrapolation of median values for weeks for which data are limited can overcome sampling shortfalls.[48]

Between 25% and 45% of women reportedly can't provide a reliable menstrual cycle history, and this leads to inaccurate estimate of gestational age.[49] If laboratory findings for NTD markers are judged to be abnormal based on the presumed gestational age, follow-up procedures include an ultrasound examination to verify gestational age.[28] Commercially available data management software programs can maintain a database of patient and sample information, perform MoM computations, apply correction factors, provide patient-specific risk assessment, and print interpretive reports in a variety of text and graphic formats [ILLUSTRATION FOR FIGURE 3 OMITTED].[50]

Alpha fetoprotein. Measurement of alpha fetoprotein (AFP) in amniotic fluid and in maternal serum has been useful in prenatal screening for open NTDs. (Closed defects don't generally produce elevated AFP levels.) The developing fetus initially produces 2 major blood proteins - albumin and AFP. The latter is similar in structure to albumin, but its function is not fully understood. Produced by the fetal yolk sack, gastrointestinal tract, and liver, AFP is present in both the fetal plasma and, via fetal urination, in the amniotic fluid. The levels of AFP in the amniotic fluid parallel, but are substantially less than, levels in the fetal plasma. Concentrations of fetal serum AFP are reported in mg/mL, while levels of amniotic fluid AFP (AFAFP) are reported in [[micro]gram]/mL. Alpha fetoprotein is currently measured by radioimmunoassay (RIA) or enzyme-linked immunosorbent assay (ELISA).[48]

At the end of the first trimester (between 10-13 weeks of gestation), the concentration of fetal AFP peaks and then declines during the second and third trimesters.[28] Alpha fetoprotein enters the maternal circulation by both diffusion from the amniotic fluid across the fetal membranes and transfer from the fetal plasma across the placenta.[51] In the maternal circulation, AFP levels rise gradually, peaking later in gestation (between 28-32 weeks) and then decline until delivery.[28] Maternal serum AFP (MSAFP) levels are reported in ng/mL or IU/mL.

The optimal time for measuring MSAFP is between 16-18 weeks of gestation, but testing can be conducted as early as 15 weeks and as late as 22 weeks.[28] Determinations of MSAFP too early in gestation are unproductive because the level is negligible in nonpregnant women and rises as pregnancy progresses. Beyond 22 weeks, the overlap in distributions of MSAFP concentrations for affected and unaffected populations limits the discriminatory power of the test.

Other conditions that may elevate MSAFP include underestimation of gestational age, multiple gestation pregnancies (twins or more), racial origin, placental changes, fetal abdominal wall defects, congenital nephrosis, fetal death, polycystic kidneys, and low birth weight. Lower than expected levels of MSAFP are noted in overestimation of gestational age, type 1 diabetics, Down syndrome, trisomy 18 (Edwards' syndrome), hydatidiform moles, and pseudopregnancy.[52] AFAFP is far more sensitive and specific for diagnosing NTDs than is MSAFP. False positives can occur if fetal blood contaminates the amniotic fluid during amniocentesis,[38] but this false positive can be properly classified as such by direct analysis of erythrocytes using either the Kleihauer-Betke test or by immunoassay for hemoglobin F in the amniotic fluid.[48]

For accurate interpretation of results, correction factors must be applied to the basic calculation for the risk of an NTD.[53] African American women have serum AFP concentrations that are 10-15% higher than other women, but they have a lower risk for NTDs. Type 1 diabetics have lower serum AFP levels but increased risk for NTDs. Maternal weight correlates positively with blood volume and is inversely related to the MSAFP. Such factors must be considered when interpreting results.[54] Ideally, separate MoM reference ranges should be established for each population.

When screening for NTDs, most laboratories select 2.0 or 2.5 MoM as the upper cutoff.[48] If the screening threshold is set at 2.0 MoM, approximately 85% of open spina bifida pregnancies will be detected when prevalence of this condition is about 1 per 1,000 births. This threshold also generates 3-4% false positives. A threshold cutoff of 2.5 MoM for the same population would detect 70-75% of affected pregnancies and yield approximately 2% false positives.[54] If the patient's MSAFP exceeds 2.0 MoM, the test is judged to be a positive result and follow-up is indicated.[28] This may include repeat testing of MSAFP, high resolution ultrasound examination, and amniocentesis. A sample diagnostic protocol summarizes the procedures commonly included in prenatal screening and diagnosis [ILLUSTRATION FOR FIGURE 4 OMITTED].

Other markers. Early in the second trimester, a decrease in maternal serum unconjugated estriol (MSuE3) is an independent predictor of NTDs.[55] During pregnancy, the fetus synthesizes large amounts of the uE3 form of estrogen. Through a complex series of enzymatic reactions, the fetal adrenal glands, liver, and placenta contribute to uE3 production, and essentially all MSuE3 originates with the fetus. The fetal adrenal glands supply the initial substrate, dehydroepiandrosterone (DHEA), which is metabolized to estriol by fetal hepatic and placental enzymes. A portion of the estriol diffuses across the placenta into the maternal circulation. When used in combination, an elevated MSAFP and a low MSuE3 are highly predictive of an NTD, especially anencephaly.[55]

When amniocentesis is performed, the amniotic fluid may be assessed for acetylcholinesterase (AChE) as well as for AFP. Normally in amniotic fluid, pseudocholinesterase is present but AChE is essentially undetectable. When nerve tissue or cerebrospinal fluid is directly exposed to anmiotic fluid, as occurs in open spina bifida, AChE is detectable in the amniotic fluid. An open NTD that might be missed by sonography should be detectable by elevated levels of amniotic fluid AFP and ACHE.[28,38] If amniotic fluid levels of AFP are increased but no AChE is detected, the presence of non-NTD fetal pathologies is suggested.[38]

Prenatal testing for NTDs: Expand cautiously

Prenatal testing has been described as a "growth segment" of the laboratory market. However, it can be an especially high-risk growth segment, because a decision to terminate a pregnancy may be made on the basis of laboratory results. Professional counseling services must be available, and the first element of any protocol should be a clearly worded consent form that advises the patient of test result limitations.

The NCCLS guidelines stress that any laboratory specializing in prenatal testing for identifying pregnancies at increased risk for NTDs must provide accurate test results and must aid in their appropriate interpretation.[48] Laboratory services should be just one element in a comprehensive prenatal screening program. Genetic counselors, along with laboratory personnel, must be able to communicate all test results directly to the physician, provide assistance in coordinating follow-up and diagnostic studies, and serve as an educational resource for physicians and patients.[56]

All prenatal screening laboratories must maintain high levels of quality control and quality assurance, as stipulated by the NCCLS.[48] In 1983, the Foundation for Blood Research (FBR) in Scarborough, ME, established the first proficiency testing program for maternal serum screening in conjunction with the New England Regional Genetics Group. Currently, the FBR and the College of American Pathologists jointly sponsor a proficiency testing program for prenatal screening to which nearly all U.S. screening programs subscribe. During the first week of November 1999, the FBR is even providing a short course designed to provide participants with a working knowledge of assay development, risk assessment, and the basics of NTD screening.[57]

Aggressive public education efforts that stress the effectiveness of folic acid to reduce the risk of NTDs should have a positive effect on birth outcomes. In addition, diagnosis of affected pregnancies continues to improve, and research continues for new markers and earlier detection techniques.

LEARNING OBJECTIVES:

1. Describe the basic sequence of events in normal neural tube development and the formation of fetal neural tube defects (NTDs).

2. Describe one genetic and one nongenetic mechanism that may increase the risk of fetal NTDs.

3. State the public health recommendations for supplementing maternal diet to reduce the risk of NTDs.

4. Explain folate's role in the metabolism of homocysteine and methionine.

5. Identify 3 biochemical markers used to screen for open NTDs.

6. Evaluate maternal serum alpha fetoprotein (MSAFP) screening data and identify 3 factors that may complicate interpretation of results.

CE test published through an educational grant from SmithKline Beecham Clinical Laboratories

Table

Determinants of hyperhomocysteinemia(*)

Specific genetic deficiencies

* 5,10-methylenetetrahydrofolate reductase

* Methionine synthase

* Cystathione beta-synthase

Demographic attributes

* Age

* Sex

* Ethnic origin

Acquired conditions

State of health

* B-vitamin deficiency (folate, [B.sub.12], and [B.sub.6])

* Impaired renal function

* End-state renal disease

* Heart and other organ transplants

* Hypothyroidism 

Lifestyle

* Smoking

* Lack of exercise

* Excessive alcohol, caffeine

* Not all determinants were discussed in this article.

Source: Table adapted from: Jacobsen DW. Homocysteine and vitamins in cardiovascular disease. Clin Chem. 1998;44(8B):1833-1843,

MATERNAL SERUM RESULTS:

AFP: 84.9 IU/mL 2.90 MoM (NTD cut-off 2.20 MoM)

uE3: 2.10 ng/mL 2.31 MoM

hCG: 43.0 IU/mL 1.46 MoM

RISK ASSESSMENT:

DOWN SYNDROME: The risk at tern is approximately 1:30300, or less than that of a 15.0-year-old. (DS risk cut-off = 1:385)

(The risk based on maternal age alone is 1:1236)

O.S.B.: The risk, based on this sample alone, is equal to 1:109

INTERPRETATION: SCREEN *** POSITIVE *** FOR RISK OF OPEN SPINA BIFIDA

The maternal serum AFP result is MODERATELY ELEVATED for a pregnancy of this gestational age, indicating a significantly increased risk of an open neural tube defect. The gestational age should be confirmed and a REPEAT SERUM SPECIMEN is recommended.

The risk of Down syndrome is LESS than the screening cut-off. The serum screen has indicated a substantially REDUCED risk compared to that based on maternal age alone.

Accuracy of gestational age is essential for valid interpretation. An ultrasound examination for gestational age is recommended.

RESULTS

AFP 35.4 IU/mL 1.54 MoM

ESTRIOL (uE3) 1.23 ng/mL 1.77 MoM

HCG 28.6 IU/mL 0.74 MoM

RISK ASSESSMENT - DOWN SYNDROME

Result

Negative

Patient Risk (at mid-trimester)

1:8020

Avg. Risk for Maternal Age

1:220

Interpretation(*)

The risk of Down syndrome is LESS than the screening cut-off. The serum screen has indicated a SUBSTANTIALLY REDUCED risk from that based on maternal age alone, sufficient to place this patient BELOW the screening cut-off.

RISK ASSESSMENT - OPEN SPINA BIFIDA

Result

Negative

Patient Risk

1:2486

Population Risk

1:1000

Interpretation(*)

The maternal serum AFP result is NOT elevated for a pregnancy of this gestational age. The risk of an open neural tube defect is less than the screening cut-off.

* Accurate estimation of gestational age is essential for valid interpretation.

References

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Sharon M. Miller is professor and associate dean, clinical laboratory sciences, College of Health and Human Sciences, Northern Illinois University, DeKalb, IL.