ALPHA-FETOPROTEIN (AMNIOTIC FL) REFLEX TO ACETYLCHOLINESTERASE

Code
000.0000
Name
ALPHA-FETOPROTEIN (AMNIOTIC FL) REFLEX TO ACETYLCHOLINESTERASE
Category
None
Department
Send-Out
Start Date
Expiration Date
Synonyms
AFP, Amniotic Fluid
CPT Codes
82106; If reflexed add 82013 and 83033
Site
SBMF
Reference Test
44032
ATLAS Test Code

Specimen Information

Type

Amniotic fluid (Sterile container)

Volume

2.5 ml

Transport Info

Refrigerated

Fasting Required?
False
Patient Instructions

Include gestational age at time of collection on the test request form
The following information is required and must accompany the sample in order for testing to be interpreted:
1) Patient's date of birth
2) Due date
3) Weeks of gestation
4) Physician's name and phone number

Reference Range

See Report

Methodology

Chemiluminescent Immunoassay
Electrophoresis

Clinical Significance

Used to help predict the risk of certain birth defects.

The combined alpha fetoprotein/human chorionic gonadotropin/unconjugated estriol test (AFP/hCG/uE3 or triple test) or the AFP/hCG test (double test) are screens used to help predict the risk of certain birth defects. Individually or together, however, these tests are not diagnostic for individual birth defects. AFP/ hCG/uE3 screening may not detect chromosome abnormalities other than Trisomy 21 and Trisomy 18. The tests tend to interact with one another, providing slightly less information than if each were independent. The triple screen may pick up more at-risk patients than the double screen (AFP/hCG), but may also produce more false positives. A maternal serum screen for women less than 35 years is considered positive for fetal Down syndrome if the risk is worse than 1/ 270 for AFP/hCG or worse than 1/190 for AFP/hCG/uE3. Addition of uE3 to the double analyte screen slightly improves the rate of detection for Down syndrome, but greatly improves the detection of Trisomy 18. AFP/hCG screening detects about 55% of Down syndrome cases, while classifying 5% of normal patients as abnormal. Further addition of estriol increases detection to 60% and false positive rate to 7%. Alpha fetoprotein (AFP) is made by the fetal liver. It is detectable in amniotic fluid and maternal blood at 10-12 weeks gestation. Screening for AFP should occur at 15-20 weeks gestation (optimum 16-18 weeks), as concentrations are rising rapidly at this time (usually 20-75 ng/mL, typical 35 ng/mL). A positive AFP result means that the amount of AFP in maternal blood is greater or less than that usually seen. It does not necessarily mean that a birth defect is present. Results are expressed in ng/mL and as "multiples of the median" (MoM), calculated as the AFP value in ng/mL divided by the median value, which is based on gestational age of the fetus. Adjustments to MoM are made for maternal weight, race, number of fetuses, and insulin-requiring diabetes. The most common cause for elevated AFP is inaccurate dating of pregnancy. A high AFP can also signal the presence of twins (50% are detected). Most important, however, are neural tube defects, present in 1-2/1000 babies. These defects include anencephaly (95% detected) and spina bifida (70-85% detected). Neural tube defects are caused by failure of neural tube fusion, leading to permanent developmental defects of the brain or spinal cord or both. In most cases, there is no skin to cover the opening, so fetal serum AFP gains access to the amniotic fluid in large concentrations. The AFP then passes from amniotic fluid to maternal blood, where it is detected in increased quantities. In addition, neural tube defects tend to appear at random, with no established familial link or family history, further reinforcing the need for prenatal AFP screening. Causes for low AFP values also include inaccurate dating of pregnancy, and most importantly, fetal Down syndrome. Down syndrome occurs in 1/800 of live births. Most cases involve 3 copies of chromosome 21 (Trisomy 21), but a small number are chromosome translocations or mosaics. AFP values are usually 30% lower in mothers with fetuses having Down syndrome, independent of the mother's age. Although the risk of chromosomal abnormalities is dramatically increased for mothers age 35 or older, 80% of children with Down syndrome are born to mothers younger than 35. Human chorionic gonadotropin (hCG) is produced by the syncytiotrophoblasts of the placenta after the embryo has implanted, usually 5-8 days after conception. Levels rise rapidly from about 5 IU/L at 8-11 days following conception, doubling every 2-3 days during the first 6 weeks of pregnancy. At 16-18 weeks, typical levels are 10,000-35,000 IU/L. Results are expressed in IU/L and as "multiples of the median" (MoM), calculated as hCG in IU/L divided by median value adjusted for gestation, maternal weight, and number of fetuses. Elevated hCG levels at about 16 weeks gestation can help predict the risk of fetal Down syndrome. The MoM for hCG is an average of 2.01 times higher when Down syndrome is present. Estriol is the principal estrogen hormone in the blood during pregnancy. Estriol precursors are produced in the fetal adrenals and liver; estriol itself is synthesized in the placenta. Estriol exists in the maternal blood as a mixture of the unconjugated form together with a number of conjugates. Levels of total serum estriol can be affected by several factors (maternal renal system and administration of ampicillin), so unconjugated estriol (uE3) is most often measured. Normally, levels of estriol increase during pregnancy. Low estriol levels are associated with Down syndrome and fetal distress. Results are expressed in ng/mL and as "multiples of the median" (MoM), calculated as uE3 in ng/mL divided by median value adjusted for gestation, maternal weight, and number of fetuses. Risk estimates are provided on all abnormal screens. The risk of a neural tube defect is based on the AFP MoM and any maternal factor that might increase the risk, such as family history of neural tube defects or insulin-requiring maternal diabetes. The risk of Down syndrome is based on maternal age, AFP MoM, hCG MoM, and uE3 MoM.

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