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Test Code MAFP1 Alpha-Fetoprotein (AFP), Single Marker Screen, Maternal, Serum


Specimen Required


Container/Tube: 

Preferred: Red top

Acceptable: Serum gel

Specimen Volume: 1 mL

Collection Instructions:

1. Do not draw specimen after amniocentesis, as this could affect results.

2. Collection tubes should be centrifuged within 2 hours of collection.

Additional Information:

1. Draw blood between 15 weeks, 0 days and 22 weeks, 6 days.

2. Initial or repeat testing is determined in the laboratory at the time of report and will be reported accordingly. To be considered a repeat test for the patient, the testing must be within the same pregnancy and trimester, with interpretable results for the same test, and both tests are performed at Mayo Clinic.


Forms

Second Trimester Maternal Screening Alpha-Fetoprotein (AFP)/QUAD Screen Patient Information (T595) is required in Special Instructions.

Useful For

Prenatal screening for open neural tube defect

Method Name

Two-Site Immunoenzymatic (Sandwich) Assay

Reporting Name

AFP Single Marker SCRN, Maternal, S

Specimen Type

Serum

Specimen Minimum Volume

0.5 mL

Specimen Stability Information

Specimen Type Temperature Time
Serum Refrigerated (preferred) 7 days
  Frozen  90 days

Reject Due To

Hemolysis

Mild OK; Gross reject

Lipemia

Mild OK; Gross OK

Icterus

NA

Other

NA

Clinical Information

Analytes:

Alpha-fetoprotein (AFP) is a fetal protein that is initially produced in the fetal yolk sac and liver. A small amount also is produced by the gastrointestinal tract. By the end of the first trimester, nearly all of the AFP is produced by the fetal liver. The concentration of AFP peaks in fetal serum between 10 to 13 weeks. Fetal AFP diffuses across the placental barrier into the maternal circulation. A small amount also is transported from the amniotic cavity.

 

The AFP concentration in maternal serum rises throughout pregnancy, from the nonpregnancy level of 0.20 ng/mL to about 250 ng/mL at 32 weeks gestation. If the fetus has an open neural tube defect (NTD), AFP is thought to leak directly into the amniotic fluid causing unexpectedly high concentrations of AFP. Subsequently, the AFP reaches the maternal circulation; thus producing elevated serum levels. Other fetal abnormalities such as omphalocele, gastroschisis, congenital renal disease, esophageal atresia, and other fetal distress situations such as threatened abortion and fetal demise also may show AFP elevations. Increased maternal serum AFP values also may be seen in multiple pregnancies and in unaffected singleton pregnancies in which the gestational age has been underestimated.

 

Lower maternal serum AFP values have been associated with an increased risk for genetic conditions such as trisomy 21 (Down syndrome) and trisomy 18 (Edwards syndrome). Risks for these syndrome disorders are only provided with the use of multiple marker screening (QUAD1 / Quad Screen [Second Trimester] Maternal, Serum).

 

Measurement of maternal serum AFP values is a standard tool used in obstetrical care to identify pregnancies that may have an increased risk for NTD. The screen is performed by measuring AFP in maternal serum and comparing this value to the median AFP value in an unaffected population to obtain a multiple of the median (MoM). The laboratory has established a MoM cutoff of 2.5 MoM, which classifies each screen as either screen-positive or screen-negative. A screen-positive result indicates that the value obtained exceeds the established cutoff. A positive screen does not provide a diagnosis, but indicates that further evaluation should be considered.

Reference Values

NEURAL TUBE DEFECTS

An AFP multiple of the median (MoM) <2.5 is reported as screen negative.

AFP MoMs ≥2.5 (singleton and twin pregnancies) are reported as screen positive.

 

An interpretive report will be provided.

Interpretation

Neural tube defects (NTD):

A screen-negative result indicates that the calculated alpha-fetoprotein (AFP) multiple of the median (MoM) falls below the established cutoff of 2.50 MoM. A negative screen does not guarantee the absence of NTDs.

 

A screen-positive result indicates that the calculated AFP MoM is 2.50 MoM or greater, and may indicate an increased risk for open NTDs. The actual risk depends on the level of AFP and the individual's pretest risk of having a child with NTD based on family history, geographical location, maternal conditions such as diabetes and epilepsy, and use of folate prior to conception. A screen-positive result does not infer a definitive diagnosis of a NTD, but indicates that further evaluation should be considered. Approximately 80% of pregnancies affected with an open NTD have elevated AFP MoM values greater than 2.5.

 

Follow up:

Upon receiving maternal serum screening results, all information used in the risk calculation should be reviewed for accuracy (ie, weight, diabetic status, gestational dating). If any information is incorrect the laboratory should be contacted for a recalculation of the estimated risks.

 

Screen-negative results typically do not warrant further evaluation.

 

Ultrasound is recommended to confirm dates for NTD screen-positive results. If ultrasound yields new dates that differ by at least 7 days, a recalculation should be considered. If dates are confirmed, high-resolution ultrasound and amniocentesis (including amniotic fluid AFP and acetylcholinesterase measurements for NTDs) are typically offered.

Cautions

Race, weight, smoking, multiple fetus pregnancy, and insulin-dependent diabetes (IDD) may affect marker concentrations. Black mothers tend to have higher alpha-fetoprotein (AFP) levels but lower risk of neural tube defects (NTDs) and are assigned to a separate AFP median set. Multiple of the medians (MoMs) are adjusted for maternal weight (to account for dilution effects in heavier mothers). The AFP is adjusted upward in IDD to account for lower values in diabetic pregnancies. Smoking results in higher second trimester maternal serum AFP. MoMs are adjusted accordingly to account for serum AFP differences in smokers.

 

The screen results are dependent on accurate information for gestation, race, insulin dependent diabetes, and weight. Inaccurate information can lead to significant alterations in the estimated risk. In particular, erroneous assessment of gestational age can result in false-positive or false-negative screen results. Because of its increased accuracy, we therefore recommend determination of gestational age by ultrasound, rather than by maternal dates alone when possible.

 

A screen-negative result does not guarantee the absence of fetal defects. A screen-positive result does not provide a diagnosis, but indicates that further diagnostic testing should be considered (an unaffected fetus may have screen-positive result for unknown reasons).

 

Valid measurements of AFP in maternal serum cannot be made after amniocentesis.

 

Triplet and higher multiple pregnancies cannot be interpreted.

 

Each center offering maternal serum screening to patients should establish a standard screening protocol, which provides pre- and post-screening education and appropriate follow-up for screen-positive results.

Clinical Reference

1. Christensen RL, Rea MR, Kessler G, et al: Implementation of a screening program for diagnosing open neural tube defects: selection, evaluation, and utilization of alpha-fetoprotein methodology. Clin Chem 1986;32:1812-1817

2. American College of Obstetricians and Gynecologists: Practice Bulletin No. 163: Screening for Fetal Aneuploidy. Obstet Gynecol 2016 May;127(5):e123-137

3. Zhang J, Lambert-Messerlian G, Palomaki GE, Canick JA: Impact of smoking on maternal serum markers and prenatal screening in the first and second trimesters. Prenat Diagn 2011 Jun;31(6):583-588

Method Description

Alpha-fetoprotein (AFP) values are compared to the median value of the unaffected population at a given gestational age and the multiple of the median (MoM) is obtained and classified as either screen-positive or screen-negative. An interpretive report will be provided.

 

The Access AFP assay is a 2-site immunoenzymatic sandwich assay. A sample is added to a reaction vessel with mouse monoclonal anti-AFP alkaline phosphatase conjugate and paramagnetic particles coated with a second mouse monoclonal anti-AFP antibody. The AFP in the sample binds to the immobilized monoclonal anti-AFP on the solid phase while, at the same time, the monoclonal anti-AFP-alkaline phosphatase conjugate reacts with different antigenic sites on the sample AFP. After incubation in a reaction vessel, materials bound to the solid phase are held in a magnetic field while unbound materials are washed away. Then the chemiluminescent substrate Lumi-Phos*530 is added to the reaction vessel and light generated by the reaction is measured with a luminometer. The light production is directly proportional to the amount of AFP in the sample. The amount of analyte in the sample is determined by means of a stored multipoint calibration curve.(Instruction manual: Beckman-Coulter Assay, Beckman Coulter, Inc, Fullerton CA 2010)

Day(s) and Time(s) Performed

Monday through Friday; 8 a.m.-4:30 p.m.

Analytic Time

1 day

Specimen Retention Time

12 months

Performing Laboratory

Mayo Clinic Laboratories in Rochester

Test Classification

This test was developed and its performance characteristics determined by Mayo Clinic in a manner consistent with CLIA requirements. This test has not been cleared or approved by the U.S. Food and Drug Administration.

CPT Code Information

82105

LOINC Code Information

Test ID Test Order Name Order LOINC Value
MAFP1 AFP Single Marker SCRN, Maternal, S 48802-3

 

Result ID Test Result Name Result LOINC Value
7058 Recalculated Maternal Serum Screen 32399-8
113146 Results Summary 32399-8
113147 Neural tube defect risk estimate 48803-1
10351 AFP 83073-7
113148 AFP MoM 23811-3
10356 INTERPRETATION 49092-0
10357 RECOMMENDED FOLLOW UP 80615-8
10248 Additional comments 48767-8
3009 Specimen collection date 33882-2
7823 Maternal date of birth 21112-8
7834 Calculated age at EDD 43993-5
26717 Maternal Weight 29463-7
26718 Maternal Weight 29463-7
IDD Insulin dependent diabetes 44877-9
RACE1 Patient race 21484-1
SMKNG Current cigarette smoking status 64234-8
10054 EDD by U/S scan 11781-2
7203 GA on collection by U/S scan 11888-5
7753 EDD by LMP 11779-6
7204 GA on collection by dates 11885-1
7830 GA used in risk estimate 21299-3
MULTF Number of Fetuses 55281-0
CHOR_ Number of Chorions 92568-5
IVFP IVF pregnancy 47224-1
PRNTD Prev Pregnancy w/ Neural Tube Defect 53827-2
PTNTD Patient or father of baby has a NTD 53827-2
INTL Initial or repeat testing 77202-0
DRPHN Physician Phone Number 68340-9
10358 GENERAL TEST INFORMATION 62364-5

NY State Approved

Yes