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

CPT CODE:

USEFUL FOR:

Prenatal screening for open neural tube defect

SPECIMEN REQUIRED:

Draw blood in a plain, red-top tube(s) or a serum gel tube(s). Spin down immediately and send 1 mL of maternal serumrefrigerated. Do not draw specimen after amniocentesis, asthis could affect resultsNote:    1. The following information is required for processing:                 A. Ordering physician name and phone number                 B. Serum draw date                 C. Date of birth (not age)                 D. Weight (indicate lbs or kg)                 E. Is patient an insulin-dependent diabetic? Select "Yes"                     if patient is on insulin prior to this pregnancy; otherwise,                     select "No."                 F. Race? Select "Black" or "Other/Non-Black/Mixed."                 G. Is this a twin pregnancy?                      1. Ultrasound estimated date of delivery (EDD) is                     required for twin pregnancies.2.  Risk estimates are not available for 3 or more    fetuses or for diabetic-twin pregnancies.3.  If 1 twin is deceased, select "No." However,    analytes from the deceased twin may affect    results and cannot be adjusted for.                 H. If this is a repeat analysis, please provide previous                       control numberI.    Please provide EDD and indicate if EDD is by last     menstrual period or ultrasound. Results will differ,    depending on method used. Ultrasound dating    increases overall screening performance and is    required for twin gestations.             2. Gestational age must be between 15 and 22 weeks;                  16 to 18 weeks is optimal. System defaults to                  ultrasound dating when provided.             3. Please complete and submit a "Second Trimester                  Maternal Screening Alpha-Fetoprotein (AFP)/Quad                  Screen Patient Information Sheet" (Supply T595) in                  Special Instructions. Forward it with the specimen,                  or send the information electronically.

TRANSPORT TEMPERATURE:

Refrig\Frozen OK\Ambient NO

CLINICAL INFORMATION:

Analytes: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 diffusesacross the placental barrier into the maternal circulation. A smallamount also is transported from the amniotic cavity.
The AFP concentration in maternal serum rises throughoutpregnancy, from the nonpregnancy level of 0.20 ng/mL to about250 ng/mL at 32 weeks gestation. If the fetus has an open neuraltube defect (NTD), AFP is thought to leak directly into the amniotic fluidcausing unexpectedly high concentrations of AFP. Subsequently,the AFP reaches the maternal circulation; thus producing elevatedserum levels. Other fetal abnormalities such as omphalocele,gastroschisis, congenital renal disease, esophageal atresia, andother fetal distress situations such as threatened abortion and fetaldemise also may show AFP elevations. Increased maternal serumAFP values also may be seen in multiple pregnancies and inunaffected singleton pregnancies in which the gestational age hasbeen underestimated.
Lower maternal serum AFP values have been associated with anincreased risk for genetic conditions such as trisomy 21 (Downsyndrome) and trisomy 18. Risks for these syndrome disorders areonly provided with the use of multiple marker screening (#81149,"Quad Screen [Second Trimester] Maternal, Serum").
Measurement of maternal serum AFP values is a standard toolused in obstetrical care to identify pregnancies that may have anincreased risk for NTD. The screen is performed by measuring AFPin maternal serum and comparing this value to the median AFP value inan unaffected population to obtain a multiple of the median (MoM). Thelaboratory has established a MoM cutoff of 2.5 MoM, which classifieseach screen as either screen-positive or screen-negative. A screen-positive result indicates that the value obtained exceeds the establishedcutoff. A positive screen does not provide a diagnosis, but indicatesthat further evaluation should be considered.

CLINICAL INTERPRETATION:

Neural tube defects:  A screen-negative result indicates that the calculated AFP 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 > or =2.50 MoM and may indicate an increased risk for open NTDs. The actual risk depends on the level of AFP and the individual's pre-test 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 >2.5.
Follow up:Upon receiving maternal serum screening results, all informationused in the risk calculation should be reviewed for accuracy (ie,weight, diabetic status, gestational dating, etc.). If any informationis incorrect the laboratory should be contacted for a recalculationof 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 least7 days, a recalculation should be considered. If dates are confirmed,high-resolution ultrasound and amniocentesis (including amnioticfluid AFP and acetylcholinesterase measurements for NTDs) aretypically offered.

REFERENCE VALUES:

NEURAL TUBE DEFECTS

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

negative. AFP MoMs >=2.5 (singleton pregnancies) and >=5.33

(twin gestation) are reported as screen positive.

An interpretive report will be provided.

Screen-positive results are called back to ordering client.