Alpha-Fetoprotein (AFP-AF), Amniotic Fluid

CPT CODE:

USEFUL FOR:

Screening test to detect open neural tube defects or other fetal abnormalities
Follow-up testing for patients with elevated serum AFP results or in conjunction with cytogenetic testing

SPECIMEN REQUIRED:

1 mL of amniotic fluid. Do not centrifuge. Send specimen refrigerated.Note:   1. The following information is required for processing:             A. Date ultrasound performed             B. Gestational dating on the day of the ultrasound             C. Collection date            2. Gestational age must be between 13 and 24 weeks; 16 to 18                weeks preferred.            3. If chromosome studies are also requested, see                 #8426 "Chromosome Analysis, Amniotic Fluid."  The specimen                for alpha-fetoprotein, amniotic fluid testing, when                requested with chromosome analysis, cannot be frozen.            4. If ordering electronically, please complete and submit                a "MayoConnect Additional Test Information Form"                (Supply T357 or see Special Instructions) with the specimen.                If not ordering electronically, please complete and submit                a "Cytogenetics/AFP Congenital Disorders Request Form"                (Supply T238) with the specimen.

TRANSPORT TEMPERATURE:

Refrig\Ambient OK\Frozen NO

CLINICAL INFORMATION:

AFP is a single polypeptide chain glycoprotein with a molecular weight of approximately 70,000 daltons. Synthesis of AFP occurs primarily in the liver and yolk sac of the fetus. It is secreted in fetal serum, reaching a peak at approximately 13 weeks gestation, after which it rapidly declines until about 22 weeks gestation and then gradually declines until term. Transfer of AFP into maternal circulation is accomplished primarily through diffusion across the placenta. Maternal serum AFP levels rise from the normal 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, AFP is thought to leak directly into the amniotic fluid causing unexpectedly high concentrations of AFP. Other fetal abnormalities such as omphalocele, gastroschisis, congenital renal disease, and esophageal atresia; and other fetal distress situations such as threatened abortion, prematurity, and fetal demise, may also show AFP elevations. Decreased amniotic fluid AFPvalues may be seen in pregnancies for which the gestational age hasbeen overestimated.

CLINICAL INTERPRETATION:

A diagnostic AFP cutoff level of 2.0 multiples of median (MoM),followed by acetylcholinesterase (AChE) confirmatory testing onpositive results, is capable of detecting 96% of open spina bifidacases with a false-positive rate of only 0.06% in non blood-stainedspecimens.
AChE analysis is an essential confirmatory test for all amniotic fluidspecimens with positive AFP results. Normal amniotic fluid doesnot contain AChE, unless contributed by the fetus as a result of opencommunication between fetal central nervous system (eg, openneural tube defects), or to a lesser degree, fetal circulation. Allamniotic fluid specimens testing positive for AFP will have the AChEtest performed. Because false-positive AChE may occur from a bloody tap, specimens with positive AChE results will also be tested for the presence of fetal hemoglobin, which may cause both elevated AFP and AChE levels.

REFERENCE VALUES:

< or =2.0 multiples of median (MoM)