generality
L'alpha-fetoprotein (AFP) is a glycoprotein substance produced, from the earliest stages of pregnancy, first by the yolk sac and then by the fetus.
During intrauterine life, the unborn child synthesizes AFP mainly in the liver, with a marginal contribution from the kidneys and gastrointestinal system. After birth, alpha-fetoprotein levels begin to decline, reaching adult-typical values ​​within a year.
What's this
Alphafetoprotein is a protein that is synthesized by the liver and yolk sac during the stages of fetal and embryonic development. This protein is present in the plasma of the unborn child in high quantities, starting from the second trimester of pregnancy, so that it is also revealed in the maternal blood.
From the moment of delivery onwards, alpha-ethoprotein levels drop rapidly, so much so that it is found only in small traces in healthy women and children.
Currently, the function of this protein in adults and during fetal development remains unclear.
The gene responsible for its expression is the AFP gene located on the q arm of chromosome 4.
Because it is measured
In pregnant women, the dosage of alphaphetoprotein in the blood is used as control screening for any congenital malformations of the neural tube (such as spina bifida or anencephaly). In addition, the examination is useful to support the diagnosis of trisomy 21 (o down syndrome).
The alphafetoprotein assay is performed together with estriol and β-hCG; the combination of these three evaluations is called tri-test and is performed between the fifteenth and twentieth week of pregnancy.
When is it prescribed?
The alphafetoprotein dosage is prescribed between the 15th and 20th week of pregnancy, in order to assess the risk of having a fetus affected by malformations or conditions such as, for example, Down syndrome. If the screening is positive, other tests are needed to confirm the diagnosis, such as ultrasound and amniocentesis.
Normal values
Values ​​in the various stages of pregnancy
During intrauterine life, AFP represents the main fetal plasma protein and, in this sense, performs functions very similar to those of albumin; its serum concentrations tend to increase until the end of the first trimester, reaching a peak of approximately 3 mg / mL between the 10th and 13th week of gestation.
Subsequently, alphafetoprotein levels drop exponentially between the 14th and 32nd week, at the end of which they amount to about 0,2 mg / mL; this decline goes hand in hand with the progressive increase in albumin synthesis (which will be the main protein molecule in the plasma), the increase in blood volume and the decreased hepatic synthesis of alpha-fetoprotein.
Regardless of the amniotic concentrations of AFP, a proportional part of this substance reaches the maternal circulation, partly passing the fetal membranes and the decidua uterus (transamniotic pathway) and partly through the transplacental pathway.
During the very early stages of pregnancy, alphaphetoprotein appears to diffuse from the fetal circulation to the amniotic fluid through the epidermis, which in these stages is not yet keratinized. However, as soon as the fetal kidneys begin to function (towards the end of the first trimester), alphaphetoprotein enters the urine of the fetus and from there passes into the amniotic fluid.
AFP Alta - Cause
By studying the trend of alphafetoprotein concentrations in amniotic fluid and maternal blood, the researchers discovered a positive correlation between high levels of AFP and some malformative pathologies, especially concerning i neural tube defects such as anencephaly (leading to fetal death) and failure to close the neural tube (spina bifida, i.e. when the vertebrae do not adequately envelop the spinal cord).
In fact, the amniotic concentration of alphafetoprotein is usually particularly low compared to that of the fetal plasma. In the presence of a neural tube defect, on the other hand, the solution of continuity that is created between the CSF and amniotic fluid determines the free passage of large quantities of AFP, so that the amniotic concentration increases significantly (together with the levels of acetylcholinesterase, a specific enzyme of the nervous tissue which under these conditions undergoes a significant increase).
Alphafetoprotein concentrations can also be measured in maternal blood as a simple screening test to identify high-risk pregnancies that need further testing, including ultrasonography. This last test, in particular, is currently preferred to the alpha-fetoprotein assay as an early screening test, both for the not exactly excellent sensitivity of the latter, and for the now excellent ability to identify ultrasound signs of chromosomopathy.
Maternal serum alphafetorpotein levels tend to increase even in the presence of a placental abruption (abruptio placentae).
In the opposite situation, that is when low values ​​of alphafetoprotein are recorded in the maternal blood, the risk that the fetus is affected by down syndrome.
Thanks to these evidences, the assay of alphafetoprotein in maternal blood represents a valid screening tool, to which many women of at-risk age are subjected between the 15th and 21st week of gestation. More specifically, alphafetoprotein levels are evaluated - in the so-called Tri-Test - together with those of other biochemical markers, such as HCG (human chorionic gonadotropin) and unconjugated estriol (placental estrogen).
Other times the dosage of inhibin A is also inserted and therefore we speak of Quadruple-Test.
In pregnant women carrying fetuses with Down syndrome, the serum levels of alphafetoprotein and unconjugated estriol decrease, while those of human chorionic gonadotropin increase.
Also in this case, these are not real diagnostic tests, but screening tests which - together with the mother's age - allow us to quantify the risk that the fetus is affected by Down syndrome. When this risk proves important, pregnant women are directed to appropriate diagnostic tests such as amniocentesis.
AFP Bassa - Cause
Alphafetoprotein values ​​can be low in the following cases:
- Gestational age lower than the presumed one (when the exact date of conception is not known);
- Abortion not yet identified.
In pregnant women carrying fetuses with a chromosomal defect that will cause Down syndrome, serum levels of alphafetoprotein and unconjugated estriol tend to be low, while those of human chorionic gonadotropin (hCG) and inhibin A increase.
How it is measured
The alphafetoprotein test is a laboratory test that involves taking a simple blood sample from a vein in the arm. AFP, hCG, Estriol and inhibin A tests can be performed on the same blood sample.
Preparation
For AFP analysis, a fast of at least 8 hours is required to avoid food interfering with the result.
Interpretation of Results
The result should be interpreted by a genetic counselor or clinician who can explain the significance of the test.
The value of alphaphetoprotein in the bloodstream strictly depends on the determination ofgestational age of the fetus. In fact, if the latter had not been calculated precisely by the gynecologist, there is a risk of considering the alpha-fetoprotein levels too high or low.
In maternal serum, the concentration of AFP in the blood begins to increase exponentially until the 32nd week, and then decreases a few days after delivery.
Elevated alphafetoprotein values ​​in pregnant women may be caused by:
- Neural tube defects (spina bifida, anencephaly);
- Problem with the placenta
- Fetal malformations (chromosomal abnormalities);
- Mother's neoplasm or liver disease.
However, factors that could raise AFP levels also include:
- Incorrect dating of pregnancy;
- Twin pregnancy.
Low levels of alphafetoprotein, on the other hand, can be associated with chromosomal disorders. To be precise, a low level of AFP along with an increased level of HCG (bi-test) has been associated with an increased risk of Down syndrome.
In any case, once these abnormal values ​​are found, the doctor will recommend further tests, such as an abdominal ultrasound or amniocentesis to measure the levels of alphafetoprotein in the amniotic fluid.
