Alpha-fetoprotein (AFP) is a protein present in the blood (precisely in its liquid part, the plasma) produced by the embryo and the fetus. In the past, its dosage was routinely offered to pregnant women to check for the presence of defects in the closure of the neural tube (for example spina bifida).
For many years, alpha-fetoprotein (AFP) associated with the dosage of two other hormones present in maternal blood (estriol and human chorionic gonadotropin) has been used to perform a prenatal screening test called tri-test. This is an "analysis currently offered only to women who were unable to carry out the combined test, a test recommended in Italy for prenatal control (screening) of the first trimester because compared to the other available tests it has greater diagnostic reliability for some congenital diseases (ie already present at birth) Both tests are free and ticket-free thanks to the Essential Levels of Assistance (LEA) provided by the National Health Service.
The AFP test is performed on the mother's blood between the 15th and 20th week of pregnancy. The diseases already present at birth (congenital) associated with an increase in AFP are mainly those related to defects in the closure of the neural tube and, sometimes, trisomies 18 and 21 (Down syndrome).However, an "alteration of the AFP dosage" alone is not sufficient for the diagnosis of these diseases, but directs the doctor to prescribe further tests such as, for example, amniocentesis or morphological ultrasound to confirm.
The dosage of alpha-fetoprotein requires a maternal blood sample, to be performed from the 15th week of pregnancy, and its analysis in the laboratory. The levels of alpha-fetoprotein during the development of the human embryo begin to increase at the end of the pregnancy. first trimester and the protein passes into the maternal blood through the placenta. Its levels drop after 32 weeks of pregnancy. During delivery and thereafter, alpha-fetoprotein levels drop rapidly.
A low level of alpha-fetoprotein has been observed in pregnancies in which the fetus is a carrier of certain chromosomal abnormalities. For example, in the case of Down Syndrome (trisomy 21), the concentrations of AFP and estriol tend to decrease while the values of other proteins such as human chorionic gonadotropin (βhCG) and inhibin A tend to increase, compared to normal ones. .
On the other hand, in pregnancies in which the fetus is affected by Edwards Syndrome (trisomy 18), the concentrations of estriol and βhCG tend to decrease, while the concentration of alpha-fetoprotein can vary.
In conclusion, high levels of alpha-fetoprotein could indicate the presence of some serious malformations of the fetus, such as spina bifida and anencephaly, which however must be confirmed by other in-depth examinations.
According to statistics reported by the American College of Obstetrician and Gynecologist (ACOG), the alpha-fetoprotein (AFP) test detects neural tube defects in about 80% of cases. fails to identify some malformations giving a false negative result.
It is advisable that the result of the test be communicated to the woman and the couple by a genetic counselor or a doctor to explain its meaning and illustrate the clinical path to follow. If the values are not normal, it is necessary, in fact, to perform a "morphological ultrasound, or an" amniocentesis, to check for the possible presence of malformations or anomalies in the fetus or its chromosomes.
Factors that can interfere with the test result
The causes that can give false positive results, that is, the increase in alpha-fetoprotein levels, are:
- wrong calculation of gestational age
- multiple pregnancies
- gestational diabetes
- detachment of the placenta
- cigarette smoke
- maternal tumors of the liver and / or ovary
- physiological increase in AFP, not related to any kind of anomaly
In addition, the reference doses of alpha-fetoprotein and therefore the interpretation of the test results must consider the ethnicity of the mother, the weight of the mother and the week of pregnancy.
Adigun OO, Khetarpal S. Alpha Fetoprotein (AFP, Maternal Serum Alpha Fetoprotein, MSAFP). StatPearls [Internet]. 2019; Feb 22