What does an abnormal AFP test mean?

Q: My blood test AFP came back abnormal. I was told not to worry, but that I needed to see a specialist. What does an abnormal AFP mean?

A: An abnormal AFP test has many possibilities. It may be an indication that the baby has a neural tube defect, such as spina bifida, a chromosomal condition such as Down’s syndrome, that the placenta may be compromised or that your baby has a different due date than previously thought. AFP is an abbreviation for Alpha-FetoProtein.

AFP is a glycoprotein produced by the fetus early in gestation (the pregnancy) by the yolk sac and later by the gastrointestinal tract and liver. By definition a glycoprotein is a compound consisting of a carbohydrate and protein. That carbohydrate is called hexosamine, an amnio sugar. Although we do not know the function of this protein, it is the major serum protein of the embryo and the early fetus. The concentration of AFP in the baby changes as the baby grows.

We do know that the highest concentration of AFP is around the 13th week of gestational development. The growing fetus passes small amounts of the protein into the amniotic fluid through urine. The mother’s bloodstream then absorbs some of this AFP. It is estimated that the mother will absorb 1/1000th of the concentration into her own blood.

Screening the mother for AFP is a routine test done around the 16 to 18th week into the pregnancy. The normal values are dependent on many factors such as: maternal age, race, weight, gestational age of the baby, the number of babies the mother is carrying, and if the mother has diabetes.

Around 50 percent of the tests are considered to be false-positive. That means that the test is falsely being reported as positive. False-positive results may be because the mother is carrying twins, inaccurate dates, a low birth weight infant, a large placenta or a sample that was contaminated. Abnormal results are also associated with the fact that the baby may be a girl. I do not know why, but it says so in the well-respected book “Williams Obstetrics.”

It is very important that you follow up with what ever your physician has advised. Usually the next step is to have a level 2 ultrasound, which is a detailed investigation of the baby’s body and the placenta.

We live in an age where there are very few surprises with pregnancies. Most people know the sex of their baby before delivery. If your baby does have a health problem, knowing about it ahead of time helps you to plan course of treatment after delivery.

This story was originally published January 15, 2017 at 1:00 PM with the headline "What does an abnormal AFP test mean?."