Precautions It is very important that the doctor know precisely how old the fetus is when the test is performed since the AFP level considered normal changes over the length of the pregnancy. Errors in determining the age of the fetus lead to errors when interpreting the test results. Since an AFP test is only a screening tool, more specific tests must follow to make an accurate diagnosis. An abnormal test result does not necessarily mean that the fetus has a birth defect. The AFP test is usually performed at week 16 of pregnancy.
Blood is drawn from a vein, usually on the inside of the elbow.
AFP can also be measured in the sample of amniotic fluid taken at the time of amniocentesis. Test results are usually available after about one week. There is no specific physical preparation for the AFP test. There is no specific aftercare involved with this screening test. When testing for cancer or liver diseases, AFP results are reported as nanograms per milliliter.