Cautions

Variables Affecting Marker Levels:

Race, weight, smoking, multiple fetus pregnancy, insulin-dependent diabetes (IDD), and in vitro fertilization (IVF) may affect marker concentrations. Black mothers tend to have higher alpha-fetoprotein (AFP) levels but lower risk of neural tube defects and are assigned to a separate AFP median set. All multiples of the median (MoM) are adjusted for maternal weight (to account for dilution effects in heavier mothers). The AFP, unconjugated estriol (uE3), and inhibin MoM are adjusted upward in IDD to account for lower values in diabetic pregnancies. Human chorionic gonadotropin (hCG) levels are higher and uE3 levels are lower in pregnancies conceived by IVF, MoM are adjusted accordingly to account for the alterations. Smoking results in higher second trimester maternal serum AFP and inhibin A levels and lower uE3 and hCG levels. MoM are adjusted accordingly to account for analyte differences in smokers.

The estimated risk calculations and screen results are dependent on accurate information for gestation, maternal age, race, IDD, and weight. Inaccurate information can lead to significant alterations in the estimated risk. In particular, erroneous assessment of gestational age can result in false-positive or false-negative screen results. Because of its increased accuracy, the determination of gestational age by ultrasound is recommended, when possible, rather than by last menstrual period.

A screen-negative result does not guarantee the absence of fetal defects. A screen-positive result does not provide a diagnosis but indicates that further diagnostic testing should be considered (an unaffected fetus may have screen-positive result for unknown reasons).

Valid measurements of AFP in maternal serum cannot be made after amniocentesis.

Triplet and higher multiple pregnancies cannot be interpreted.

Each center offering maternal serum screening to patients should establish a standard screening protocol that provides pre- and post-screening education and appropriate follow-up for screen-positive results.

In a small percentage of samples, there is potential for alkaline phosphatase associated positive interference in the Beckman Access uE3 assay. This potential interference does not appear to be related to the amount of alkaline phosphatase in the patient sample. A falsely elevated uE3 test result can lead to inaccurately underestimating the relative risk of chromosomal abnormalities, such as trisomy 21 and 18.

In rare cases, some individuals can develop antibodies to mouse or other animal antibodies (often referred to as human anti-mouse antibodies [HAMA] or heterophile antibodies), which may cause interference in some immunoassays. Caution should be used in interpretation of results, and the laboratory should be alerted if the result does not correlate with the clinical presentation.