Wallach's Interpretation of Diagnostic Tests: Pathways to Arriving at a Clinical Diagnosis (1199 page)

   Second-trimester measurement of AFP is used to assess risk of fetal neural tube defects.

   Screening is offered to all women, regardless of age, to provide more accurate risk information than provided by age alone.

   Limitations

   Risk for trisomy 13 is
not
calculated; however, trisomy 13 pregnancies are typically associated with ultrasound anomalies detectable with second-trimester ultrasound definition are chromosomally normal, and some affected pregnancies will be missed.

SECOND-TRIMESTER SCREENING (MATERNAL SERUM SCREENING; QUAD SCREEN)

   Definition

   Performed between 15 and 22 weeks of gestation, the quadruple screen combines maternal age plus four serum biochemical markers: hCG, inhibin A, AFP, and unconjugated estriol to assess the risk of trisomy 21 and trisomy 18.

   Use

   Risk assessment for trisomy 21 (Down syndrome), trisomy 18, and open neural tube defects.

   Interpretation

   Trisomy 21 profile typically has high levels of hCG and inhibin A with low levels of AFP and unconjugated estriol.

   Trisomy 18 is associated with low levels of hCG, AFP, and estriol. (Inhibin A does not contribute to trisomy 18 risk profile.)

   Different centers use different cutoffs, balancing detection rate against number of invasive procedures performed. A cutoff of 1:270 (approximately 5% positive screening rate) detects approximately 80% of trisomy 21 and trisomy 18 pregnancies.

   Limitations