Wallach's Interpretation of Diagnostic Tests: Pathways to Arriving at a Clinical Diagnosis (1152 page)

   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

   Detects fewer affected pregnancies than combined first-semester plus second-trimester screening modalities.

   Does not permit first-trimester decision making regarding termination of affected pregnancies.

COMBINED FIRST-TRIMESTER AND SECOND-TRIMESTER SCREENING (INTEGRATED/SEQUENTIAL SCREENING)
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   Definition

   Integrated screening combines first-semester and second-trimester screening to give one result after the second-trimester screen is completed.

   Sequential screening gives the risk after the first trimester if risk is higher than a specific cutoff and gives the combined risk after the second trimester if first-trimester risk was not higher than the cutoff. It can be further divided into stepwise and contingent.

   Stepwise screening: Women with risk above a certain cutoff following the first-trimester screen are offered invasive diagnostic testing directly, whereas women below cutoff are offered second-trimester screening.

   Contingent screening: Women with high risk are offered diagnostic testing, women with intermediate risk are offered second-trimester screening, and women with low risk have no further testing.

   Some centers prefer to divide patients into two groups only: Those with high risk who will be offered invasive testing directing and those who will proceed to second-trimester testing.

   Use

   Risk assessment for trisomy 18, trisomy 21, and neural tube defects.