Can a quadruple marker test (Quad screen) be performed after 24 weeks of gestation?

Quadruple Marker Testing in Pregnancy After 24 Weeks

The quadruple marker test should not be performed after 24 weeks of gestation as it is only validated for use between 15 and 20.9 weeks of pregnancy. 1

Timing of Quadruple Marker Testing

• The optimal time for quadruple marker screening is between 16 to 18 weeks of gestation 1
• Testing can be performed between 15 and 20.9 weeks, but not earlier or later 1
• If a sample is drawn before 15 weeks, a new sample should be collected at the appropriate gestational age 1

Components and Purpose of Quadruple Screening

The quadruple test includes four serum markers:

• Alpha-fetoprotein (AFP)
• Human chorionic gonadotropin (hCG)
• Unconjugated estriol (uE3)
• Dimeric inhibin-A (INH-A) 1

This screening test is designed to:

• Detect open neural tube defects (75-90% detection rate) 1
• Screen for Down syndrome (approximately 75% detection rate in women under 35, over 80% in women 35 and older) 1, 2
• Identify trisomy 18 (at least 70% detection rate) 1

Interpretation and Accuracy Considerations

• The laboratory must be informed of the exact gestational age at the time of sample collection for accurate interpretation 1
• Ultrasound dating of the pregnancy reduces false-positive rates and increases detection rates 1
• Test accuracy is improved when the laboratory is informed of maternal weight, race, presence of insulin-dependent diabetes, number of fetuses, and family history of neural tube defects 1
• If there is a discrepancy in gestational age of 2 or more weeks after an ultrasound examination, the test result must be reinterpreted 1

Alternative Screening Options After 24 Weeks

For patients beyond 24 weeks who have not had aneuploidy screening:

• Detailed ultrasound examination is recommended to assess for structural anomalies 1
• Evaluation of soft markers on ultrasound can provide information about aneuploidy risk 1
• Cell-free DNA testing (NIPT) can be performed after 10 weeks throughout pregnancy, with a 99% detection rate for trisomy 21 1

Clinical Implications

• Performing quadruple screening outside the validated window (15-20.9 weeks) would yield unreliable results that could lead to inappropriate clinical decisions 1
• False positive or negative results are more likely when testing is performed outside the recommended gestational age window 1
• Patients who have missed the window for quadruple screening should be counseled about alternative screening and diagnostic options 1

Common Pitfalls to Avoid

• Attempting to interpret quadruple marker results from samples drawn after 20.9 weeks 1
• Failing to accurately date the pregnancy before testing, which can significantly affect result interpretation 1
• Not considering alternative screening methods when the optimal window for quadruple screening has passed 1
• Relying solely on maternal age for risk assessment, which has a lower detection rate (51%) and higher false-positive rate (14%) compared to quadruple screening 2