Here is the clear, direct answer to your question: The second-trimester quadruple screen measures four maternal serum biomarkers—AFP, hCG, uE3, and Inhibin A. In Down syndrome, AFP and uE3 are low (~0.75 MoM) while hCG and Inhibin A are high (~2.0 MoM). In Trisomy 18, AFP, uE3, and hCG are all markedly low (medians ~0.65, 0.43, and 0.36 MoM respectively), creating a distinct triple-depression pattern.
The quadruple screen’s diagnostic power comes from the opposing high/low analyte shifts between Down syndrome and Trisomy 18. Recognizing these divergent profiles is critical—not just for clinical interpretation, but for IVD manufacturers designing assays that must maintain precision across both suppressed and elevated concentration extremes.
How the Second-Trimester Quadruple Screen Works
The quadruple test is a multi-analyte maternal serum screening panel performed between 15 and 22 weeks of gestation. It combines four immunoassay measurements with maternal age to calculate risk scores for fetal aneuploidies.
The Four Core Analytes
- Alpha-fetoprotein (AFP): A fetal glycoprotein produced in the yolk sac and liver. In unaffected pregnancies, maternal serum levels rise steadily through the second trimester.
- Human chorionic gonadotropin (hCG): A placental hormone that peaks early in pregnancy and then declines. The assay typically measures the intact or free beta subunit.
- Unconjugated estriol (uE3): A placental-derived hormone that increases as pregnancy progresses. It reflects feto-placental function.
- Dimeric inhibin A (InhA): A placental glycoprotein hormone that remains relatively stable in maternal serum during the second trimester.
Together, these markers provide an integrated biochemical fingerprint that shifts predictably in the presence of certain chromosomal abnormalities.
Contrasting Biomarker Profiles: Down Syndrome vs. Trisomy 18
The most valuable diagnostic information lies not in any single marker, but in the pattern of how all four deviate from normal medians.
The Down Syndrome (Trisomy 21) Signature
Down syndrome causes a characteristic “two low, two high” profile.
- AFP is reduced to approximately 0.75 multiples of the median (MoM), about 25% lower than expected.
- uE3 is similarly decreased to around 0.75 MoM.
- hCG is significantly elevated, typically reaching 2.0 MoM or higher.
- Inhibin A is also doubled to a median of roughly 2.0 MoM.
This combined pattern—suppressed fetal proteins and placental hormone overproduction—enables a detection rate of approximately 80% at a 5% false-positive threshold when used alongside maternal age.
The Trisomy 18 (Edwards Syndrome) Signature
Trisomy 18 produces a completely opposite pattern of suppression, often described as a triple depression.
- AFP is low, with medians around 0.65 MoM—even more reduced than in Down syndrome.
- uE3 is extremely low, often reported at 0.43 MoM.
- hCG is severely depressed to approximately 0.36 MoM, a stark contrast to the elevation seen in Down syndrome.
- Inhibin A is typically not factored into the classic Trisomy 18 pattern, as its concentration does not show the same consistent deviation; the core discriminators are the three suppressed markers.
The depth of this suppression makes Trisomy 18 the condition most often associated with a “screen-positive” result driven by very low analyte values, rather than elevated ones.
Understanding the Biological Rationale
Why do these patterns differ so dramatically? Down syndrome is linked to trophoblastic hyperplasia and delayed placental maturation, which directly drives the overproduction of hCG and Inhibin A while the fetal components (AFP, uE3) remain under-expressed. In contrast, Trisomy 18 tends to produce a small, growth-restricted placenta with globally diminished synthetic function, leading to suppressed output of nearly all placental and fetal serum markers. This fundamental difference is precisely what clinical algorithms exploit.
Critical Limitations and Trade-offs to Keep in Mind
A purely objective evaluation requires acknowledging the real-world complexities that underlying MoM values can obscure.
The Fragility of MoM Calculations
Individual patient medians are affected by maternal weight, smoking, diabetes, ethnicity, and gestational dating errors. Even a 12% difference in AFP per 10 kg weight shift can misclassify risk if corrections are not rigorously applied. For IVD developers, this demands built-in software corrections and highly robust calibration curves.
Incomplete Marker Information for Trisomy 18
Because hCG is supressed rather than elevated in Trisomy 18, some older “triple screen” algorithms that rely on hCG as a positive flag will miss these cases entirely. The full quadruple panel with Inhibin A helps recover some of that lost sensitivity, but never fully closes the gap.
Analytical Range Challenges for Assay Manufacturers
The dynamic range needed to accurately quantify both 0.3 MoM and 2.5 MoM from a single assay is extreme. Antibody pairs must demonstrate consistent linearity and minimal cross-reactivity across wide concentration spans, and raw material lot variability can quickly erode the precision needed to separate a 0.43 MoM uE3 from a 0.75 MoM.
Making the Right Choice for Your Goal
How you use this information depends on where you sit in the diagnostic chain.
- If your primary focus is developing a second-trimester IVD kit: Select antibody pairs validated across the full clinical spectrum, from severely suppressed levels (Trisomy 18) to highly elevated ones (Down syndrome hCG/InhA), and ensure your calibrators bracket these extremes.
- If your primary focus is optimizing clinical laboratory screening algorithms: Prioritize meticulous median normalization for your population, incorporate rigorous ultrasound dating, and never rely on a single cutoff—use combined risk models that consider all four markers simultaneously.
- If your primary focus is training or provider education: Emphasize that “low” in the Trisomy 18 panel looks profoundly different from “low” in Down syndrome, and that the diagnostic significance of a 0.36 MoM hCG is entirely distinct from a 2.0 MoM hCG even though both are “abnormal.”
Master the pattern differences, and you transform four simple serum concentrations into a powerful, life-impacting diagnostic window.
Summary Table:
| Biomarker | Unaffected Median | Down Syndrome (T21) | Trisomy 18 (T18) | Key Biological Shift |
|---|---|---|---|---|
| AFP | 1.0 MoM | Low (~0.75 MoM) | Severely Low (~0.65 MoM) | Fetal synthetic under-expression |
| hCG | 1.0 MoM | Elevated (~2.0 MoM) | Severely Suppressed (~0.36 MoM) | Trophoblastic hyperplasia (T21) vs. placental insufficiency (T18) |
| uE3 | 1.0 MoM | Low (~0.75 MoM) | Extremely Low (~0.43 MoM) | Diminished feto-placental output |
| Inhibin A | 1.0 MoM | Elevated (~2.0 MoM) | Variable / Not core factor | Trophoblastic overproduction in T21 |
Developing second-trimester maternal screening immunoassays requires high-affinity antibody pairs and raw materials that maintain strict precision across wide dynamic ranges—from severely suppressed 0.3 MoM levels to 2.5+ MoM spikes.
At CamelBio, we provide diagnostic manufacturers, clinical labs, and research institutes with one-stop access to premium IVD raw materials, custom technical services, and expert regulatory consulting—covering every stage from concept to clinic.
Contact CamelBio today to discover how our raw material solutions can enhance your assay reliability and performance!