Test Code: 803139

Clinical Use

  • Prenatal risk assessment for neural tube defects, Down syndrome, and trisomy 18

Clinical Background

Prenatal testing is routinely offered to pregnant women for neural tube defects, Down syndrome, and trisomy 18 risk assessment. Neural tube risk assessment is based on Alpha-fetoprotein (AFP) alone, whereas Down syndrome and trisomy 18 risk assessments are based on multiple marker combinations that may include maternal age, AFP, human chorionic gonadotropin (hCG), and unconjugated Estriol (uE3).

Recent studies have demonstrated the utility of adding inhibin A to Down syndrome risk assessment. During pregnancy, inhibin is secreted from both the corpus luteum and the placenta. In Down syndrome pregnancies, inhibin A levels are 2-fold higher than in unaffected pregnancies, leading to the detection of approximately 40% of Down syndrome fetuses with a 5% false positive rate. When combined with maternal age, AFP, hCG, and uE3, the detection rate increases to approximately 75%. Addition of inhibin A improves the detection rate by approximately 10% relative to commonly used marker combinations.

Specimen Requirements

  • 2 mL refrigerated serum from a serum separator tube or a plain red-top tube (1 mL minimum).
  • Neural tube defect screening: Collect sample between 15 and 21 weeks gestation. Optimal draw time is 16-18 weeks gestation.
  • Down syndrome screening: Collect sample between 14 and 22 weeks gestation.
  • Collection date, maternal birth date (mm/dd/yy), race, weight, gestational age and how it was obtained, insulin-dependent diabetic status, single or multiple gestation and history of Neural tube defects must be provided for interpretation of results.
  • This information can be submitted on our Maternal Serum Screen Information Sheets (supply #8662) which are available through our warehouse.

The report for this test includes a risk for Neural tube defects, Down syndrome and Trisomy-18 [Includes Alpha-fetoprotein, HCG, Unconjugated Estriol, Inhibin-A].

CPT Codes*

82105; 82677; 84702; 86336


Concentrations of AFP, hCG, uE3, and inhibin-A are determined using the analytical methods stated below. The multiple of the median (MoM) is calculated for each. Separate medians are used to calculate the MoM for African American and Asian populations. MoM values for all analytes are adjusted for maternal weight; the AFP MoM is adjusted for insulin dependent diabetic status.

  • Down syndrome: Risk is based on all four MoM values combined with maternal age at the time of delivery.
  • Trisomy 18: Risk is based on hCG, uE3, and AFP MoM values combined with maternal age at the time of delivery.
  • Neural tube defect: Risk is based on AFP MoM only.

Based on risk, results will report as “Screen Positive” or “Screen Negative”.

Analytical methods used are as follows:

  • AFP:  Immunochemiluminescence assay
  • hCG:  Immunochemiluminescence assay
  • uE3:  Immunochemiluminescence assay
  • Inhibin A:  Enzyme immunoassay

Interpretive Information

  Normal Risk Increased Risk
Open neural tube defect    
  • Singleton pregnancy
AFP MoM <2.5 AFP MoM ≥2.5
  • Twin pregnancy

AFP MoM <4.0

AFP MoM ≥4.0

  • Insulin-dependent diabetic

AFP MoM <1.9

AFP MoM ≥1.9

Down syndrome Risk <1:270 Risk ≥1:270
Trisomy 18 Risk <1:100 

Risk ≥1:100

*The CPT codes provided are based on AMA guidelines and are for informational purposes only. CPT coding is the sole responsibility of the billing party. Please direct any questions regarding coding to the payor being billed.


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