Test ID: SEQU
Sequential Maternal Screening, Part 1, Serum

Prenatal screening for Down syndrome:

-Part 1 (first trimester): nuchal translucency (NT), pregnancy-associated plasma protein A (PAPP-A)

-Part 2 (second trimester): alpha-fetoprotein (AFP), unconjugated estriol (uE3), human chorionic gonadotropin (hCG), and inhibin A

Prenatal screening for trisomy 18:

-Part 1 (first trimester): NT, PAPP-A

-Part 2 (second trimester): AFP, hCG, uE3

Prenatal screening for open neural tube defects:

-Part 2 (second trimester): AFP

Sequential maternal screening is a 2-part test, with first- and second-trimester components. It requires a nuchal translucency (NT) measurement and blood draw in the first trimester. If the result from Part 1 indicates a risk for Down syndrome that is higher than the screen cutoff, the screen is completed and a report is issued. If Part 1 results are negative, an additional blood draw in the second trimester is required (see SEQF/60700 Sequential Maternal Screen, Part 2, Serum). If the second specimen is not received for sequential screening, the results are uninterpretable and no maternal risk will be provided.

See Sequential Maternal Serum Screening Algorithm in Special Instructions.

• NT/CRL Data for First Trimester/Sequential Maternal Screening
• First Trimester/Sequential Maternal Screening Patient Information Sheet
• Sequential Maternal Serum Screening Algorithm

Two-Site Immunoenzymatic (Sandwich) Assay

Approval to send specimens for sequential screening is required and may take up to 5 business days to complete. Part 1 of sequential screening involves an ultrasound measurement, in addition to the blood specimen. The ultrasound measurement requires advanced skill; therefore, results are only accepted from sonographers who have been approved by Mayo Medical Laboratories. Do not send specimens if the sonographer is not NT-certified or before completing the approval process. To begin the application process, visit the Sonographer Approval Process page at:

http://www.MayoMedicalLaboratories.com/customer-service/forms/maternal-screening.html or complete the NT/CRL Data for First Trimester/Sequential Maternal Screening form in Special Instructions.

When Part 1 is negative, Part 2 must be completed in order to receive an interpretable result. If collecting a second-trimester specimen is expected to be difficult, order first-trimester screening instead (see 1STT/87857 First Trimester Maternal Screen).

The ultrasound and blood draw must be completed within a gestational window of 10 weeks, 0 days and 13 weeks, 6 days, which corresponds to a crown-rump length (CRL) range of 31 mm to 80 mm.

Container/Tube: 

Preferred: Red top

Acceptable: Serum gel

Specimen Volume: 1 mL

Collection Instructions: Immediately spin down.

Forms: First Trimester/Sequential Maternal Screening Patient Information Sheet (Supply T593) is required; in Special Instructions.

Maternal serum screening has historically been used in obstetric care to identify pregnancies that may have an increased risk for certain birth defects, such as Down syndrome and trisomy 18. Screening in the second trimester has been available in some version (ie, alpha fetoprotein [AFP] test, triple screen, quad screen) for decades. Screening in the first trimester became an established alternative over the last decade.

More recently, sequential screening, which has an improved detection rate as compared to either first- or second-trimester screening, has become a standard option. Sequential screening has a higher detection rate because information about a pregnancy is collected in both trimesters, which provides a greater opportunity for detecting problems.

Sequential Maternal Screening, Part 1, Serum involves an ultrasound and a blood draw. The ultrasound measurement is of the back of the fetal neck, where fluid tends to accumulate in babies who have chromosome conditions, heart conditions, and other health problems. This measurement, referred to as the nuchal translucency (NT), is difficult to perform accurately. Therefore, NT data is accepted only from NT-certified sonographers. Along with the NT measurement, a maternal serum specimen is drawn and 1 pregnancy-related marker is measured (pregnancy-associated plasma protein A [PAPP-A]). The results of the ultrasound measurement and blood work are then entered, along with the maternal age and demographic information, into a mathematical model that calculates Down syndrome and trisomy 18 risk estimates.

If the result from the first-trimester Sequential Maternal Screening, Part 1, Serum indicates a risk for Down syndrome that is higher than the screen cutoff, the screen is completed and a report is issued. In that event, the patient is typically offered counseling and diagnostic testing (ie, either chorionic villus sampling or amniocentesis). When the screen is completed after Sequential Maternal Screen Part 1, a neural tube defect (NTD) risk is not provided. For a stand-alone neural tube defect-risk assessment, order #81169 Alpha-Fetoprotein (AFP), Single Marker Screen, Maternal, Serum.

If the risk from the first trimester is below the established cutoff, an additional serum specimen is drawn in the second trimester for Sequential Maternal Screen, Part 2, which includes tests for AFP, unconjugated estriol (uE3), human chorionic gonadotropin (hCG), and inhibin A. Once that specimen is processed, information from both trimesters is combined and a report is issued. If results are positive, the patient is typically offered counseling and diagnostic testing (ie, amniocentesis).

Nuchal Tanslucency (NT)

The NT measurement, an ultrasound marker, is obtained by measuring the fluid-filled space within the nuchal region (back of the neck) of the fetus. While fetal NT measurements obtained by ultrasonography increase in normal pregnancies with advancing gestational age, fetuses with Down syndrome have larger NT measurements than gestational age-matched normal fetuses. Increased fetal NT measurements can, therefore, serve as an indicator of an increased risk for Down syndrome. 

Pregnancy-Asociated Pasma Potein A (PAPP-A)

PAPP-A is a 187-kDA protein comprised of 4 subunits: 2 PAPP-A subunits and 2 pro-major basic protein (proMBP) subunits. PAPP-A is a metalloproteinase that cleaves insulin-like growth factor-binding protein-4 (IGFBP-4), dramatically reducing IGFBP-4 affinity for IGF1 and IGF2, thereby regulating the availability of these growth factors at the tissue level. PAPP-A is highly expressed in first-trimester trophoblasts, participating in regulation of fetal growth. Levels in maternal serum increase throughout pregnancy. Low PAPP-A levels before the fourteen week of gestation are associated with an increased risk for Down syndrome and trisomy 18.

An interpretive report will be provided. See Interpretation section for more details.

Maternal screens provide an estimation of risk, not a diagnosis. A negative result indicates that the estimated risk falls below the screen cutoff. A positive result indicates that the estimated risk exceeds the screen cutoff.

Down syndrome:

Sequential Maternal Screening, Part 1, Serum results are negative when the calculated risk is below 1/50 (2%). If Part 1 is negative, submit an additional specimen in the second trimester (order #60700 Sequential Maternal Screening, Part 2, Serum).

Sequential Maternal Screening, Part 2, Serum results are negative when the calculated risk is below 1/270 (0.37%). Negative results mean that the risk is less than the established cutoff; they do not guarantee the absence of Down syndrome.

Results are positive when the risk is greater than the established cutoff (ie, >1/50 in Sequential Maternal Screening, Part 1, Serum and >1/270 in Sequential Maternal Screening, Part 2, Serum). Positive results are not diagnostic.

When both Sequential Maternal Screening Part 1 and Part 2 are performed with a screen cutoff of 1/270, the combination of maternal age, nuchal translucency (NT), pregnancy-associated plasma protein A (PAPP-A), alpha-fetoprotein (AFP), unconjugated estriol (uE3), human chorionic gonadotropin (hCG), and inhibin A has an overall detection rate of approximately 90% with a false-positive rate of approximately 3% to 4%. In practice, both the detection rate and false-positive rate vary with maternal age.

Trisomy 18:

In Part 1, trisomy 18 results are only reported if the Down syndrome risk is positive.

In Part 2, the screen cutoff for trisomy 18 is 1 in 100 (1%). Risks that are > or =1% are screen-positive; positive results are not diagnostic. Risks <1% are screen-negative; negative results do not guarantee the absence of trisomy 18.

Use caution when revising positive results with earlier dating. Babies with trisomy 18 tend to be small, which can lead to underestimation of gestational age and an increased chance of missing a true-positive.

When both Sequential Maternal Screening Part 1 and Part 2 of sequential screening are performed with a screen cutoff of 1/100, the combination of maternal age, PAPP-A, AFP, unconjugated estriol, and hCG, has an overall detection rate of approximately 90% with a false-positive rate of approximately 0.1%.

Neural tube defect (NTD):

Risk assessment for NTD is only available after completion of Part 2 of the sequential maternal screen. See #60700 Sequential Maternal Screening, Part 2, Serum for details.

Follow-up:

Verify that all information used in the risk calculation is correct (maternal date of birth, gestational dating, etc). If any information is erroneous, contact the laboratory for a revision.

Screen-negative results typically do not warrant further evaluation.

If the results are positive, the patient is typically offered counseling, ultrasound, diagnostic testing, and possibly, referral to genetics counseling or a high-risk clinic.

Nuchal translucency (NT) measurements must be obtained from NT-certified sonographers.  NT-measurement quality indicators will be monitored on a regular basis. Sonographers will be contacted if there is ongoing deviation in the quality indicators.

Incorrect or incomplete information may significantly alter results.

A negative screen does not guarantee the absence of fetal defects.

A positive screen does not provide a diagnosis, but indicates that diagnostic testing should be considered (an unaffected fetus may have a screen-positive result for unknown reasons).

The use of these markers to screen for Down syndrome or trisomy 18 is not an approved FDA procedure.

In twin pregnancies, the risk for Down syndrome is approximated, using twin-adjusted medians. A specific risk for trisomy 18 cannot be calculated. In cases where 1 twin has demised, results may be unreliable.

Results are not available for triplets or higher-multiple pregnancies, or for diabetics carrying twins.

Each center offering maternal serum screening to patients should establish a standard screening protocol, which provides pre- and post-screening education and appropriate follow-up for screen-positive results.

1. Malone FD, Canick JA, Ball RH, et al: First-trimester or second-trimester screening, or both, for Down's syndrome. N Engl J Med 2005 Nov 10;353(19):2001-2011 

2. Screening for fetal chromosomal abnormalities. ACOG Practice Bulletin No. 77. American College of Obstetricians and Gynecologists. Obstet Gynecol 2007;109:217-227

3. Wald NJ, Rodeck C, Hackshaw AK, et al: SURUSS in Perspective. Semin Perinatol 2005;29:225-235

4. Palomaki GE, Steinort K, Knight GJ, et al: Comparing three screening strategies for combining first- and second-trimester Down syndrome markers. Obstet Gynecol 2006 Feb;107(2 Pt 1): 367-375

5. Palomaki GE, Neveux LM, Knight GJ, et al: Maternal serum-integrated screening for trisomy 18 using both first- and second-trimester markers. Prenat Diagn 2003 Mar;23(3):243-247

Sequential Maternal Screening, Part 1, Serum includes measuring the nuchal translucency (NT) and pregnancy-associated plasma protein A (PAPP-A). The NT and PAPP-A are compared to median values for a given gestational age and a multiple-of-the-median (MoM) is calculated for each. The MoM results are entered into a multivariate algorithm that includes the mother's age to derive risk factors for Down syndrome and trisomy 18. If the calculated risks exceed the screen cutoff, the results are reported and the screen is ended. If the results from the first part of screening fall below the screen cutoff, the results are held until the second sample is analyzed. PAPP-A is performed on the Beckman Access using an automated immunoenzymatic assay with paramagnetic separation and chemiluminescent detection. (Package insert: PAPP-A, Beckman-Coulter Access, 2007)

Monday through Friday; 5 a.m.-5 p.m., Saturday; 6 a.m.-1 p.m.

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