TEST CATALOG ORDERING & RESULTS SPECIMEN HANDLING CUSTOMER SERVICE EDUCATION & INSIGHTS
Test Catalog

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

Useful For Suggests clinical disorders or settings where the test may be helpful

First-trimester prenatal screening for Down syndrome (trisomy 21) and trisomy 18

Testing Algorithm Delineates situations when tests are added to the initial order. This includes reflex and additional tests.

Sequential maternal screening is a 2-step test, with first- and second-trimester components. It requires a nuchal translucency (NT) measurement and blood collection 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 the results from part 1 are negative, an additional blood collection in the second trimester is required (see SEQB / Sequential Maternal Screening, Part 2, Serum). If the second specimen is not received for sequential screening, the results are uninterpretable and no maternal risk will be provided.

 

The following are available in Special Instructions:

-Sequential Maternal Serum Screening Testing Process

-Prenatal Aneuploidy Screening and Diagnostic Testing Options Algorithm

Clinical Information Discusses physiology, pathophysiology, and general clinical aspects, as they relate to a laboratory test

Maternal serum screening is used to identify pregnancies that may have an increased risk for certain birth defects, such as trisomy 21 (Down syndrome), neural tube defects (NTD) and trisomy 18. Various options for maternal serum screening are available and include: first trimester, second trimester, and cross-trimester. Sequential screening is a type of cross-trimester screening which has an improved detection rate as compared to either first- or second-trimester screening. Sequential screening combines biochemical and ultrasound markers (nuchal translucency: NT) measured in both trimesters of the pregnancy.

 

SEQA / Sequential Maternal Screening, Part 1, Serum involves an ultrasound and a blood draw. The ultrasound measurement, referred to as the NT measurement, 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 collected to measure pregnancy-associated plasma protein A (PAPP-A). The results of the ultrasound measurement and blood work, along with the maternal age and demographic information, are used to calculate Down syndrome and trisomy 18 risk estimates.

 

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. In that event, the patient is typically offered counseling and diagnostic testing. When the part 1 screen is completed, NTD risk is not provided. For a stand-alone NTD-risk assessment, order MAFP1 / 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 collected in the second trimester for SEQB / Sequential Maternal Screen, Part 2, Serum. The blood sample is tested for AFP, unconjugated estriol (uE3), human chorionic gonadotropin (hCG), and inhibin A. The information from both trimesters is combined and a report is issued. If results are positive, the patient is typically offered counseling and diagnostic testing.

 

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, Down syndrome and trisomy 18 fetuses 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 and trisomy 18.

 

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 fourteenth week of gestation are associated with an increased risk for Down syndrome and trisomy 18.

Reference Values Describes reference intervals and additional information for interpretation of test results. May include intervals based on age and sex when appropriate. Intervals are Mayo-derived, unless otherwise designated. If an interpretive report is provided, the reference value field will state this.

An interpretive report will be provided.

Interpretation Provides information to assist in interpretation of the test results

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 (trisomy 21):

First-trimester 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 SEQB / Sequential Maternal Screening, Part 2, Serum).

 

Second-trimester 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, > or =1/50 in the first trimester and > or =1/270 in the second trimester). Positive results are not diagnostic.

 

When both sequential maternal screening parts 1 and 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 1/100 (1%). Risks that are greater or equal to 1% are screen-positive; positive results are not diagnostic. Risks less than 1% are screen-negative; negative results do not guarantee the absence of trisomy 18.

 

Use caution when revising trisomy 18 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 sequential maternal screening parts 1 and 2 are performed, the overall detection rate is approximately 90% with a false-positive rate of approximately 0.1% using a screen cutoff of 1/100.

 

Neural Tube Defect:

Risk assessment for neural tube defects (NTD) is only available after completion of part 2 of the sequential maternal screen. See SEQB / 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.

Cautions Discusses conditions that may cause diagnostic confusion, including improper specimen collection and handling, inappropriate test selection, and interfering substances

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 screen-negative result does not guarantee the absence of fetal defects. A screen-positive result does not provide a diagnosis, but indicates that further diagnostic testing should be considered (an unaffected fetus may have screen-positive result for unknown reasons). In fact, given the low prevalence of Down syndrome, the majority of women with a positive screen will not have a Down syndrome fetus.

 

In twin pregnancies, the risk for Down syndrome is approximated, using twin-adjusted medians. In cases where one twin has demised, results may be unreliable.

 

Results are not available for triplets or higher-multiple pregnancies.

 

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.

Clinical Reference Recommendations for in-depth reading of a clinical nature

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. Prenatal Diagnostic Testing for Genetic Disorders. ACOG Practice Bulletin No. 163. American College of Obstetricians and Gynecologists. Obstet Gynecol. 2016 May;127(5):979-981. doi: 10.1097/AOG.0000000000001439

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

6. Yarbrough ML, Stout M, Gronowski AM: Chapter 69: Pregnancy and Its Disorders. In Tietz Textbook of Clinical Chemistry and Molecular Diagnostics. Sixth Edition. 2018. pp. 1655-1696

Special Instructions Library of PDFs including pertinent information and forms related to the test