Test Catalog

Test Id : 1STT1

First Trimester Maternal Screen, Serum

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

Prenatal screening for trisomy 21 (Down syndrome) and trisomy 18

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

Method Name
A short description of the method used to perform the test

Immunoenzymatic Assay

NY State Available
Indicates the status of NY State approval and if the test is orderable for NY State clients.


Reporting Name
Lists a shorter or abbreviated version of the Published Name for a test

First Trimester Maternal Screen

Lists additional common names for a test, as an aid in searching

Combined Screen

First Trimester Screen


Specimen Type
Describes the specimen type validated for testing


Ordering Guidance

This test does not screen for neural tube defects. If risk assessment for neural tube defects is desired, collect specimen between 15 weeks, 0 days and 22 weeks, 6 days of gestation for an alpha-fetoprotein single marker screen; order MAFP1 / Alpha-Fetoprotein (AFP), Single Marker Screen, Maternal, Serum.


QUAD screening (QUAD1 / Quad Screen [Second Trimester] Maternal, Serum) is not recommended following first-trimester screening.


Necessary Information

Approval to send specimen for first-trimester screening is required and may take up to 5 business days to complete. Nuchal translucency (NT) measurements are only accepted from NT-certified sonographers. Do not send specimen to Mayo Clinic Laboratories if the sonographer is not NT-certified or before completing the application process. See Maternal Screening: Sonographer Approval Process link or complete the NT/CRL Data for First Trimester/Sequential Maternal Screening.


Question ID Description Answers
DRPH1 Physician Phone Number
MTWT Patient Weight
LBKGS Units (lbs or kg) lbs
SONON Sonographer Name
SONOC Sonographer Code
DT3 Scan Date
NUMF Number of Fetuses 1
3 or more
CHOR Number of Chorions Monochorionic
Not applicable
NT_B NT Twin
IDD_ Insulin dependent diabetes None
B_RCE Patient Race Black
EGGDN IVF Egg Donor Date of Birth
EGGFZ IVF Egg or Embryo Freeze Date
PRHX Prev Down (T21) / Trisomy Pregnancy No
If MAFP- Not applicable
SMKN1 Current cigarette smoking status Non-smoker
INTL1 Initial or repeat testing Initial
SONOD Sonographer reviewer ID

Specimen Required
Defines the optimal specimen required to perform the test and the preferred volume to complete testing

Collection Container/Tube:

Preferred: Serum gel

Acceptable: Red top

Submission Container/Tube: Plastic vial

Specimen Volume: 1 mL

Collection Instructions: Centrifuge and aliquot serum into plastic vial within 2 hours of collection

Additional Information:

1. Blood draw and ultrasound must be completed between 10 weeks, 0 days and 13 weeks, 6 days of gestation, which corresponds to a crown-rump length (CRL) range of 31 to 80 mm.

2. Initial or repeat testing is determined in the laboratory at the time of report and will be reported accordingly. To be considered a repeat test for the patient, the testing must be within the same pregnancy and trimester, with interpretable results for the same test and both tests are performed at Mayo Clinic.

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


Specimen Minimum Volume
Defines the amount of sample necessary to provide a clinically relevant result as determined by the Testing Laboratory

0.75 mL

Reject Due To
Identifies specimen types and conditions that may cause the specimen to be rejected

Gross hemolysis Reject
Gross lipemia OK
Gross icterus OK

Specimen Stability Information
Provides a description of the temperatures required to transport a specimen to the performing laboratory, alternate acceptable temperatures are also included

Specimen Type Temperature Time Special Container
Serum Refrigerated (preferred) 7 days
Frozen 90 days
Ambient 7 days

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

Prenatal screening for trisomy 21 (Down syndrome) and trisomy 18

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

Multiple marker serum screening has become a standard tool used in obstetric care to identify pregnancies that may have an increased risk for certain birth defects such as Down syndrome (trisomy 21) and trisomy 18 (Edward syndrome). Since early 2000s, first-trimester screening has been established as an alternative option of equal or better performance when compared to second-trimester screening programs.


The first-trimester screen is performed by measuring analytes human chorionic gonadotropin (hCG) and pregnancy-associated plasma protein A (PAPP-A) in maternal serum that are produced by the fetus and the placenta. Additionally, the nuchal translucency (NT) measurement, a sonographic marker shown to be effective in screening fetuses for Down syndrome and trisomy 18, is included in the risk calculation. A mathematical model is used to calculate a risk estimate by combining serum concentrations to hCG and PAPP-A, NT measurement, and maternal demographic information. The laboratory establishes a specific cutoff for each condition, which classifies each screen as either screen-positive or screen-negative. A screen-positive result indicates that the value obtained exceeds the established cutoff. A positive screen does not provide a diagnosis but indicates that further evaluation should be considered.



hCG is synthesized by placental cells starting very early in pregnancy and serves to maintain the corpus luteum and, hence, progesterone production during the first trimester. Thereafter, the concentration of hCG begins to fall as the placenta begins to produce steroid hormones and the role of the corpus luteum in maintaining pregnancy diminishes. Increased total hCG levels are associated with an increased risk for Down syndrome. Low levels of hCG are associated with an increased risk for trisomy 18.



PAPP-A is a 187 kDa protein comprised of 4 subunits: 2 PAPP-A subunits and 2 pro-major basic protein 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 14th week of gestation are associated with an increased risk for Down syndrome and trisomy 18.



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 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.

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.


Calculated screen risks <1/230 are reported as screen negative.

Risks > or =1/230 are reported as screen positive.



Calculated screen risks <1/100 are reported as screen negative.

Risks > or =1/100 are reported as screen positive. A numeric risk for trisomy 18 risk is provided with positive results on non-diabetic, non-twin pregnancies.

An interpretive report will be provided.

Provides information to assist in interpretation of the test results


A screen-negative result indicates that the calculated risk is below the established cutoff of 1/230 for Down syndrome and 1/100 for trisomy 18. A negative screen does not guarantee the absence of trisomy 18 or Down syndrome. Screen-negative results typically do not warrant further evaluation.



When a Down syndrome risk cutoff of 1/230 is used for follow-up, the first trimester maternal screen has an overall detection rate of approximately 85% with a false-positive rate of 5%. In practice, both the detection rate and false-positive rate increase with age, thus detection and positive rates will vary depending on the age distribution of the screening population.

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

Upon receiving maternal serum screening results, all information used in the risk calculation should be reviewed for accuracy (eg, maternal date of birth, demographics, sonographic information). If any information is incorrect, the laboratory should be contacted for a recalculation of the estimated risks.


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 fetus with Down syndrome.


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


In rare cases, some individuals can develop antibodies to mouse or other animal antibodies (often referred to as human anti-mouse antibodies [HAMA] or heterophile antibodies), which may cause interference in some immunoassays. Caution should be used in interpretation of results and the laboratory should be alerted if the result does not correlate with the clinical presentation.


Variables Affecting Marker Levels:

-All serum marker multiple of medians are adjusted for maternal weight (to account for dilution effects). The estimated risk calculations and screen results are dependent on accurate information for gestation, maternal age, and weight. Inaccurate information can lead to significant alterations in the estimated risk.

-In twin pregnancies, the risk for Down syndrome is calculated using twin-adjusted medians. Risks for triplets and higher multiples cannot be calculated.

-Nuchal translucency (NT) measurements must be obtained from a trained and certified sonographer. NT quality indicators are monitored by the performing laboratory on a regular basis. Institutions will be contacted if there is ongoing deviation in the quality indicators.

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. American College of Obstetricians and Gynecologists: Practice Bulletin No. 163: Screening for Fetal Aneuploidy. Obstet Gyneco,l 2016 May;127(5):e123-137

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

4. Yarbrough ML, Stout M, Gronowski AM: Pregnancy and its disorders. In: Rifai N, Horvath AR, Wittwer CT, eds. Tietz Textbook of Clinical Chemistry and Molecular Diagnostics. 6th ed. Elsevier; 2018:1655-1696

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

Method Description
Describes how the test is performed and provides a method-specific reference

The first-trimester screen for Down syndrome and trisomy 18 includes pregnancy-associated plasma protein A (PAPP-A), total human chorionic gonadotropin (hCG), and nuchal translucency measurement.


The Beckman Access HCG5 assay is an automated 2-site, mouse monoclonal antibody-based immunoenzymatic sandwich assay with paramagnetic separation and chemiluminescent detection. Testing is performed using the Beckman Coulter DxI.(Package insert: Access HCG5. Beckman Coulter, Inc; 2018)


The Access PAPP-A assay is a 2-site immunoenzymatic sandwich assay. Testing is performed using the Beckman Coulter DxI.(Unpublished Mayo method)

PDF Report
Indicates whether the report includes an additional document with charts, images or other enriched information


Day(s) Performed
Outlines the days the test is performed. This field reflects the day that the sample must be in the testing laboratory to begin the testing process and includes any specimen preparation and processing time before the test is performed. Some tests are listed as continuously performed, which means that assays are performed multiple times during the day.

Monday through Friday

Report Available
The interval of time (receipt of sample at Mayo Clinic Laboratories to results available) taking into account standard setup days and weekends. The first day is the time that it typically takes for a result to be available. The last day is the time it might take, accounting for any necessary repeated testing.

4 to 6 days

Specimen Retention Time
Outlines the length of time after testing that a specimen is kept in the laboratory before it is discarded

9 months

Performing Laboratory Location
Indicates the location of the laboratory that performs the test


Several factors determine the fee charged to perform a test. Contact your U.S. or International Regional Manager for information about establishing a fee schedule or to learn more about resources to optimize test selection.

  • Authorized users can sign in to Test Prices for detailed fee information.
  • Clients without access to Test Prices can contact Customer Service 24 hours a day, seven days a week.
  • Prospective clients should contact their Regional Manager. For assistance, contact Customer Service.

Test Classification
Provides information regarding the medical device classification for laboratory test kits and reagents. Tests may be classified as cleared or approved by the US Food and Drug Administration (FDA) and used per manufacturer instructions, or as products that do not undergo full FDA review and approval, and are then labeled as an Analyte Specific Reagent (ASR) product.

This test was developed, and its performance characteristics determined by Mayo Clinic in a manner consistent with CLIA requirements. This test has not been cleared or approved by the US Food and Drug Administration.

CPT Code Information
Provides guidance in determining the appropriate Current Procedural Terminology (CPT) code(s) information for each test or profile. The listed CPT codes reflect Mayo Clinic Laboratories interpretation of CPT coding requirements. It is the responsibility of each laboratory to determine correct CPT codes to use for billing.

CPT codes are provided by the performing laboratory.


LOINC® Information
Provides guidance in determining the Logical Observation Identifiers Names and Codes (LOINC) values for the order and results codes of this test. LOINC values are provided by the performing laboratory.

Test Id Test Order Name Order LOINC Value
1STT1 First Trimester Maternal Screen 49086-2
Result Id Test Result Name Result LOINC Value
Applies only to results expressed in units of measure originally reported by the performing laboratory. These values do not apply to results that are converted to other units of measure.
DT3 Scan Date 34970-4
CRL1 CRL 11957-8
NT_ NT 49035-9
NUMF Number of Fetuses 55281-0
CRL2 CRL Twin 11957-8
NT_B NT Twin 49035-9
CHOR Number of Chorions 92568-5
IDD_ Insulin dependent diabetes 44877-9
B_RCE Patient Race 21484-1
IVF IVF 47224-1
26428 Recalculated Maternal Serum Screen 43995-0
26411 Specimen Collection Date 33882-2
26412 Maternal Date of Birth 21112-8
26429 Calculated Age at EDD 43993-5
26413 Maternal Weight 29463-7
26880 Maternal Weight 29463-7
26430 GA on Collection by U/S Scan 11888-5
26426 PAPP-A 48407-1
26427 THCG 32166-1
26434 Down Syndrome Screen Risk Estimate 43995-0
26435 Down Syndrome Maternal Age Risk 49090-4
26436 Trisomy 18 Screen Risk Estimate 43994-3
26437 Interpretation 49588-7
26438 Additional Comments 48767-8
26439 Recommended Follow Up 80615-8
601798 Results Summary 50679-0
601515 PAPP-A MoM 76348-2
601516 THCG MoM 32166-1
601517 NT MoM 49035-9
601518 NT Twin MoM 49035-9
SMKN1 Current cigarette smoking status 64234-8
PRHX Prev Down (T21) / Trisomy Pregnancy 53826-4
INTL1 Initial or repeat testing 86955-2
SONON Sonographer Name 49088-8
SONOC Sonographer Code No LOINC Needed
SONOD Sonographer Reviewer ID 49089-6
DRPH1 Physician Phone Number 68340-9

Test Setup Resources

Setup Files
Test setup information contains test file definition details to support order and result interfacing between Mayo Clinic Laboratories and your Laboratory Information System.

Excel | Pdf

Sample Reports
Normal and Abnormal sample reports are provided as references for report appearance.

Normal Reports | Abnormal Reports

SI Sample Reports
International System (SI) of Unit reports are provided for a limited number of tests. These reports are intended for international account use and are only available through MayoLINK accounts that have been defined to receive them.

SI Normal Reports | SI Abnormal Reports

Test Update Resources

Change Type Effective Date
Test Changes - Specimen Information 2023-02-14
Test Changes - Specimen Information 2021-12-14