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First Trimester Down's Syndrome Screen

First Trimester Screen for Down's Syndrome, First Trimester Screen for Down Syndrome, First Trimester Down's Syndrome Screen, PAPP-A

Test Codes

Antrim #31425, EPIC: LAB5775, SOFT: DOWN1

Department

Special Chemistry

Instructions

• The specimen is to be drawn at 11-13 weeks gestational age and should be accompanied by a completed prenatal screening.
• An NT measurement between 11-13 weeks is required for performance of this screen.

Specimen Collection Criteria

Collect: One Gold-top SST tube. (Minimum Whole Blood: 4.0 mL)

Physician Office/Draw Specimen Preparation

Let specimen clot 30-60 minutes then immediately centrifuge to separate serum from cells. Refrigerate (2-8°C or 36-46°F) the centrifuged collection tube within two hours of collection. (Minimum Serum: 1.0 mL)

Preparation for Courier Transport

Transport: Centrifuged collection tube, refrigerated (2-8°C or 36-46°F). (Minimum Serum: 1.0 mL)

Rejection Criteria

• Plasma specimens.
• Severely hemolyzed, lipemic or icteric specimens. 
• Specimens not collected and processed as indicated. 

In-Lab Processing

Let specimen clot 30-60 minutes then immediately centrifuge to separate serum from cells. Room temperature is acceptable for a maximum of two hours. (Minimum Serum: 1.0 mL)

Storage

Specimen Stability for Testing:

Centrifuged SST Tubes and Microtainers^® with Separator Gels
Room Temperature (20-26°C or 68-78.8°F): 2 hours
Refrigerated (2-8°C or 36-46°F): 7 days
Frozen (-20°C/-4°F or below): Unacceptable

Red-top Tubes and Microtainers^® without Separator Gels
Room Temperature (20-26°C or 68-78.8°F): 2 hours
Refrigerated (2-8°C or 36-46°F): 24 hours
Frozen (-20°C/-4°F or below): Unacceptable

Serum Specimens (Pour-Overs)
Room Temperature (20-26°C or 68-78.8°F): 2 hours
Refrigerated (2-8°C or 36-46°F): 7 days
Frozen (-20°C/-4°F or below): Unacceptable

Specimen Storage in Department Prior to Disposal:

Refrigerated (2-8°C or 36-46°F): 7 days

Laboratory

Royal Oak Special Testing Laboratory

Performed

Monday – Friday.
Results available within 2 business days.

Reference Range

The screen includes quantitation of pregnancy associated plasma protein A (PAPP-A) and hCG. The result of each test is reported as an MoM (multiple of the median) that is adjusted for the patient weight and is calculated for the appropriate gestational age.

Results of the assays are combined with the NT measurement to provide an overall risk of fetal Down's syndrome. The cut-off used for the First Trimester Screen is 1:230.

Test Methodology

Chemiluminescent Immunoassay.

Interpretation

Down's Syndrome
In comparison to a normal pregnancy, fetal Down's syndrome results in lower PAPP-A and higher hCG and NT results. Because the calculated MoM for each test takes into account the patient's gestational age, it is essential that gestational age information is based on an ultrasound measurement. Because this is a screen for Down's syndrome, a number of positive results will be determined by further investigation to be false positives. Use of the First Trimester provides a detection rate similar to the integrated screens (85%) but with a greater false positive rate (5%).

Donated Eggs
If a pregnancy has resulted from use of a donated ovum, the age of the egg donor is used to calculate Down syndrome and Trisomy 18 risk assessments. It is therefore essential that the egg donor's date of birth or age is given to the lab with other patient information.

Open Neural Tube Defect (ONTD) Screening
ONTD screening cannot be performed by the First Trimester Screen.

Clinical Utility

The First Trimester Screen is a screen used to determine the risk of a woman having a Down's syndrome fetus. It is not a diagnostic test. The risk of Trisomy 18 is also calculated from patient age and test results; the risk is reported as increased if it is greater than 1:100.

CPT Codes

LOINC:  PAPP-A 48407-1

Contacts

Last Updated

1/11/2023