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Sequential Screen

PAPP-A

Test Codes

First Trimester: EPIC: LAB5782, SOFT: SEQ1, Second Trimester/Final: EPIC: LAB5783, SOFT: SEQ2

Department

Special Chemistry

Instructions

• The sequential screen requires nuchal translucency (NT) measured by an ultrasonographer approved by Clinical Pathology.
• A completed prenatal screening requisition form must accompany all samples.
• If an NT measurement cannot be obtained the test will be converted to a serum integrated screen.

Specimen Collection Criteria

Collect: This test requires:

• One Gold-top SST tube at 11-13 weeks gestational age. (Minimum Whole Blood: 4.0 mL) 
• One Gold-top SST tube at 15-22 weeks gestational age. (Minimum Whole Blood: 4.0 mL)

If 1^st trimester risk for Down Syndrome increased, a 2^nd trimester blood draw will NOT be obtained.

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. Specimens for uE₃ (15-22 weeks gestational age) should not be frozen as this may have a detrimental effect on the assay. (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. Specimens for uE₃ (15-22 weeks gestational age) should not be frozen as this may have a detrimental effect on the assay. (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

First Trimester Testing (PAPP-A) performed: Monday – Friday. 
Second Trimester Testing (AFP, hCG, uE3, Inhibin A) performed: Monday – Friday.
Results available when all patient test results and history have been completed.

Reference Range

The screen includes quantitation of PAPP-A (pregnancy-associated plasma protein A) in the 1^st trimester and AFP, hCG, unconjugated estriol (uE₃), and inhibin A in the 2^nd trimester. The result of each test is reported as an MoM (multiple of the median) that is adjusted for the weight, race when appropriate and gestational age. An adjustment for insulin-dependent diabetes is made when appropriate. The risk cut off for Down's syndrome is 1:270. The risk of an open neural tube defect is determined by comparing to the absolute cut-off for AFP of 2.2 MoM.

Test Methodology

Chemiluminescent Immunoassay.

Interpretation

Based on the nuchal thickness (NT) measurement, pregnancy-associated plasma protein A (PAPP-A) and patient age, the sequential screen is designed to identify women who are at a very high risk for carrying a Down syndrome fetus in the 1^st trimester. A cut-off of 1 in 30 will be used to indicate whether the risk is increased.

If the Down syndrome risk is greater than 1 in 30, a final result that includes the calculated risk will be reported. Because of the early gestational age, a risk for open neural tube defect (ONTD) cannot be reported.

If the Down syndrome risk is less than 1 in 30, a report will be issued indicating that the laboratory has performed PAPP-A testing and that we are now waiting for the 2^nd trimester sample. No Down's syndrome risk will be included in the report.

Once a 2^nd trimester sample is received, alpha fetoprotein, hCG, unconjugated estriol and inhibin A will be performed. A final report that includes risks for both Down syndrome and ONTD will be issued; this is analogous to the full integrated screen report. A final risk cut-off (2^nd trimester) of 1 in 110 will be used for the sequential screen. The cut-off of 1 in 110 is also used for the integrated screen.

Using the risk cut-offs noted above (1 in 30 for 1^st trimester and 1 in 110 for 2^nd trimester), it is estimated that the false positive rate for Down's syndrome using the sequential screen will be slightly greater than the current integrated screen.

NOTE:

• If an NT cannot be performed, the test will be converted to a serum integrated screen.
• hCG will not be performed on the 1^st trimester sample - studies have shown that it does not add significantly to sequential screen performance.
• Risks for Trisomy 18 (if greater than 1 in 100) and for Smith Lemli Opitz syndrome (if greater than 1 in 50) will be reported as they occur.
• We will continue only to accept NT measurements from Beaumont Fetal Imaging or from ultrasonographers who are approved by Clinical Pathology.

Clinical Utility

The sequential screen is a screen used to determine the risk of a woman having a Down's syndrome fetus. It is not a diagnostic test. It is estimated that approximately two-thirds of Down's syndrome cases will be identified in the 1^st trimester and an additional one-third in the 2^nd trimester. The overall detection rate is estimated to be approximately 90%. The AFP level alone is used to calculate the risk of the fetus having an open neural tube defect (ONTD). Although screening for ONTD can be performed between 15 and 22 weeks gestational age, the optimal time is between 16 and 18 weeks gestational age.

Reference

1. Malone, F.D. et al. First-Trimester or Second-Trimester Screening, or both, for Down's syndrome. New England J. Med (2005). 353: 2001-2011.
2. Palomaki G et al. Comparing three screening strategies for combining first and second trimester Down syndrome, syndrome markers. Obstet Gynecol 2006;107:367-75.

CPT Codes

84163 (PAPP-A), 82105 (AFP), 82677 (uE₃), 84702 (hCG), 86336 (Inhibin A).
LOINC:  PAPP-A 48407-1

Contacts

Last Updated

1/13/2023