Maternal Serum Screen, 1st and 2nd trimester (Blood)

Diagnostic Use

This test estimates the chance that a pregnancy will be affected by conditions including T21 (Down syndrome), T18 (Edward syndrome), T13 (Patau syndrome), Turner syndrome and triploidy. This allows for more informed decision making re: whether referral to a maternal foetal medicine service for more definitive testing may be useful.

Note that this is not a diagnostic test. This screening test helps to categorise pregnancies as either increased chance (1:5 – 1:300 chance) or low chance (1:301 -1:100,000 chance). For example, in the theoretical scenario where 100 pregnancies each had a 1:100 chance of T21, only one would be expected to actually be affected by T21 and the remaining 99 would be expected to be unaffected. Further, a fraction of true cases are missed by the screening test.

1. First Trimester Screening:
   • Collection Timing: Blood must be collected when the patient is between 9 weeks and 13 weeks 6 days gestation. Nuchal translucency is to be performed within the same time frame however is best taken between 12 weeks and 13 weeks 6 days.
   • Components: Measurement of PAPP-A (pregnancy-associated plasma protein-A) and free B-hCG (beta-human chorionic gonadotropin).
   • Additional Component: Nuchal translucency measurement.
   • Purpose: Assess the risk of Down syndrome and other chromosomal conditions.
   • Availability: Funded by the NSU to New Zealand residents or patient with an appropriate visa. Non- residents are not funded, however is available at a cost to the patient.
2. Second Trimester Screen:
   • Collection Timing: Blood must be collected when the patient is between 14 weeks and 20 weeks 6 days gestation.
   • Components: Measurement of AFP (alpha-fetoprotein), free B-hCG, unconjugated estriol (UE3), and Inhibin A.
   • Additional Component: Nuchal translucency measurement, if provided.
   • Purpose: Assess the risk of Down syndrome and other chromosomal conditions.
   • Availability: Funded by the NSU to New Zealand residents or patient with an appropriate visa. Non- residents are not funded, however is available at a cost to the patient.
3. Serum Integrated Screening:
   • Components: Measurement of PAPP-A in the first trimester, followed by measurements of AFP, unconjugated estriol, free B-hCG, and Inhibin A in the second trimester.
   • Integration: Results from both trimesters are combined to produce a comprehensive risk assessment.
   • Purpose: Assess the risk of Down syndrome and other chromosomal conditions when a nuchal translucency measurement could not be obtained or provided.
   • Availability: Funded by the NSU to New Zealand residents or patient with an appropriate visa. Non- residents are not funded, however is available at a cost to the patient.
4. Fully Integrated Screening:
   • Components: Measurement of PAPP-A in the first trimester, followed by measurements of AFP, unconjugated estriol, free B-hCG, and Inhibin A in the second trimester.
   • Additional Component: Nuchal translucency measurement.
   • Integration: Results from both trimesters are combined to produce a comprehensive risk assessment.
   • Purpose: Assess the risk of Down syndrome and other chromosomal conditions.
   • Availability: Not funded by NSU, however is available at a cost of $190 + GST to the patient
5. Further Note: These costs do not include the radiographers fee where an ultrasound scan has been performed

Interpretation

It is essential to note that these screening tests provide a risk assessment and are not diagnostic . Further diagnostic tests, such as amniocentesis or chorionic villus sampling (CVS), may be recommended for a definitive diagnosis if an increased chance result is obtained from screening.

Reference Intervals

Results are given as a chance of the conditions screened for.

The screening threshold is 1:300 for aneuploidies, for example;

– Low chance would be 1:301 -1:100,000.

– Increased chance would be 1:5 – 1:300.

An accurate estimated date of delivery (EDD) is critical to this risk estimate.