Using Serum Screening in Saskatchewan

A tool to adjust the risk for Aneuploidy and Neural Tube Defects
Dr George Carson
Director Maternal-Fetal Medicine 

Regina-Qu'Appelle Health Region Screening for aneuploidy

A crude screening tool has been available in the past for estimating the risk of genetic abnormality in any pregnancy. It has been almost entirely dependent on the age of the mother, and has generally been used to recommend that all pregnant women over a specific age should be offered the option of amniocentesis.

The Serum Screening Test has been introduced as a refinement to this screening as a means to advise women of the possible risk that they have a pregnancy complicated by Trisomy-21, Trisomy-18 or an open neural tube defect. It is not a definitive diagnostic test, but rather a tool to aid the pregnant patient in deciding whether or not to undergo amniocentesis, with its associated risks. Therefore, if the patient has another sufficient indication and would have amniocentesis in any case, or would not act on amniocentesis results whatever they are, then serum screening is irrelevant and unnecessary.

The test depends upon the fact that certain substances made by the fetus and appearing in the blood of the mother occur in characteristically different median amounts in pregnancies in which the patient carries a fetus with Trisomy 21 or 18, or open neural tube defect than in unaffected pregnancies. The substances currently used in this testing are:

1. Alpha-feto protein

2. Beta HCG

3. Estriol

According to the amounts of these substances present, compared to those at the same gestational age in a pregnancy in which the fetus has none of these abnormalities, the risk that one of these abnormalities exists may be calculated.

Incorporating Ultrasound Data in the Risk Calculation

In total patient management one should consider all of the relevant information. In the issue of the risk of an anomaly the information includes the background risk, the results of biochemical testing, and the findings on ultrasound. There are a series of structural changes or "markers" which may be seen in normal fetuses, but occur more commonly in those with aneuploidy. Thus the presence of markers increases, and their absence decreases the chance that the particular fetus that has been imaged has a trisomy.

Uptake of serum screening in Saskatchewan

The table below shows the use of Serum Screening in Saskatchewan since the introduction of the test.

Steps for the Proper Utilization of Serum Screening.

1. Provide prenatal counselling and obtain informed consent;

• What information can the test provide, and what can it not do?
• What would the woman do if "increased risk" were found?
• What would the woman do if amniocentesis were done and aneuploidy found?

2. Date the pregnancy reliably (as in all pregnancies)

• Normal results vary markedly even week by week by gestational age, so the interpretation of the meaning of the amount of substances measured is critically dependent upon the gestational age.
• Options available for management depend upon the gestational age at which a definitive diagnosis was made if the testing proceeds to this diagnosis.

3. Do the testing at the early end of the gestational age range.

• The range is from 15 - 20 weeks.
• Testing done just after 15 weeks gestation provides the answer by 16 weeks and allows amniocentesis to be done, in indicated and chosen, and the results to be available at 19 weeks

4. If an "increased risk" result is obtained;

• ACT PROMPTLY, and do not wait until the next prenatal visit (unless it is tomorrow). The pre-test counselling should have covered all potential results and actions.
• Confirm the dates if previously established by menstrual dating only
• If dating has already been done by an otherwise indicated ultrasound (e.g for uncertain dates or nuchal translucency testing, proceed using that ultrasound confirmed date.

5. If, after known accurate dating, the result remains "increased risk" refer for/provide additional counselling and decision making about amniocentesis.

6. Use all data available to define the magnitude of risk.

• Background data, which includes maternal age and previous aneuploidy or neural tube defect.
• The serum screen result.
• "Markers" or Lack of "markers" for aneuploidy.

7. If after comprehensive risk assessment informed consent is given, proceed to amniocentesis.