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Grand Rounds / Articles

The following presentation is from the POGO 2003 Conference held in Saskatoon. Participants rated this session highly.

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.

U/S Finding

Criteria

Likelihood Ratio

Structural Defect

cardiac defect, cystic hygroma with or without hydrops, duodenal atresia

25

Nuchal Thickening

>5mm AP

18.5*

Echogenic Bowel

subjectively increased, grades 2 or 3

5.5*

Short Humerus

observed / predicted _0.9

2.5

Short Femur

observed / predicted _0.91

2.3*

Echogenic intracardiac focus

present or absent

2

Renal Pyelectasis

>3mm AP

1.6*

Normal Ultrasound

none of the above

0.4

Uptake of serum screening in Saskatchewan

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

Tests performed May 2001 to October 2002
Urban Rural
Saskatoon/North 742 284
Regina/South 1,513 861

Steps for the Proper Utilization of Serum Screening.

1. Provide prenatal counselling and obtain informed consent;

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

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

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

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.

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

Diagram: Going Down the Serum Screen Road