New choices in prenatal screening for Down syndrome
A FASTER trial investigator discusses first-trimester combined screening, integrated screening, and findings from the trial
The terms screen-negative and screen-positive are applied to test results to convey the most accurate balance between detection and false-positive rates. However, these terms indicate only risk categories; within these categories, the specific risk assigned is of the greatest value in counseling the patient. For example, a woman with a screen-positive result may have a risk of 1 in 200 or a risk of 1 in 10.
These patient-specific risks are extremely accurate. Thus, a woman with the lower risk (ie, 1 in 200) may choose to forego invasive testing, whereas a woman with the much higher risk (1 in 10) may want definitive diagnosis.
Similarly, women with screen-negative results can have very different risks—as low as 1 in 50,000 or less and as high as being almost screen-positive. Again, it is helpful to counsel each woman using the specific risk calculated for her.
Clinical guidelines
Clinicians have an obligation to help pregnant women choose the best and safest options in diagnosis and treatment.It is not enough to simply offer a menu of therapies or tests and let the patient choose. A clinician would never let the patient decide which medications are safest and most effective, and the same should hold true for screening tests.
The following guidelines may help the clinician and patient make the most informed decision:
- If nuchal translucency ultrasound is available and the gravida presents by 11 to 13 gestational weeks, the integrated test is the safest, most effective screening method to assess Down syndrome risk.
- If a woman wants the earliest prenatal diagnosis, first-trimester combined screening, using nuchal translucency and serum markers, is appropriate.
- If nuchal translucency is unavailable, the serumonly version of the integrated test is the best screening method.
- If a woman presents after 13 weeks’ gestation, the second-trimester quad marker test is best.
Each hybrid test makes sense in theory. However, no one knows yet whether they will work as anticipated once they are implemented clinically, and the appropriate risk cutoffs have not yet been determined. It also is unclear whether women whose risks fall on the edge of the various groupings would be interested in waiting for the integrated test to be completed.
These tests remain investigational.
Dr. Canick is a consultant to and receives grant/research support from Diagnostic Systems. He holds patents on use of estriol in prenatal screening (nos. 5506150 and 5605843).
