Ongoing sonographer training: regular quality checks needed for first-trimester NT screenings

OB/GYN News, May 15, 2004 by Miriam E. Tucker

NEW YORK -- Ongoing sonographer training and quality control programs are essential for achieving accurate and consistent nuchal translucency measurements in the first trimester, Dr. Fergal D. Malone said at an obstetrics symposium sponsored by Columbia University and New York Presbyterian Hospital.

"Nuchal translucency is a great advance in aneuploid detection, but it requires significant resources for quality control and training. It's not good enough to just go to a weekend course," said Dr. Malone, director of obstetric and gynecologic ultrasound and director of perinatal research at Columbia Presbyterian Medical Center.

Even well-trained sonographers need to have their measurements reviewed periodically--preferably by computer--as techniques tend to drift over time, he noted.

Currently, there is very little coordination of this approach in the United States. The Society for Maternal-Fetal Medicine and other organizations are working on a quality control program for first-trimester nuchal translucency (NT) screening. Guidelines could be available by this summer, he said.

Indeed, data have shown wildly disparate results from center to center. In a prospective study of 3,991 pregnancies at high risk for Down syndrome at 16 centers, the success of first-trimester NT measurement in predicting outcome varied from 61% to 100%, while the trisomy 21 detection rate varied from 0% at one center to 100% at another (N. Engl. J. Med. 338[14]:955-61, 1998).

The sonographers in that study had received no special training, and the centers did not have quality control measures in place, Dr. Malone noted.

Establishment of gestational age-based cutoffs will be necessary, since NT in creases by about 17% per week during the first trimester (Prenat. Diagn. 16[7]:629-34, 1996). This can be done using the 95th percentile or multiples of the appropriate gestational median (MoMs).

Another controversial matter yet to be decided is whether a given generic NT median is valid across different populations, different centers, or even different sonographers within a single center. In the 15-center Scottish NT trial involving 17,229 pregnant women attending routine antenatal clinics, the median for NT was 1.0 MoM, but the individual center medians ranged from 0.7 to 1.4 MoM.

This variation would translate to a difference in Down syndrome risk of 1 in 1,400 to 1 in 285. "That's a significantly different risk calculation for the same patient," Dr. Malone said.

Use of center-specific medians could reduce the amount of variance by about 15% (Prenat. Diagn. 23[5]:389-92, 2003).

In the meantime, he offered some important pointers for obtaining an accurate NT measurement:

* Focus on the midsagittal plane, not the oblique plane.

* Make sure the fetus's neck is in a neutral position. If it's flexed, the NT measurement will be underestimated by up to 0.4 mm, while an extended fetal neck will overestimate the value by as much as 0.6 mm.

* Zoom the image in as far as possible. If the fetus fills up less than 75% of the frame, the image will be too small for adequate evaluation.

* Be sure you're visualizing the separate amnion and not mistaking it for skin. The latter will result in overestimation of the NT value. When it's hard to tell, wait until the fetus moves. If the line moves with the fetus, it's skin. If it stays still, it's amniotic membrane.

* Place the calipers on the inner, not outer, borders of the NT space.

BY MIRIAM E. TUCKER

Senior Writer