Quality assessment of routine nuchal translucency measurements: a North American laboratory perspective

Abstract

Purpose: To assess nuchal translucency measurements that were performed as part of routine prenatal screening for Down syndrome. Methods: Collect ultrasound measurements of nuchal translucency and crown rump length provided by individual sonographers over a 6-month period to six North American prenatal screening laboratories, along with the laboratory's nuchal translucency interpretation in multiples of the median. For sonographers with 50 or more observations, compute three nuchal translucency quality measures (medians, standard deviations, and slopes), based on epidemiological monitoring. Results: Altogether, 23,462 nuchal translucency measurements were submitted by 850 sonographers. Among the 140 sonographers (16%) who submitted more than 50 observations, 76 (54%) were found to have all three quality measures in the target range. These 140 sonographers collectively accounted for 14,210 nuchal translucency measurements (61%). The most common single measure to be out of range was nuchal translucency multiples of the median, found for 29 of the 140 sonographers (21%). Nuchal translucency (NT) is defined as the collection of fluid behind the fetal neck occurring in the first trimester of pregnancy. 1-3 When measured and interpreted correctly, it is the most discriminatory marker for Down syndrome that has been reported in a routine setting. Univariately, NT measurements can identify about 60% of Down syndrome pregnancies in the first trimester at a 5% false-positive rate. 4 Detection increases to between 80% and 85%, when NT is combined with first trimester biochemical measurements of pregnancyassociated plasma protein-A (PAPP-A) and human chorionic gonadotropin (the intact, total, or free-␤ subunit forms). 4 When NT measurements are incorporated into the integrated test (NT and PAPP-A measurements in the first trimester) and the quadruple test (␣-fetoprotein, unconjugated estriol, human chorionic gonadotropin and dimeric inhibin-A in the second trimester), 85-90% detection is achievable at a false-positive rate of 2% or less. 5,6 Acquiring the skill to properly measure NT requires specialized training and oversight. 7 Recognizing this, several national and international programs have been established to train and qualify sonographers, and also to assess their ability to consistently and accurately quantify the NT thickness. 8 -11 The hands-on performance training is usually evaluated through still images submitted to one, or a panel of, expert sonographers. Several research trials using NT measurements have shown that such training, although necessary, is not sufficient to assure reproducibility of absolute measurements among sonographers. For example, both the Serum, Urine and Ultrasound Screening Study (SURUSS) 5 and First and Second Trimester Evaluation of Risk (FASTER) study 12 found that use of sonographer-specific reference data (medians) resulted in improved screening performance.In the current study, information was sought from screening laboratories regarding how they deal with NT measurements

Technical standards and guidelines: Prenatal screening for Down syndrome that includes first-trimester biochemistry and/or ultrasound measurements

“…41 Target ranges for the three parameters are (1) 15% to 35%, (2) 0.90 to 1.10 MoM, and (3) standard deviations of 0.08 to 0.13. 65 Neither the percent per week increase nor logarithmic standard deviation are under the direct control of the screening laboratory. However, the laboratory can adjust the median values when the median MoM for a given sonographer is outside of the expected range.…”

Richtlinie der Gendiagnostik-Kommission (GEKO) für die Anforderungen an die Durchführung der vorgeburtlichen Risikoabklärung sowie an die insoweit erforderlichen Maßnahmen zur Qualitätssicherung gemäß 23 Abs. 2 Nr. 5 GenDG

“…Die vorgeburtliche Risikoabklärung insgesamt sowie jede einzelne ihrer Komponenten (Ultraschall, laboratoriumsmedizinische Untersuchungen, Algorithmus) müssen einer kontinuierlichen Qualitätssicherung unterzogen werden. Durch eine kontinuierliche Qualitätssicherung kann das Problem der Unterschätzung der Nackentransparenz deutlich verringert und so die Entdeckungsrate erhöht werden [17,18,19]. Eine externe Qualitätssicherung ist insbesondere für den Bereich Ultraschall wesentlich, weil es sich bei der fetalen Nackentransparenz um eine Messung im Bereich um 0,1 mm handelt und deren Genauigkeit direkt von der Qualifikation des Anwenders abhängt [17].…”

Assessment of sonographer nuchal translucency measurement performance – central tendency and dispersion

“…The UKNSP estimated that a deviation of 10% on a serum markers MoM was sufficient to change the screen positive rates by approximately 2% [8]. Assessment of NT scan quality at a single laboratory, however, indicated that only 55% of the 140 sonographers who had performed 50 or more scans within a 6 month period had NT measurement distributions whose median were within 10% of the expected values [9]. The recent study by Kagan et al demonstrated the effects of departures from the expected NT distribution on DRs and FPRs [10].…”