Audit on nuchal translucency thickness measurements in Flanders, Belgium: a plea for methodological standardization

Contingent non-invasive prenatal testing: an opportunity to improve non-genetic aspects of fetal aneuploidy screening

BACKGROUND

Several countries today struggle with suboptimal performances in many aspects of the fetal aneuploidy screening process and consider introducing non-invasive prenatal screening (NIPT) as a solution. In this study, costs and benefits of different scenarios for contingent NIPT screening in Belgium are evaluated with respect to partial redistribution of the national screening budget into quality improving measures for those screening activities that will be maintained when full NIPT screening is implemented.

METHODS

Data from the Belgian National Institute for Health and Disability Insurance and the Study Centre for Perinatal Epidemiology were used in modeled calculations of medical and economic impact of NIPT after prior conventional screening (1) at thresholds 1:300 and 1:600, and (2) at current and improved screening sensitivity.

RESULTS

Contingent NIPT screening under current screening conditions would maintain today's 7.9(0)/000 live birth prevalence of Down syndrome (LBPD) at an 11% reduction of overall short-term costs. Lowering the screening threshold to 1:600 or increasing sensitivity by 10% would reduce LBPD to 7(0)/000 at a maximum 3% increase of overall short-term costs.

CONCLUSION

Today, in Belgium and in many other countries, full NIPT screening is considered too expensive for immediate introduction into the national fetal aneuploidy screening program. Contingent NIPT screening is both clinically and economically beneficial. A temporary contingent NIPT protocol allows for reinvesting economic savings into optimization of those screening aspects, which are to be maintained in parallel to full NIPT screening.

Nuchal Translucency Quality Review (NTQR) program: first one and half million results

OBJECTIVE

To evaluate the performance of first-trimester nuchal translucency (NT) measurement by providers (physician-sonologists and sonographers) within the Nuchal Translucency Quality Review (NTQR) program.

METHODS

After training and credentialing providers, the NTQR monitored performance of NT measurement by the extent to which an individual's median multiple of the normal median (MoM) for crown-rump length (CRL) was within the range 0.9-1.1 MoM of a published normal median curve. The SD of log10 MoM and regression slope of NT on CRL were also evaluated. We report the distribution between providers of these performance indicators and evaluate potential sources of variation.

RESULTS

Among the first 1.5 million scans in the NTQR program, performed between 2005 and 2011, there were 1 485 944 with CRL in the range 41-84 mm, from 4710 providers at 2150 ultrasound units. Among the 3463 providers with at least 30 scans in total, the median of the providers' median NT-MoMs was 0.913. Only 1901 (55%) had a median NT-MoM within the expected range; there were 89 above 1.1 MoM, 1046 at 0.8-0.9 MoM, 344 at 0.7-0.8 MoM and 83 below 0.7 MoM. There was a small increase in the median NT-MoM according to providers' length of time in the NTQR program and number of scans entered annually. On average, physician-sonologists had a higher median NT-MoM than did sonographers, as did those already credentialed before joining the program. The median provider SD was 0.093 and the median slope was 13.5%. SD correlated negatively with the median NT-MoM (r = -0.34) and positively with the slope (r = 0.22).

CONCLUSION

Even with extensive training, credentialing and monitoring, there remains considerable variability between NT providers. There was a general tendency towards under-measurement of NT compared with expected values, although more experienced providers had performance closer to that expected.

Quality of first-trimester measurement of crown-rump length

OBJECTIVE

The purpose of this study was to evaluate the quality of crown-rump length (CRL) measurements in scans judged to be of high quality for the measurement of the nuchal translucency.

STUDY DESIGN

We analyzed prospective data on 68,250 scans by 1913 sonographers that were collected by the French College of Fetal Echography (CFEF) national practice assessment program for the first-trimester scan. The scans were evaluated according to the CFEF image scoring method (CFEF-ISM), which includes items to measure the quality of the scan for both nuchal translucency and the CRL measurements. The scans were classified into 4 quality groups with the use of the full CFEF-ISM score and then a shortened version of the CFEF-ISM that excluded the item on quality of CRL measurement. The proportion of scans with an inaccurate CRL measurement was compared across the different quality groups.

RESULTS

Overall, 21.67% of scans were of insufficient quality for CRL measurement. Among 23,764 "excellent" scans according to the full CFEF-ISM, 965 scans (4.06%) had insufficient CRL quality vs 9.24% of scans with "excellent" quality on the short CFEF-ISM (relative risk, 2.27; 95% confidence interval, 2.11-2.44; P < .001).

CONCLUSION

High scores of the quality of nuchal translucency measurement do not guarantee accurate measurement of crown-rump length. Specific measures are needed to evaluate and to improve the quality of the measurement of crown-lump length.

Likelihood ratio-based quality control for nuchal translucency measurements at 11-14 weeks of gestation

OBJECTIVE

To develop a quantitative quality control process of nuchal translucency (NT) measurement at 11-14 weeks of gestation based on the mixture model, characterized by analysis of the corresponding distribution of the likelihood ratio (LR).

METHODS

Based on the published mixture model for NT measurement in the first trimester, we simulated the expected distribution of NT and the corresponding LR. This approach was then tested for the quality control of 15 048 NT measurements performed by four operators trained and certified by The Fetal Medicine Foundation. The new quality control approach based on LR was compared with existing processes based on NT plots, multiples of the median (MoMs) and analysis of the 95(th) centile.

RESULTS

Each operator contributed 2176-4730 examinations. Median, 5(th) and 95(th) centile of NT values ranged from 1.4 to 1.6, 1.0 to 1.0 and 2.2 to 2.6 mm, respectively. Median of NT-MoM values ranged from 0.83 to 0.95. Analysis of the distribution of NT measurements confirmed departure from the assumptions of the delta-NT and NT-MoM models. Analysis of LR distributions demonstrated a significant difference between observed and expected distributions for all operators (P < 10(-4) ).

CONCLUSION

An LR-based quality control process is feasible at 11-14 weeks of gestation. Because it is more sensitive to measurement bias around the critical area of the 95(th) centile, its use should be encouraged.

Nuchal Translucency Measurements for First-Trimester Screening: The ‘Price’ of Inaccuracy

OBJECTIVE

First-trimester aneuploidy screening is in transition from the phase of 'development' to that of 'diffusion.' As with all other technologies, there is risk of diminished performance until newer centers are well experienced. Typically, inexperienced sonographers under-measure nuchal translucency (NT), and experience, training and continual monitoring of data are needed to ensure that measurements do not deviate. Here we assess the impact of systematic under-measurement on abnormality detection.

STUDY DESIGN

Actual NT measurements from 13,887 normal fetuses, 82 with trisomy 21 (T21) and 61 with other abnormalities (OA) with birth outcome data were mathematically modified to show either a 25% or 0.5-mm decrease in measurement. The impact upon sensitivity and screen-positive rates were assessed.

RESULTS

Reducing the NT measurements of T21 and OA cases lowers the sensitivity from 81.7 and 70.5%, respectively, to 67.1 and 62.3% (p < 0.01). If normals are correspondingly lowered, then the screen-positive rates are reduced from 6.9 to 2.0 and 2.5%. To maintain the same screen-positive rates and sensitivity, the risk threshold would have to be increased from 1/300 to 1/556.

CONCLUSION

Minor inaccuracies in NT measurements as small as 25% or 0.5 mm will have very significant negative impacts upon abnormality detection, reducing detection rates by 18% (81.7 to 67.1%). Just as it is completely accepted that laboratory measurements require standardization and quality assurance, NT measurements, because they are used in an algorithm, need to be treated with the same rigor. That way the published data from centers that have developed such screening can be applied by other operators at other sites when counseling their patients.

Sequential triage in the first trimester may enhance advanced ultrasound scanning in population screening for trisomy 21

OBJECTIVE

To design a trisomy 21 screening protocol for sequential triage in the first trimester, and to evaluate whether it reduces the need for advanced ultrasound scanning to such an extent that this could be dealt with by a limited number of well-trained sonographers only.

METHODS

Screening results of 31 trisomy 21 affected pregnancies and 16 096 unaffected pregnancies from the first trimester screening program of Algemeen Medisch Laboratorium in Antwerp, Belgium, were used to define high-risk, intermediate-risk and low-risk groups. A serum screening result (age, pregnancy-associated plasma protein-A (PAPP-A) and free beta-human chorionic gonadotropin (beta-hCG)) of >or=1 : 30 and/or a nuchal translucency thickness (NT) measurement of >or= 3.5 mm were classified as high risk. A serum screening result of < 1 : 1000 together with an NT of < 3.5 mm were classified as low risk. Other results were considered intermediate risk, for which further advanced ultrasound screening would be indicated. This protocol was then evaluated prospectively in another population of 13 493 first-trimester pregnancies.

RESULTS

Of the total population, 1.9% was identified as being high risk (14 trisomy 21 pregnancies and 222 unaffected pregnancies; prevalence, 1 : 17), 59.6% was identified as being low risk (three trisomy 21 pregnancies and 9615 unaffected pregnancies; prevalence, 1 : 3206) and 38.4% was identified as being intermediate risk (10 trisomy 21 pregnancies and 6190 unaffected pregnancies; prevalence, 1 : 620). A similar distribution was found in the prospective arm of the study. There was no reduction of overall screening performance compared with our current first-trimester combined screening program. The number of intermediate-risk pregnancies was sufficiently low as to enable advanced ultrasound scanning by well-trained sonographers only.

CONCLUSION

In population screening for fetal trisomy 21, sequential triage in the first trimester can be achieved using very simple methods. Pregnancies at high or at low risk can be identified easily and the number of pregnancies at intermediate risk can be reduced sufficiently to enable advanced ultrasound scanning by well-trained sonographers only. A prospective study is needed to evaluate the performance of this approach and to compare its results with current combined or integrated screening algorithms.

Population screening for fetal trisomy 21: easy access to screening should be balanced against a uniform ultrasound protocol

OBJECTIVES

To evaluate the performance of a first-trimester fetal aneuploidy screening program, with a documented underestimation of nuchal translucency thickness measurements (NT) compared to the Fetal Medicine Foundation (FMF) reference range.

METHODS

We analysed the data of Algemeen Medisch Laboratorium (AML) in Antwerp, Belgium, on combined screening with pregnancy-associated plasma protein-A (PAPP-A), free beta-human chorionic gonadotropin (FB-hCG) and NT. NT-multiples of the median (MoM), relative to the FMF reference range, were used for risk calculations.

RESULTS

The proportion of first-trimester screening tests in the total of serum screening tests increased from 1.3% (125/9424) in 2000 to 53.1% (6577/12 377) in 2003. Only 11.4% (1514/13 267) of NT measurements were performed according to FMF criteria. The 80.8% (21/26) trisomy 21 (T21) detection rate (DR) at cut off 1:300 resulted from maternal serum screening. NT measurements did not add to this DR, but reduced the false-positive rate from 16.8% (2212/13181) to 8.6% (1130/13181). Only 23.8% (5/21) of T21 detections were by FMF trainees.

CONCLUSION

Easy access to screening and maternal serum parameters accounted for the majority of T21 detections in our first-trimester combined screening program.