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

Quality control of ultrasonography markers for Down's syndrome screening: a retrospective study by the laboratory

OBJECTIVES

Quality control of ultrasonography markers is necessary to ensure greater efficacy of prenatal aneuploidy screening. The aim of this study was to assess the quality of the crown-rump length (CRL) and nuchal translucence (NT) measurement accuracy by the laboratory according to quality indicators.

METHODS

Retrospective observational study on 4,908 single-foetus pregnant women who underwent prenatal aneuploidy screening in the first trimester of pregnancy. Euploid foetuses with CRL between 45 and 84 mm were included, while those with NT≥3.5 mm were excluded. CRL measurement was considered to be accurate if the median multiples of the median (MoM) for pregnancy-associated plasma protein A (PAPP-A) was between 0.90 and 1.10. Fifteen sonographers participated in the study, six of whom comprised the control group. Systematic error for a sonographer was considered when CRL measurement was greater than ±2 mm with respect to the control group. Quality for NT was assured by means of the WHIRI method and each sonographer cumulative sum control chart (CUSUM).

RESULTS

For CRL accuracy, five sonographers underestimated the measurements, while another four overestimated them, with no statistical differences. For smaller sized foetuses, all sonographers met the established specifications. Regarding NT control, three sonographers did not meet the quality criteria for the median MoM. All sonographers met the specifications for the logarithmic standard deviation of the NT MoM levels. Thirteen sonographers met the CUSUM specifications.

CONCLUSIONS

Evaluation of a quality control of ultrasonography parameters by laboratory professionals is necessary to avoid under- or overestimation tendencies for CRL and NT measurements. CUSUM is a useful tool for the immediate correction of errors in NT measurements.

[The left outflow tract: Normal features]

Congenital heart disease remains an important cause of perinatal morbidity and mortality. Screening for these is based on a good knowledge of normal fetal heart anatomy and ultrasound views to be performed. After recommending the use of the four chambers and the right outflow tract views in 2005, CNEOF proposes recently adding the left outflow tract assessment. The use of this one should sensitize the operator to the notion of sweeping essential to obtain all these views and make it possible to improve the detection of the conal VSD and conotruncal pathologies.

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.

Repeat measurements of nuchal translucency at 11-14 weeks of gestation: when do we need them?

OBJECTIVE

Previous studies have recommended that several repeat measurements of nuchal translucency (NT) be obtained to optimize sensitivity of the screening process. However, truncation is applied within the risk calculation for small NT measurements; therefore, repeating NT measurements in the lower range may be unnecessary. The aim of this study was to determine the optimal number of NT measurements and whether this is dependent on the initial value obtained.

METHODS

We simulated the expected distribution of sets of five repeat NT measurements and their corresponding likelihood ratios (LR) for a variety of crown-rump length (CRL) values at 11-14 weeks' gestation, based on the published mixture model for first-trimester NT measurements and on published data regarding NT measurement variability. The ratio between LR obtained from the highest and the initial measurement in each set were computed, with a ratio of 1 indicating that repeat measurements would have no effect on risk estimation. We calculated NT cut-offs below which a change in estimated LR would occur in fewer than 10% of cases if repeat measurements were obtained, and we tested this approach on a real first-trimester screening dataset.

RESULTS

The simulations performed indicated that repeating NT measurements when the first NT obtained ranged between ≤ 0.9 and ≤ 1.6 mm at CRLs of 45 and 84 mm, respectively, is not useful. When applied to a real dataset, our approach allowed avoiding repetition of measurements in 47/165 cases (28%).

CONCLUSIONS

Although there is some variability in NT measurements, repeat assessment is not useful to optimize screening performance for the smallest NT measurements due to lower truncation limits that are applied in risk calculation. Our study provides NT cut-off values, in relation to CRL, below which it appears that there is no need to repeat measurements once a good quality image has been obtained.