Is It Time To Change Our Reference Curve For Femur Length ... - PLOS

Are they effective at identifying measurements <5th or >95th percentile?

From the scatter diagram of the 5th, 50th, and 95th percentile curves calculated from each superimposed reference chart (Fig 2), we were able to obtain a rough direct impression. The overall results for the classification of the fetuses using the 5th and 95th percentiles from each of the three reference curves for each parameter are shown in Tables 5 and 6 (see Tables 5 and 6).

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Fig 2. Scatter diagrams with the 5th, 50th, and 95th percentile curves calculated from and superimposed on each reference chart of FL.

The 5th (bottom line), 50th (middle line), and 95th (top line) centile curves for fetal femur length using each of the reference equations of Leung TN,et al (a), Chitty LS, et al (b),Papageorghiou, AT, et al (c), superimposed on the scatter diagram of FL and GA.

https://doi.org/10.1371/journal.pone.0159733.g002

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Table 5. Results of screening for fetuses with measurements < 5th percentile.

https://doi.org/10.1371/journal.pone.0159733.t005

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Table 6. Results of screening for fetuses with measurements > 95th percentile.

https://doi.org/10.1371/journal.pone.0159733.t006

When using the reference equations from Leung TN, Pang MW et al, the observed Z-scores for the 5th percentile and 95th percentile were -0.8983 and 2.6627. A total of 710 measurements that were actually less than the 5th percentile were missed diagnoses, and 3,479 measurements were wrongly classified as larger than the 95th percentile. The sensitivity of screening fetuses with measurements < 5th percentile was only 23.1%, although the specificity was 100%, and the Youden index was 0.231. Thus, the value was too low to be used as a diagnostic test. The sensitivity and specificity of screening for fetuses with measurements >95th percentile were 100% and 80.2%, respectively, and the Youden index was 0.802.

The observed Z-scores for the 5th percentile and 95th percentile when using the reference equations from Chitty LS, Altman DG et al were -1.530 and 1.2746. A total of 280 measurements that were actually less than the 5th percentile and 586 measurements that were actually larger than the 95th percentile were missed diagnoses. The sensitivity of screening fetuses with measurements < 5th percentile and > 95th percentile was 72.1% and 39.8%, respectively, and the specificity was 100% for both percentiles. Thus, the Youden index was 0.721 for identifying measurements that were <5th percentile and 0.398 for identifying measurements that were >95th percentile.

Finally, when using the reference equations from Papageorghiou AT et al (2014), the observed Z-score for the 5th and 95th percentile were -1.6622 and 2.9296. Only 25 measurements were wrongly classified as less than the 5th percentile, and 3,436 measurements were wrongly classified as greater than the 95th percentile. The sensitivity of screening fetuses with measurements < 5th percentile and > 95th percentile were both 100%, and the specificity was 99.9% and 81.5%, respectively. Thus, the Youden index was 0.999 for identifying measurements that were <5th percentile and 0.815 for identifying measurements that were >95th percentile.