Validation of diameter measurements by ultrasound: intraobserver and interobserver variations assessed in vitro and in fetal sheep

Objectives Blood flow evaluation in the fetus is hampered by inaccuracy in the measurement of vessel diameter. The impact of biological variation on r eproducibility studies is unknown. The present study evaluated diameter mea surements carried out With modern ultrasound equipment by assessing inter- and intraobserver variations. Design Repeated measurements analyzed by a general factorial model of analy sis of variance. Methods Three observers measured repeatedly (six or more rimes for each tub e and session) silicone tube implants (0.6, 2.2 and 6 mm), applying ultraso und imaging in eight fetal sheep, and the same silicone tubes in vitro. Ana lysis of variance was carried out on 199 observations made in vitro and 537 in vivo. Results The upper 95% confidence limit for the intra- and interobserver var iations was higher for measurements made in vivo than in vitro. It runs hig hest for the largest diameter and varied between 0.10 and 0.38 mm. When the diameter was determined as an average of repeated measurements, the error was reduced: six measurements in vivo had upper 95% confidence limits for i ntraobserver variation of 0.04 and 0.09 min for diameters of CI. 6 and 6 mm , respectively Conclusions The results show that ultrasound diameter measurements have a h igh reproducibility even for vessels of small dimensions when repeated meas urements are taken with high-frequency ultrasound under favorable condition s. The low variation described in the present investigation of silicone tub es, compared to previous studies, suggests that diameter changes of living vessels represent a separate source of measurement variation.