A prospective, blinded comparison of the linear versus nonlinear transducer for amniotic fluid index measurements

Objective: The purpose of this study was to determine whether a nonlinear ultrasound transducer could be used to reliably measure the amniotic fluid index (AFI) compared with a linear transducer. Methods: Two hundred pregnant women between 22 and 41 weeks gestational age had blinded AFI measurements with a linear and a nonlinear transducer. The measurements were compared by correlation coefficient, calculation of sensitivity and specificity, McNemar's test of significance of change, and coefficient of agreement. Results: The mean value of AFI for each transducer was nearly identical. The mean difference between paired measurements was 2.6 cm or 24.8%. There was no trend in the direction of difference between transducer measurements. When looking for significantly low AFI measurements (≤5 cm), the sensitivity of the nonlinear transducer was 85% and the specificity 93%. Conclusions: The difference in AFI measurements shown in this study comparing the nonlinear and the linear transducer are similar to intraobserver differences reported by other authors. The clinical variation inherent in the method of measuring AFI limits precise reproducibility. The nonlinear transducer seems to offer an acceptable alternative to the linear transducer for measurement of the AFI.