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 nonlin ear ultrasound transducer could be used to reliably measure the amniot ic fluid index (AFI) compared with a linear transducer. Methods: Two h undred pregnant women between 22 and 41 weeks gestational age had blin ded AFI measurements with a linear and a nonlinear transducer. The mea surements were compared by correlation coefficient, calculation of sen sitivity and specificity, McNemar's test of significance of change, an d coefficient of agreement. Results: The mean value of AFI for each tr ansducer was nearly identical. The mean difference between paired meas urements was 2.6 cm or 24.8%. There was no trend in the direction of d ifference between transducer measurements. When looking for significan tly low AFI measurements (less than or equal to 5 cm), the sensitivity of the nonlinear transducer was 85% and the specificity 93%. Conclusi ons: The difference in AFI measurements shown in this study comparing the nonlinear and the linear transducer are similar to intraobserver d ifferences reported by other authors. The clinical variation inherent in the method of measuring AFI limits precise reproducibility. The non linear transducer seems to offer an acceptable alternative to the line ar transducer for measurement of the AFI.