ULTRASONOGRAPHIC ASSESSMENT OF AMNIOTIC-FLUID DOES NOT REFLECT ACTUALAMNIOTIC-FLUID VOLUME

OBJECTIVE: Our objective was to compare the ability of two methods of amniotic fluid assessment (two-diameter amniotic fluid pocket versus t he amniotic fluid index) to predict oligohydramnios (actual amniotic f luid volume <500 ml) or polyhydramnios (actual amniotic fluid volume > 1500 ml). STUDY DESIGN: The amniotic fluid index and the two-diameter amniotic fluid pocket were assessed before amniocentesis and determina tion of amniotic fluid volume with the dye (aminohippurate sodium) - d ilution technique. To assess the detection of either oligohydramnios o r polyhydramnios, the areas under the receiver-operator characteristic curves (+/-SE) were estimated by the point-to-point trapezoidal metho d of integration. Prediction limits were calculated by regression anal ysis of amniotic fluid index or two-diameter amniotic fluid pocket ver sus actual amniotic fluid volume and determination of 95th percentile ranges for amniotic fluid volume. RESULTS: We studied 144 patients wit h a mean (+/-SD) gestational age of 31.7 +/- 5.5 weeks; mean (+/-SD) a mniotic fluid index and two-diameter amniotic fluid pocket were 12.6 /- 6.1 cm and 21.2 +/- 18.4 cm(2), respectively. Mean (+/-SD) actual a mniotic fluid volume was 722 +/- 735 ml (range 101 to 4318 ml). The ar eas under the four receiver-operator characteristic curves were not si gnificantly different from the nondiagnostic line (p < 0.05). Regressi on slopes (r values) for amniotic fluid index and two-diameter amnioti c fluid pocket versus actual amniotic fluid volume were 0.34 and 0.23, respectively. Calculation of the prediction limit for 95% confidence that oligohydramnios is absent requires that the amniotic fluid index be 30 cm and the two-dimension amniotic fluid pocket be 90 cm(2), both -thresholds of which are currently considered to represent clinical po lyhydramnios. CONCLUSIONS: Both amniotic fluid index and two-dimension amniotic fluid pocket appear to be inaccurate predictors of actual ol igohydramnios or polyhydramnios when compared with dye-dilution calcul ations of actual amniotic fluid volume.