Superiority of the four-quadrant sum over the single-deepest-pocket technique in ultrasonographic identification of abnormal amniotic fluid volumes

Abstract

Ultrasonographic estimation of amniotic fluid volume is an important adjunct to assessment of fetal well-being. However, no single method has emerged as most predictive. Many ultrasonographers use subjective or single-pocket guidelines in evaluation of amniotic fluid volume. We therefore assessed the ability of a maximum-vertical-pocket technique to identify patients with abnormal volumes of amniotic fluid. A total of 1168 patients was studied. Oligohydramnios (n = 76) was defined as an amniotic fluid index <5th percentile for gestational age (70 to 98 mm). Hydramnios (n = 51) was defined when the amniotic fluid index was >95th percentile (185 to 249 mm). The ability of the maximum-vertical-pocket technique to identify cases with oligohydramnios by amniotic fluid index was poor: sensitivity was 42% and positive predictive value was 51%. Fifty-eight percent of cases with oligohydramnios by amniotic fluid index had "normal" values according to the single-pocket technique. The correlation coefficient between amniotic fluid index and maximum vertical pocket was 0.51, R2 = 24%. The correlation between the maximum vertical pocket and amniotic fluid index was somewhat better in cases of polyhydramnios (r = 0.79, R2 = 62%), but the sensitivity, positive predictive value, and false-negative rates were high as in oligohydramnios. These data suggest that the maximum-vertical-pocket technique has adequate specificity but is a poor screening tool. The results demonstrate that use of gestation-specific norms for the amniotic fluid index identifies abnormal amniotic fluid volumes more efficiently than the single-pocket technique.