Mathematical models to correlate amniotic fluid index and amniotic fluid volume.

PURPOSE: To describe a predictable relationship that relates amniotic fluid index (AFI) to amniotic fluid volume (AFV) and improve the accuracy of AFI to detect true oligohydramnios. METHODS: Data from 42 parturients (group I) who underwent measurements of amniotic fluid sonographically (amniotic fluid index) as well as by dye-dilution technique was used to relate AFI to AFV. Subsequently, 22 consecutive women (group II) were used to test the accuracy of the equation to predict true oligohydramnios. RESULTS: In group II, 11 of 22 patients had true oligohydramnios and the sensitivity, specificity, positive and negative predictive values of AFI < or = 5.0 to detect a confirmed AFV < 500 mL were 0%, 91%, 0%, and 48%, respectively. These values of AFI, when used in conjunction with the equation, improved to 73%, 55%, 62%, 67%, respectively. With AFI and the equation, significantly more patients in group II with true oligohydramnios (8 of 11) could be detected than with using AFI alone (0 of 11; p = 0.002). CONCLUSION: AFI is poor predictor of true oligohydramnios. Using the mathematical model, the detection rate of oligohydramnios is significantly improved.