Amnioreduction: How much to drain?

Objectives: To assess the relationship between the volume of amniotic fluid removed and the change in amniotic fluid index (AFI) and calculate an equa tion describing this association. Materials and Methods: A retrospective an alysis of 19 amnioreduction procedures performed in our unit. Multiple regr ession analysis was used to assess the effect of gestational age and pre-pr ocedure AFI on the change in AFI (Delta AFI) after adjusting for the volume removed. Results: As expected, a significant ;linear relationship was foun d between the change in AFI and the volume removed (r = 0.82, n = 19, p < 0 .0001). Delta AFI was not dependent on the gestational age or the pre-proce dure AFI. The equation describing the association between the volume remove d and Delta AFI was: volume = (Delta AFI - 2.26)/0.008, which is close to 7 cm Delta AFI for every 100 mi removed. Discussion: Using the described equ ation, it is possible to predict the required volume to be removed in order to achieve a particular Delta AFI, which may reduce the need to interrupt the procedure to measure the AFI. However, the limitation of AFI as a semiq uantitative assessment of the liquor volume, together with its inter- and i ntra-observer variations mean this equation should be used only as a guide.