Ovenstone Factor in the management of Rh sensitization

Several methods have been developed for analyzing amniotic fluid to aid in the management of rhesus isoimmunization. Because all methods attempt to predict the severity of fetal hemolysis based on the original findings of Bevis and on the amount of unconjugated bilirubin in amniotic fluid as measured by its optical density at 450 millimicron, they share two possible sources of error: calculation of exact gestational age and accurately reading bloody or contaminated amniotic fluid. The Ovenstone Factor measures the derivative of the bilirubin curve against wavelength and thereby avoids some of the contaminating pigments at 450 millimicron. This paper compares the predictive accuracy of two methods, Liley versus Ovenstone Factor, in 78 samples of fluid from 46 patients. Predictions were correct in 80% of infants using Ovenstone Factor and 67% using the Liley curve, and incorrect in only 7% and 13%, respectively. The Ovenstone Factor, with its simplicity and high predictive accuracy, is an excellent additional tool in the management of Rh-sensitized pregnancies.