Amniotic Fluid Optical Density
Analysis of amniotic fluid for bilirubin is helpful in evaluating the severity of hemolytic disease of the newborn. Amniotic fluid specimens are usually collected after 28 weeks gestation. An absorbance curve, from 560 nm to 350 nm, is performed on amniotic fluid using a recording spectrophotometer.
A normal curve shows gradually increasing absorbance with decreasing wavelengths and is essentially a straight line between 560 and 350 nm. An abnormal curve shows a hump at 450 nm due to the absorbance of bilirubin. The net absorbance, or delta A450, is calculated by measuring the difference between the patient’s curve and a theoretical baseline drawn between 560 and 350 nm. The delta A450 is then plotted against the gestational age on a Liley graph.More recently, a Queenan curve has replaced the Liley curve, because it is based on a correlation of delta A450 with actual cord blood hematocrits to assess risk (Am J Obstet Gynecol 1993; 168:1370-6).
The specimen must be immediately protected from light to prevent degradation of bilirubin. Contamination of amniotic fluid with maternal blood increases the A450. Fetal blood contamination of amniotic fluid causes an elevation of both hemoglobin and bilirubin. Meconium stained amniotic fluid results in a positive interference. Oligohydramnios and polyhydramnios may produce false positive and false negative values, respectively. Contamination of amniotic fluid with maternal urine will produce erroneous results.
Specimen requirement is a minimum of 3 mL of amniotic fluid.
