Amniotic Fluid Optical Density

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 A_450, 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.

This test has been largely replaced by ultrasound and Doppler studies to assess blood flow in the middle cerebral artery of the fetus. Elevated peak systolic velocity is a strong indicator of fetal anemia. 

References

Scott F, Chan FY. Assessment of the clinical usefulness of the 'Queenan' chart versus the 'Liley' chart in predicting severity of rhesus iso-immunization. Prenat Diagn. 1998 Nov;18(11):1143-8. PMID: 9854722.

Spinnato JA et al. Hemolytic disease of the fetus: a comparison of the Queenan and extended Liley methods. Ob Gyn, 1998:92(3);441-445.