Amniotic fluid optical density (OD) analysis is the most common way to measure bilirubin present in amniotic fluid. Bilirubin is a pigment acquired by the amniotic fluid during its circulation through the gastrointestinal tract and may be found in amniotic fluid as early as the 12th week of gestation. It reaches its highest concentration between 16 and 30 weeks. As the pregnancy continues, the amount of bilirubin progressively decreases until it finally disappears near term. Bilirubin levels increase in the presence of erythroblastotic fetuses and fetuses with anencephaly or intestinal obstruction.

This measurement is used to monitor the fetal state in an Rho-negative pregnant women who has a rising anti-Rho antibody titer. The rising titer is synonymous with Rh erythroblastosis fetalis or hemolytic disease of the newborn. This determination usually is not made before 20–24 weeks’ gestation because no therapy is available to the fetus before that time. Close to term, the amniotic fluid bilirubin pigment concentration normally decreases in the absence of Rh sensitization.

Determination of OD is only one of several laboratory methods used to measure bilirubin. The degree of hemolytic disease falls into three zones (zones 1, 2, and 3) using OD measurement and a wavelength (absorbance) of 450 nm (the Liley or Diazo method). Difficulty in interpretation occurs frequently. Findings must be interpreted by a knowledgeable person who can recognize pitfalls. Pertinent clinical information and other laboratory data also may be taken into consideration. After 30 weeks’ gestation, the Liley test result is usually combined with an assessment of FLM (L/S ratio) to assist in the decision of whether to induce delivery.

1. Collect 5–10 mL of amniotic fluid in a lightproof container. Label specimen with the patient’s name, date, and test(s) ordered.

2. Send the fluid to the laboratory immediately.

3. Be aware that the specimen may be refrigerated for up to 24 hours. It can be frozen if a longer time will elapse before analysis.

4. Avoid blood in the specimen. If initial aspiration produces a bloody fluid, the needle should be repositioned to obtain a specimen free of red cells. If a blood-free specimen cannot be obtained, the specimen must be examined at once before hemolysis occurs.

5. Indicate weeks of gestation on the laboratory request form.

1. If OD = 0.28–0.46 (zone 1, low zone, 2+ OD) at 28–31 weeks, the fetus will not be affected or will have very mild hemolytic disease.

2. If OD = 0.47–0.90 (zone 2, middle zone, 3+ OD), there is a moderate effect on the fetus. The fetal age and the trend in OD indicate the need for intrauterine transfusion and early delivery.

3. If OD = 0.91–1.0 (zone 3, high zone, 4+ OD), the fetus is severely affected, and fetal death is a possibility. In this case, a decision concerning delivery or intrauterine transfusion, depending on the age of the fetus, should be made. After 32–33 weeks of gestation, early delivery and extrauterine treatment are preferred.

4. An OD <0.04 indicates fetal maturity and well-being.

5. Increased OD is found in:

   1. Erythroblastosis fetalis

   2. Other fetal hemolytic diseases

   3. Maternal infectious hepatitis

   4. Maternal sickle cell crisis

Pretest Patient Care

1. Explain the test purpose and the meaning of test results.

2. Follow guidelines in Chapter 1 for safe, effective, informed pretest care.

1. Blood, hemoglobin, or meconium in the specimen can produce inaccurate results.

2. Maternal use of steroids interferes with the test.

3. Exposure of the amniotic fluid to light compromises the test.

4. Fetal acidosis interferes with the test.

Normal

• Low: 0.28–0.46 OD at 28–31 weeks

• <0.04 OD toward end of pregnancy indicates fetal maturity with low risk of abnormal red blood cell breakdown.