Amniotic fluid is formed in the membranous sac that surrounds the fetus. The total volume of fluid at term is 500 to 2,500 mL. In amniocentesis, fluid is obtained by ultrasound-guided needle aspiration from the amniotic sac. This procedure is generally performed between 14 and 16 weeks’ gestation for accurate interpretation of test results, but it also can be done between 26 and 35 weeks’ gestation if fetal distress is suspected. Amniotic fluid is tested to identify genetic and neural tube defects, hemolytic diseases of the newborn, fetal infection, fetal renal malfunction, or maturity of the fetal lungs (see monograph titled “Lecithin/Sphingomyelin Ratio”). Several rapid tests are also used to differentiate amniotic fluid from other body fluids in a vaginal specimen when premature rupture of membranes (PROM) is suspected. A vaginal swab obtained from the posterior vaginal pool can be used to perform a rapid, waived procedure to aid in the assessment of PROM. Nitrazine paper impregnated with an indicator dye will produce a color change indicative of vaginal pH. Normal vaginal pH is acidic (4.5 to 6.0) and the color of the paper will not change. Amniotic fluid has an alkaline pH (7.1 to 7.3) and the paper will turn a blue color. False-positive results occur in the presence of semen, blood, alkaline urine, vaginal infection, or if the patient is receiving antibiotics. The amniotic fluid crystallization or Fern test is based on the observation of a fern pattern when amniotic fluid is placed on a glass slide and allowed to air dry. The fern pattern is due to the protein and sodium chloride content of the amniotic fluid. False-positive results occur in the presence of blood urine or cervical mucus. Both of these tests can produce false-negative results if only a small amount of fluid is leaked. The reliability of results is also significantly diminished with the passage of time (greater than 24 hr). AmniSure is an immunoassay that can be performed on a vaginal swab sample. It is a rapid test that detects placental alpha microglobulin-1 protein (PAMG-1), which is found in high concentrations in amniotic fluid. AmniSure does not demonstrate the high frequency of false-positive and false-negative results inherent with the pH and fern tests.
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