Amniotic Fluid Cytology

What is Amniotic Fluid Cytology?

Amniotic Fluid Cytology is a quick diagnostic test used to determine whether the amniotic sac (bag of waters) has ruptured prematurely. The test examines a smear of fluid taken from the external cervical os for the presence of “ferning,” a characteristic microscopic pattern.

Synonyms

• Premature Rupture of Bag of Waters (BOW)
• Premature Rupture of Fetal Membranes (PROM)

Commonly Included Test

A simple smear from the external os in patients suspected of having ruptured their amniotic sac prematurely.

Specimen & Collection

• Specimen: Amniotic fluid
• Sampling Time: Within 15 minutes
• Collection Procedure:
  • Collect fluid from the external os using a simple spatula or wooden tongue depressor
  • Spread onto a plain or frosted glass slide
  • Fix immediately with 95% ethyl alcohol

Causes for Rejection

• Unlabeled slide

Use of the Test

This test is used to confirm the leakage of amniotic fluid from the cervical os in suspected premature rupture of membranes (PROM).

Reference Range

• Normal (no rupture): Smooth, even microscopic pattern
• Positive for rupture: Branching “fern-like” patterns due to high sodium and estrogen concentration in amniotic fluid

Contraindications

• Significant vaginal bleeding — test should be avoided

Methodology

An alcohol-fixed smear is stained using the Papanicolaou (Pap) stain and examined under a microscope. The appearance of fern-like crystalline structures confirms the presence of amniotic fluid, as a result of its high sodium and estrogen content.

Additional Diagnostic Information

Distinguishing between amniotic fluid and maternal urine may be necessary in certain cases:

• Amniotic fluid: Higher protein, urea, and creatinine levels comparable to serum
• Maternal urine: Higher concentrations of urea and creatinine than amniotic fluid
• Note: These values may vary depending on the stage of pregnancy

Turnaround Time

Results are typically available the same day, often within minutes.

References

• Cunningham FG, MacDonald PC, Leveno KJ, et al. Williams Obstetrics, 19th ed. Norwalk, CT: Appleton and Lange, 1993, p.373.
• Jacobs, Demott, Finley, Horvat, Kasten.JR, Tilzer. Laboratory Test Handbook, Lexi-Comp Inc, 1994.