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Test Code FERN Fern Test; Amniotic Fluid Crystallization Test

Additional Codes


Powerchart: Fern Test

Epic: LAB469 Fern Test, Vaginal Fluid

Test Method

The “Fern” test is based upon the ability of amniotic fluid to form a microscopic crystalline pattern suggestive of fern leaves when the fluid specimen is allowed to air dry on a glass slide; the phenomenon is due to the interaction of high concentrations of electrolytes and protein in amniotic fluid relative to other fluids that may be present in the posterior vagina.

Specimen Requirement

Vaginal swab

Additional Specimen Collection Information

  1. A vaginal swab obtained from the posterior vaginal pool. As when handling any bodily fluid, Universal Precautions must be observed.
  2. Position the patient in the dorsal lithotomy position.
  3. Avoiding the use of lubricant or antiseptic, place a sterile speculum into the vaginal vault.
  4. Using a sterile cotton swab, obtain a sample of the vaginal secretion from the posterior vaginal pool.
  5. Smear a thin layer of the fluid obtained on the center portion of a clean glass microscope slide.
  6. Label the slide using a printed label. Indicate on the label the date/time and initials of person collecting the sample.


Testing is performed Monday through Sunday.
Routine: Typically completed within 4 hours after the specimen arrives at the testing site.
STAT: Typically completed within 1 hour after the specimen arrives at the testing site.

Reference Values

Not applicable

CPT Code Information

87210 Smear, primary source with interpretation; wet mount for infectious agents (eg, saline, India ink, KOH preps)

Useful For

The procedure allows for the qualitative assessment/detection of premature amniotic fluid leakage; i.e., PROM (Premature Rupture of Membranes).

LOINC Code Information

54452-8 Ferning [Presence] in Cervical mucus