Research Day

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Introduction: The fern test was first described by Kardos and Tamasi in 1955 as a test used to determine rupture of amniotic membranes. The test is done by collection of fluid from the vagina which is then allowed to dry for 10 minutes on a slide. The slide is then inspected with microscopy for evidence of ferning. Accurate determination of rupture of membranes is important due to its significant implications on the management of obstetrical patients. Case: EC is a 23 year old G1P0 at 40 and 3/7 weeks gestation who presented to an outpatient facility with a chief complaint of vaginal fluid leaking. Approximately 12 hours prior she had leaked sufficient vaginal fluid to wet her underwear, but not sufficient to soak through her pants. She denied loss of vaginal fluid since then. A speculum exam revealed a small amount of pooling in the vaginal vault. A sample of this fluid was obtained. Fern test was negative and vaginal pH was 4.5. Her history was concerning for premature rupture of membranes (PROM) so she was further evaluated with an Amnisure test which was positive. She was subsequently admitted with PROM for induction of labor. Discussion: The fern test has been noted to have a sensitivity as high as 100%, with most studies reporting a sensitivity at least 90% or greater. These statistics can be very misleading as most of these studies were done in patients who were in labor. The sensitivity of the fern test is much lower in patients who are not in labor. A more recent study reported a sensitivity of 51% and a specificity of 70% for detecting PROM in patients who were not in labor. It was noted that sensitivity was slightly higher and specificity slightly lower for medical students and residents when compared to gynecologists. The sensitivity of the Amnisure test has been reported to be 98% to 99% with specificity ranging from 88% to 100%. Rupture of membranes has previously been diagnosed either by visualization of fluid leaking from the cervical os or having 2 of the following: vaginal pooling, positive fern test, or positive nitrazine test. Conclusion: The fern test is only one of several methods that can be used to assess rupture of membranes. Base on this recent study, ferning should not be relied on to rule out rupture of membranes in a nonlaboring patient with clinical history suspicious for PROM.



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