Amniotic Fluid Ferns at ANY gestational age
Throughout my career heard so many reasons why the fern slide didn’t fern in the apparently ruptured patient.
“She’s only 19 weeks, they don’t fern this early.”
“There’s blood in the sample, that makes it not fern.”
“She has chorio, that makes it not fern.”
Over the years, this has driven me a bit crazy, because the real reason that the slide is not ferned is not any of these reasons. The reason there is no fern on the slide is because the fluid on the slide is not amniotic fluid.
Clearly there are some reasonable questions to be asked about what will cause or prevent ferning in a vaginal pool sample. Does gestational age affect it? Does blood affect it? How long does one need to wait?
Asking these questions is great. What is not great is making crap up about the answers when these questions have been clearly answered in the literature.
“Amniotic fluid was obtained from 38 patients between 16-38 weeks’ gestational age who underwent amniocentesis. Amniotic fluid was immediately mixed with freshly obtained, heparinized fetal cord blood in varying concentrations (blood to amniotic fluid 1:5, 1:10, 1:20). The slides were examined microscopically for the presence of ferning. All samples were fern-positive, but many had atypical ferns described as “skeletonized.” As the concentration of blood to amniotic fluid increased, the number of atypical ferns increased (32 of 38 at 1:5, 22 of 38 at 1:10, and nine of 38 at 1:20). We conclude that the presence of blood may alter the morphology of the fern, but does not act as a contaminant that would affect the accuracy of the test.”
“Thirty-six specimens of amniotic fluid across gestational ages (16 to 42 weeks) were tested. The fern test was unaffected by meconium at any concentration and by blood at dilutions of 1:10 or greater. When blood and amniotic fluid were mixed in equal amounts, ferning was not present. Arborization of amniotic fluid was unaffected by pH alterations.”
“Samples were obtained from amniocenteses between 14 and 42 weeks. Part I: Of 112 samples allowed to dry on a slide for 3 minutes only, 86.6% were ferning positive and 100% were nitrazine positive. Flame-drying increased the presence of ferning to 96.4%. Part II: 363 samples were allowed to dry completely for up to 10 minutes. All samples were ferning and nitrazine positive….These tests may be reliably performed at gestations of 12 to 41 weeks.”
MORAL: Amniotic fluid will fern at any gestational age, in varying pH, and in the presence of a moderate amount of blood. If you have fluid on the slide, and the slide doesn’t fern, you need to seriously doubt the diagnosis of ruptured membranes.