Ferning of amniotic fluid contaminated with blood.

The diagnosis of rupture of the membranes by traditional methods of nitrazine paper determination and fern crystallization has been reported to be 93-98% accurate. Although the presence of contaminants in amniotic fluid has been reported to reduce this accuracy, the literature is conflicting concerning these effects. We examined the effect of blood contamination on the accuracy of diagnosing ruptured membranes. 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.