The ferning test for detection of amniotic fluid contamination in umbilical blood samples

There is currently no standard means of detecting amniotic fluid contamination in fetal blood obtained by percutaneous umbilical blood sampling. The ferning test is proposed for this purpose. An in vitro model, using centrifuged serial dilutions of neonatal cord blood and amniotic fluid, showed that ferning of the supernatant occurs in proportion to the degree of contamination. A system for grading ferning was devised, and tested in a blind trial. For the detection of amniotic fluid contamination of 10% or more, the method showed a sensitivity of 98% and a specificity of 81%. The method may be useful in percutaneous umbilical blood sampling, and in the diagnosis of spontaneous rupture of membranes with vaginal bleeding.