Value of the fern test to confirm or reject the diagnosis of ruptured membranes is modest in nonlaboring women presenting with nonspecific vaginal fluid loss.

The strength of the fern test to differentiate between amniotic and nonamniotic fluid in vaginal discharge was determined in 51 term women in labor with ruptured membranes and compared with that in a group of 120 nonlaboring subjects, presenting with nonspecific vaginal fluid loss. Sensitivity and specificity in the laboring group were 98.0 and 88.2%, respectively, in agreement with previous reports. In contrast, in the non-laboring group sensitivity and specificity were only 51.4 and 70.8%, respectively. The result of the fern test predicted the actual state of the membranes correctly in 63% and incorrectly in 29% of these patients. In 16 or 39 subjects with ruptured membranes (approximately 40%), the outcome of the fern test was negative. The modest diagnostic strength of the fern test in the present study compared with previously reported data is at least in part due to differences in study population (laboring versus nonlaboring) and to the fact that observers were deprived of relevant clinical information. It is concluded that the fern test should be granted supportive rather than conclusive value in diagnosing ruptured membranes in nonlaboring women presenting with nonspecific vaginal fluid loss.