Alpha-fetoprotein as a tool to distinguish amniotic fluid from urine, vaginal discharge, and semen

OBJECTIVE:: To estimate whether alpha-fetoprotein (AFP) can be used to distinguish amniotic fluid absorbed in sanitary pads from other similarly absorbed substances (semen, urine, and normal vaginal discharge). METHODS:: A prospective cohort study. Urine and amniotic fluid specimens were collected from 52 pregnant women admitted for labor. Semen specimens were collected from 17 men undergoing infertility evaluation. Alpha-fetoprotein concentrations were measured directly from urine, amniotic fluid, and semen and from pads instilled with samples from these specimens. Alpha-fetoprotein concentrations were also measured from pads absorbed with normal vaginal discharge collected from 27 pregnant women. RESULTS:: Alpha-fetoprotein levels in amniotic fluid (245.38±21.03 ng/mL, n=52) were significantly higher than those measured in maternal urine (0.84±0.17 ng/mL, n=52, P<.001), or semen (1.52±0.35 ng/mL, n=17, P<.001). The same trend was seen when AFP was extracted from pads: amniotic fluid levels (19.44±1.98 ng/mL, n=52) were significantly higher than those of urine (undetectable, n=52), semen (undetectable, n=17), or normal vaginal discharge (0.53±0.16 ng/mL, n=27, P<.001). Receiver operator characteristic curve analysis demonstrated 96.2% sensitivity and 100% specificity for distinguishing the presence of amniotic fluid from normal vaginal discharge on sanitary pads (cutoff 3.88 ng/mL, area under the curve 0.99). CONCLUSION:: When the diagnosis of rupture of membranes is in doubt, AFP levels can assist in differentiating amniotic fluid from other bodily fluids. A method that utilizes sanitary pads and an assay for AFP quantification may be an accurate and convenient way to confirm the diagnosis of rupture of membranes.