Effect of blood contamination on amniotic fluid detection in vitro using immunoassays

Objective: To determine the accuracy of Actim PROM^®, Amnisure^®, and ROM Plus^® tests for detecting amniotic fluid proteins in the setting of blood contamination.Methods: IGFBP-1 and AFP are proteins present in high concentrations in amniotic fluid, and are detected by three commercially-available immunoassays used for diagnosing ruptured membranes: Actim PROM^®, Amnisure^®, and ROM Plus^®. We used whole blood samples and diluted these with amniotic fluid (containing known concentrations of amniotic fluid proteins) to whole blood levels of 50, 20, 10, 5, and 1%. Actim PROM^®, Amnisure^®, and ROM Plus^® tests were performed on each sample in duplicate according to package insert instructions. Results were interpreted independently at 5, 10, 15, and 20 min by two obstetricians who were blinded to the concentrations of blood and amniotic fluid proteins in each sample. Results of each test were determined to be true positive, false negative, false positive, or true negative based on physician interpretation and whether amniotic fluid had been spiked into the samples in detectable concentrations. Overall accuracy, intraobserver concordance, and interobserver concordance, sensitivity, specificity, and predictive values for each test were calculated. Fisher exact test was used to compare test characteristics, with a p-value of <.05 considered significant.Results: Out of 120 tests performed, there were no false positive results for any test. Overall, ROM Plus^® had better accuracy (97.9%) than Amnisure^® (80.7%) or Actim^® PROM (78.3%). Intra- and interobserver concordance were similar for all three tests (98-100%). ROM Plus^® had significantly higher sensitivity than Amnisure^® and Actim^® PROM (p < .0001). There was no significant difference in sensitivity between Amnisure^® and Actim^® PROM (p = .51).Conclusion: ROM Plus^® maintains strong test characteristics for the detection of amniotic fluid proteins in the setting of blood contamination, and performs significantly better than Amnisure^® and Actim^® PROM tests in the presence of blood.