PROK1 and congenital rubella syndrome: The cut-off level of PROK1 for CRS was 56.9 pg/mL (area under the curve (AUC), 0.62; negative likelihood ratio, 0.56; positive likelihood ratio, 1.48; and diagnostic odds ratio, 2.45; sensitivity, 70.6%; specificity, 50.2%) using the ROC curve (Additional file 1).