We performed an inter-assay comparison among three immunoassay platforms for midpregnancy testing of sFlt-1, PlGF and the sFlt-1/PlGF ratio, which are established markers for pre-eclampsia. Maternal blood was collected 19–22 weeks’ gestation. Raw data values were converted to multiples of the median (MoM). PlGF and sFlt-1 values among platforms were highly correlated (p < 0.001). There was significant variation in raw data values for PlGF and sFlt-1 among platforms, eliminated following conversion to MoM. When directly comparing raw data values among platforms, platform-specific reference ranges values should be used. MoM values were equivalent among platforms, allowing direct inter-assay result comparison.