An improved casting substrate is provided for use in a composite membrane for use as a fluid filtration medium. The composite membrane has good adhesion between the casting substrate and the microporous membrane while maintaining high permeability. The casting substrate comprises a single spunbond layer or a multiple spunbond layers formed of thermoplastic fibers, optionally comprising at least one meltblown layer. The casting substrate allows the membrane casting solution to penetrate the casting substrate uniformly to a controlled depth without penetrating through the entire thickness of the substrate. The casting substrate has good uniformity of mean pore size, basis weight, thickness and surface roughness.