Abstract: A test strip includes a first layer and an overlying differential flow-retarding layer that reduces the fraction of a component of a liquid specimen percolated therethrough. An inert backing, preferably having an aperture therethrough, overlies the flow-retarding layer. For the testing of a blood specimen, the flow-retarding layer is formed of glass fibers that reduce the fraction of red blood cells in the percolate, leaving primarily, but not entirely, plasma in the percolate. The first layer includes chemicals that react with a component of the plasma such as glucose in a measurable reaction, preferably a visually measurable reaction. In using this test strip, the blood specimen is placed into the aperture of the backing and permitted to percolate through the flow-retarding layer such that a fraction of the red blood cells is removed from the percolate.