on the substrate in time depending on the concentration of the select particles in the test liquid some of the second particles are able to diffuse all the way to the diagnostic substrate and complex with its exposed layer of select particles wherever such complexing occurs a second and visible layer is formed such second layer formation occurs first at the thin end of the zone of test liquid where there is a smaller quantity of select particles separating the second particles from the diagnostic substrate as more time is allowed for diffusion the complexing event will proceed along the diagnostic substrate opposite thicker regions of this zone having defined the change in thickness of the liquid filled zone along the longitudinal extent of the containing means a reading of the extent length and or area of the double layer formation on the diagnostic substrate provides a measure of the concentration of select particles present in the test liquid depending upon the diagnostic substrate employed and the method of its use the second layer may be visible with good contrast to the unaided eye or may be removed and its content so removed or its absence detected in the preferred arrangement the containing means is disposed in a generally planar orientation extending along a flat surface of the diagnostic substrate is.