Methods and devices for analyzing fluid variables such as viscosity, surface tension, analyte concentration, and the presence of particulates or aggregates are provided. The devices analyze fluid variables by measuring cessation of fluid flow through the medium or a change in flow rate due to an inherent property of the fluid variable, or due to a modification of the fluid variable as the fluid migrates through the medium. The lateral flow devices measure relative rates of capillary flow down converging arms of a common element of porous media in which at least one of the arms has been modified by the addition of flow-modifying agents which affect the flow rate in response to the concentration of the analyte. The fluid fronts will converge at a position determined by their relative flow rates, which is thus indicative of a fluid variable, such as analyte concentration, in the sample fluid.