A method of assessing amplification of nucleic acids comprising: providing a porous substrate comprising a plurality of amplification reagents distributed throughout the porous substrate wherein said amplification reagents comprise: a) isothermal nucleic acid amplification reagents; b) DNA polymerase and one or more nucleic acid primers; or c) RNA polymerase, and said porous substrate is formed from a solid porous material selected from paper, cellulose, porous polymer sheets, glass fibers, quartz fibers and nitrocellulose membranes, and wherein said porous substrate is provided wet or saturated with liquid wherein said liquid is an appropriate buffer; applying a sample comprising nucleic acids to a predetermined initial location on the porous substrate; allowing the sample to react with the amplification reagents on the porous substrate for a duration of time to generate amplification products of a nucleic acid sequence of interest, wherein the amplification products move within the porous substrate by reaction diffusion and comprise one or more detectable moieties configured to provide a signal when the amplification products are generated during amplification; and determining an initial concentration of the sample based on a time-dependent progress of the signal within the substrate based on a fit to data correlating to a reference sample, wherein said determining comprises a) plotting the signal using the distance on the porous substrate at which said signal crosses a predetermined threshold and based on a fit to data correlating to the reference sample; or b) plotting the signal using the time at which said signal crosses a predetermined threshold at a predetermined distance on the porous substrate and based on a slope and fit to data correlating to the reference sample. providing a porous substrate comprising a plurality of amplification reagents distributed throughout the porous substrate wherein said amplification reagents comprise: a) isothermal nucleic acid amplification reagents; b) DNA polymerase and one or more nucleic acid primers; or c) RNA polymerase, and said porous substrate is formed from a solid porous material selected from paper, cellulose, porous polymer sheets, glass fibers, quartz fibers and nitrocellulose membranes, and wherein said porous substrate is provided wet or saturated with liquid wherein said liquid is an appropriate buffer; applying a sample comprising nucleic acids to a predetermined initial location on the porous substrate; allowing the sample to react with the amplification reagents on the porous substrate for a duration of time to generate amplification products of a nucleic acid sequence of interest, wherein the amplification products move within the porous substrate by reaction diffusion and comprise one or more detectable moieties configured to provide a signal when the amplification products are generated during amplification; and determining an initial concentration of the sample based on a time-dependent progress of the signal within the substrate based on a fit to data correlating to a reference sample, wherein said determining comprises a) plotting the signal using the distance on the porous substrate at which said signal crosses a predetermined threshold and based on a fit to data correlating to the reference sample; or b) plotting the signal using the time at which said signal crosses a predetermined threshold at a predetermined distance on the porous substrate and based on a slope and fit to data correlating to the reference sample. a) plotting the signal using the distance on the porous substrate at which said signal crosses a predetermined threshold and based on a fit to data correlating to the reference sample; or b) plotting the signal using the time at which said signal crosses a predetermined threshold at a predetermined distance on the porous substrate and based on a slope and fit to data correlating to the reference sample. <EOS>