The invention includes a method of assaying an analyte in a sample in a portable, handheld microfluidic reader. The method includes the steps of: inserting the sample in the reader; capturing the analyte with a first antibody having a DNA tag attached thereto; capturing the analyte in the sample with a second antibody attached to a surface or having a magnetic nanoparticle (MNP) attached thereto; where a sandwich including the magnetic nanoparticle, first and second antibodies, the analyte and the DNA tag is formed; replicating the DNA tag using isothermal amplification to a predetermined amount of DNA tags detectable by a detector sufficient to overcome the minimal mass sensitivity limitations of the detector; and measuring the amount of replicated DNA tags using the detector. The invention also includes an apparatus or handheld portable field microfluidic reader in which the method is performed.