A video processing method improves the repeatability of point-of-care (POC) mean pixel intensity (MPI) analysis, for example by rejecting low-quality input data using image features. Using these features, the method selects high-quality frames and achieves an improvement in correlation and standard error for the MPI-absorbance relationship. In another example, a mobile device (e.g., smartphone) based image processing method produces an MPI output with smaller variances, lower limits of detection (LODs), and a higher dynamic range than existing methods. This method has improved performance in the presence of additive and multiplicative ambient light noise.