Methods for evaluating microsatellite instability (MSI) analyze nucleic acid sequence reads corresponding to a plurality of marker regions for MSI. The marker regions may include long homopolymers and/or short tandem repeats (STRs). For a target homopolymer, a histogram of homopolymer signal values is calculated based on flow space signal measurements for the homopolymer region in the sequence reads. A score per marker based on features of the histogram of homopolymer signal values is determined for each marker region corresponding to the target homopolymers. For a target STR, the method includes calculating a histogram of repeat lengths for sequence reads corresponding to the marker region of the target STR. A score per STR marker is calculated based on features of the histogram of repeat lengths. A plurality of per marker scores may be combined to form a total MSI score for the sample.