Antioxidants play a role in counteracting free radicals and reactive oxygen species and are thought to help prevent or slow the progression of many chronic diseases, such as cancer, diabetes mellitus, cardiovascular disease, and neurodegenerative diseases. Herein we report a simple way to make a colorimetric assay for measuring total antioxidant capacity (TAC) on craft paper-based analytical devices (cPADs) suitable for sub-μL volume blood samples. We incorporated a microfluidic separation mechanism for isolation of plasma from interfering blood cells. The whole diagnostic process, including cPAD construction, plasma sample preparation, assay, and image thresholding analysis, can be completed in fifteen minutes. We applied our approach toward the measurement of TAC in mice that model Huntington's disease (HD), a fatal, neurodegenerative movement disorder. Results revealed that TAC was significantly elevated in R6/2 HD model mice compared to their age-matched wild-type (WT) controls. We expect that this method, carrying a simple, fast, and sensitive assay on low-cost and disposable paper, will meet the potential needs for point-of-care (POC) testing of TAC, as well as other disease biomarkers in blood and other types of bodily fluids where limited volumes of samples are obtainable.