The diagnosis and severity of acute graft versus host disease (GVHD) is typically based on clinical and laboratory observations that are often subjective or inconclusive. For instance, a skin rash may be graded as GVHD by one observer and attributed to an allergic drug reaction by a second observer. Biopsies of involved organs may be useful but can be associated with significant risk and may also return conflicting or inconclusive results. For many years investigators have attempted to identify biomarkers to predict and diagnose GVHD [2, 3] and ideally herald the onset and severity before clinical symptoms develop allowing for early intervention and better, more effective, and targeted immunosuppressive therapies. To date, however there are no validated diagnostic or prognostic noninvasive tests for acute GVHD.