BACKGROUND: Acute graft versus host disease (aGVHD) affects approximately 30-60% of patients after allogeneic hematopoietic stem cell transplantation (HCT) and our ability to predict who develops this complication and their response to treatment is limited. Fecal calprotectin has recently gained popularity as an effective marker of GI inflammation in patients with Inflammatory Bowel Disease (IBD). METHODS: Fecal calprotectin and albumin were evaluated as prognostic and predictive markers of aGVHD in 60 adult and pediatric HCT patients. Stool samples were sent for calprotectin quantification prior to starting conditioning, at day 14 post-HCT, at day 28 post-HCT, and at onset of aGVHD +/-2 days. RESULTS: Fecal calprotectin did not differentiate patients with GI-GVHD and non-GI GVHD and did not vary based on severity. However, in patients with steroid-refractory GI aGVHD, significantly higher fecal calprotectin levels were noted. At onset of lower-GI symptoms, steroid refractory patients (n=3) had a mean fecal calprotectin level of 449ug/g (range 116-1111ug/g) and a mean albumin of 1.93g/dL (range 1.6-2.3g/dL) compared with a mean fecal calprotectin of 24ug/g (range 16-31ug/g) and a mean albumin of 3.3g/dL (range 2.3-3.9g/dL) in steroid responsive patients (n=9) (fecal calprotectin p=0.032, albumin p=0.027). CONCLUSION: Patients with steroid-refractory GI aGVHD had higher fecal calprotectin levels and lower albumin levels than patients with steroid-responsive disease. We recommend further studies to evaluate non-invasive tests with fecal calprotectin in combination with albumin in predicting steroid refractory disease at onset of symptoms to potentially identify patients that may benefit from upfront escalation in GVHD treatment.