BACKGROUND: Fecal calprotectin is a marker used to differentiate inflammatory bowel disease versus irritable bowel syndrome and is relevant in the diagnosis of ulcerative colitis and Crohn's disease. Markedly elevated calprotectin from stool samples provides evidence of colonic inflammation to support the diagnosis of pancolitis. This report is the first to demonstrate the clinical significance of fecal calprotectin in supporting the diagnosis of pancolitis induced by the anti-viral drug, Biktarvy (bictegravir/emtricitabine/tenofovir alafenamide). CASE REPORT: A 62-year-old male on Biktarvy for his HIV diagnosis was admitted to internal medicine with abdominal pain, bloody diarrhea and pancolitis. His white blood cell count was 15.8 (4.0-11.0x10(9)/L), neutrophil count was 9.7 (2.0-7.5 x 10(9)/L), monocyte count was 1.2 (0.2-0.8 x 10(9)/L), granulocyte count was 1.5 (</=0.1 x 10(9)/L) and hemoglobin was 163 (140-180 g/L). The patient had a C-reactive protein of 229 (</=11.0 mg/L). Serology and blood culture were negative for microbial testing and abdomino-pelvic computed tomography findings were unremarkable. A bloody stool collected had a fecal calprotectin level of 1,159 (<50 microg/g). CONCLUSIONS: This case highlights how anti-retroviral therapies such as Biktarvy may elicit medication-induced gastrointestinal symptoms, which may underlie the cause of bloody diarrhea and pancolitis, and consequently a grossly elevated fecal calprotectin.