Inflammatory Bowel Disease (IBD): Elevated calprotectin is a strong indicator of intestinal inflammation, which is a hallmark of IBD, including Crohn's disease and ulcerative colitis. The negative stool testing helps to rule out infectious causes, making IBD a prime consideration.

Irritable Bowel Syndrome (IBS): Although IBS is a diagnosis of exclusion, some patients with IBS can have elevated calprotectin levels, especially if they have a significant inflammatory component to their disease. However, the presence of elevated calprotectin might suggest an alternative diagnosis.
Microscopic Colitis: This condition can cause chronic diarrhea and may have normal or near-normal colonoscopy findings but abnormal histology. Elevated calprotectin could be seen due to the inflammation present in microscopic colitis.
Celiac Disease: An autoimmune reaction to gluten can lead to chronic diarrhea and elevated inflammatory markers like calprotectin, even if stool tests are otherwise negative.

Colon Cancer: Although less likely, it's crucial not to miss colon cancer, especially in older adults. Chronic diarrhea can be a symptom, and elevated calprotectin indicates inflammation, which could be associated with a tumor.
Lymphocytic Colitis: A subtype of microscopic colitis, it's essential to consider due to its potential to cause chronic diarrhea and the need for specific treatment.
Giardiasis (if stool testing was not comprehensive): While the question states negative stool testing, if the testing did not include specific assays for Giardia (such as antigen testing or PCR), it could be missed. Giardiasis can cause chronic diarrhea and may not always be detected by routine stool exams.

Eosinophilic Gastroenteritis: A rare condition characterized by eosinophilic infiltration of the gastrointestinal tract, which can cause chronic diarrhea among other symptoms. Elevated calprotectin could reflect the underlying inflammation.
Common Variable Immunodeficiency (CVID): Patients with CVID can have gastrointestinal symptoms, including chronic diarrhea, due to impaired immune function. Elevated calprotectin might indicate an underlying inflammatory process.
Whipple's Disease: A rare, systemic bacterial infection that can cause chronic diarrhea, malabsorption, and weight loss. Although stool tests are negative, Whipple's disease requires a high index of suspicion for diagnosis, often through small bowel biopsy.