Chronic Infectious Diarrhea: Given the duration of non-bloody diarrhea without abdominal pain in an elderly patient, chronic infectious causes such as giardiasis, cryptosporidiosis, or bacterial overgrowth should be considered. The absence of bloody stools and normal inflammatory markers (ESR, CRP) supports this possibility.

Irritable Bowel Syndrome (IBS): Although IBS can occur at any age, its diagnosis in the elderly might be less common due to the higher prevalence of other gastrointestinal diseases. However, the absence of alarm features (e.g., blood in stool, significant weight loss, anemia) and normal laboratory tests make IBS a plausible diagnosis.
Medication-induced Diarrhea: Elderly patients often take multiple medications, some of which can cause diarrhea as a side effect (e.g., laxatives, antibiotics, antacids, and certain cardiovascular drugs). A thorough review of the patient's medication list is essential.
Diverticulitis or Diverticulosis: While diverticulitis typically presents with abdominal pain, diverticulosis can be asymptomatic or cause changes in bowel habits, including diarrhea, without pain.

Colorectal Cancer: Although less likely given the absence of alarm features like blood in stool or significant weight loss, colorectal cancer must be considered, especially in an elderly patient. The lack of abdominal pain does not rule out this diagnosis.
Inflammatory Bowel Disease (IBD): While less common for IBD to first present in the elderly, it is crucial not to miss this diagnosis due to its significant implications on quality of life and the need for specific treatment.
Ischemic Bowel Disease: This condition can present with chronic diarrhea and is more common in the elderly due to atherosclerotic disease. It's a critical diagnosis not to miss due to its potential for severe complications.

Microscopic Colitis: This condition is characterized by chronic, watery diarrhea without abdominal pain, more common in elderly women but can also occur in men. The diagnosis requires a colonoscopy with biopsy.
Lymphocytic Colitis: Similar to microscopic colitis, it presents with chronic diarrhea and is diagnosed through colonoscopy and biopsy.
Neuroendocrine Tumors: Certain neuroendocrine tumors, like VIPomas, can cause severe, chronic diarrhea. These are rare but important to consider in the differential diagnosis due to their potential impact on the patient's quality of life and the specific treatment required.