Endoscopic procedures with tissue biopsy are the only way to definitively diagnose ulcerative colitis. Other types of tests can help rule out complications or other forms of inflammatory bowel disease, such as Crohn's disease.
Lab tests:
Blood tests. Your doctor may suggest blood tests to check for anemia — a condition in which there isn’t enough red blood cells to carry adequate oxygen to your tissues — or to check for signs of infection.
Stool studies. White blood cells or certain proteins in your stool can indicate ulcerative colitis. A stool sample can also help rule out other disorders, such as infections caused by bacteria, viruses and parasites.
Endoscopic procedures:
Colonoscopy. This exam allows your doctor to view your entire colon using a thin, flexible, lighted tube with a camera on the end. During the procedure, your doctor can also take small samples of tissue (biopsy) for laboratory analysis. A tissue sample is necessary to make the diagnosis.
Flexible sigmoidoscopy. Your doctor uses a slender, flexible, lighted tube to examine the rectum and sigmoid colon — the lower end of your colon. If your colon is severely inflamed, your doctor may perform this test instead of a full colonoscopy.
Courtesy: Mayo Clinic