You can detect microscopic levels of blood with an at-home hemocult test, which is available at pharmacies and can be done every year after age 40. Don't worry-it's actually not as gross as it sounds; just follow the directions on the label. This test, as many at-home tests do, has false-positives, so you'll need to follow up with your physician (this week, not next year) if you do have a positive reading.
Blood can be a sign of many things, but it's important to look for it so that a colonoscopy-recommended every three years for people over 50, or over 40 for people with a family history of colon cancer-can catch the pre-cancerous polyps before they lead to colon cancer.
Find out more about this book:YOU: The Owner's Manual, Updated and Expanded Edition: An Insider's Guide to the Body that Will Make You Healthier and Younger