Occult blood in stool at 45 year old checkup; what to look for?

Occult blood test. Fecal occult blood test is done to see if you have evidence of bleeding in the GI tract. The sensitivity will be very low if it is done only once. The sensitivity will be much higher if that is done in 3 separate occasion. Your md likely did it for a good reason- eg- you have significant family history of colon cancers; or there are suspicious sign/symptoms that have to be evaluated further etc.
You need. To see a GI doctor or coloretal surgeon and atleast get a colonoscopy and or upper endoscopy if indicated. Occult blood in the stool is not normal and needs a thorough workup and evaluation. Don't ignore this!