First of all you have to use the most sensitive reagen- guaiac based.
If you just do it once- the sensitivity to find blood in the stool is only 10-25%. If you use it three times as recommended- the sensitivity will go up to around 80-90%. The specificity/accuracy of the test is low- as many other conditions can cause blood in the stool. Colonoscopy
is more sensitive and more specific/accurate.