Chronic bad breath may have any of a variety of causes (for more information, see
http://www.medpedia.com/questions/948-what-is-the-best-way-to-minimize-bad-breath ). You mentioned the possibility of bacteria in the stomach or kidney; these and other infections can be associated with bad breath, and are often detectable through markers in the blood. The most commonly used marker for infection detectable through a blood test is the white blood cell (WBC) count; an increase or decrease in WBCs from normal may be a sign of infection. Should an infection be detected, it may be treated with appropriate antibiotics, which may cure the bad breath. Additionally, the "full body check" you mentioned is often intended to discover unrelated problems as well; this often requires blood testing to be considered "complete".