If you test negative for all three, then you have anergy , which means your immune system is to damaged to respond to common antigens. Some mutations of TB are resistant to some of the common drugs. There are susceptibility tests that can show which drugs will work on a particular strain. The biggest reason for TB treatment failure is a failure to take the medicines. Some with late-stage AIDS have trouble absorbing the drugs.
Doctors can sometimes run tests to see if malabsorption is a problem.
This information is designed to support, not replace, the relationship that exists between you and your doctor.