The terms "preventive therapy" and "chemoprophylaxis" have been used for decades and are preferred in the UK because it involves giving medication to people who have no active disease and are currently well, the reason for treatment is primarily to prevent people from becoming unwell. The term "latent tuberculosis treatment" is preferred in the US because the medication does not actually prevent infection: it prevents an existing silent infection from becoming active. The feeling in the US is that the term "treatment of LTBI" promotes wider implementation by convincing people that they are receiving treatment for disease. There are no convincing reasons to prefer one term over the other. It is essential that assessment to rule out active TB is carried out before treatment for LTBI is started. To give LTBI treatment to someone with active TB is a serious error: the TB will not be adequately treated and there is a risk of developing drug-resistant strains of TB. There are several treatment regimens available:

9H—Isoniazid for 9 months is the gold standard and is 93% effective. 6H—Isoniazid for 6 months might be adopted by a local TB program based on cost-effectiveness and patient compliance. This is the regimen currently recommended in the UK for routine use. The US guidance exclude this regimen from use in children or persons with radiographic evidence of prior tuberculosis (old fibrotic lesions). (69% effective)
6 to 9H2—A twice-weekly regimen for the above two treatment regimens is an alternative if administered under Directly observed therapy (DOT).
If it can be shown that they are fully compliant with their medication, then the probability that they have another diagnosis (perhaps in addition to the diagnosis of TB) is very high. These patients should have their diagnosis carefully reviewed and specimens obtained for TB culture and sensitivity testing. Patients who get better and then get worse again should likewise be questioned very closely about adherence to treatment. If adherence is confirmed then they should be investigated for resistant TB (including MDR-TB), even if a specimen has already been obtained for microbiology before commencing treatment. Prescription or dispensing errors will account for a proportion of patients who fail to respond to treatment. Immune defects are a rare cause of non-response. In a tiny proportion of patients, treatment failure is a reflection of extreme biological variation and no cause is found. Treatment relapse
Patients are said to relapse if they improve while on treatment, but become ill again after stopping treatment. Patients who experience only a transient improvement while on treatment, or who never respond to treatment are said to have failed treatment and are discussed above. There is a small relapse rate associated with all treatment regimens, even if the treatment has been taken religiously with 100% compliance (the standard regimen 2HREZ/4HR has a relapse rate of 2 to 3% under trial conditions). The majority of relapses occur within 6 months of finishing treatment. Patients who are more likely to relapse are those who took their medication in an unreliable and irregular fashion.