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.
Other abbreviations have been widely used (for example, the notations RIF, RFP, and RMP have all been widely used for rifampicin, and the combination regimens have notations such as IRPE, HRZE, RIPE, and IREP that are variously synonyms or near-synonyms, depending on dosage schedules), but for clarity, the semistandardized abbreviations used above are used in the rest of this article. In this system, which the World Health Organization (WHO) supports, "RIPE" is "RHZE". (Both have mnemonic potential, as tuberculosis is named after tubercles (small tubers), and a tuber can be ripe and can be a rhizome. )Drug regimens are similarly abbreviated in a semistandardised manner. The drugs are listed using their single letter abbreviations (in the order given above, which is roughly the order of introduction into clinical practice). A prefix denotes the number of months the treatment should be given for; a subscript denotes intermittent dosing (so 3 means three times a week) and no subscript means daily dosing. Most regimens have an initial high-intensity phase, followed by a continuation phase (also called a consolidation phase or eradication phase): the high-intensity phase is given first, then the continuation phase, the two phases divided by a slash.So,

2HREZ/4HR3means isoniazid, rifampicin, ethambutol, pyrazinamide daily for two months, followed by four months of isoniazid and rifampicin given three times a week.In the US only, streptomycin is not considered a first line drug by ATS/IDSA/CDC because of high rates of resistance. The WHO have made no such recommendation.