treatment must be distinguished from patients who relapse. Patients who responded to treatment and appeared to be cured after completing a course of TB treatment are not classed as treatment failures, but as relapses and are discussed in a separate section below. Patients are said to have failed treatment if they fail to respond to treatment (cough and sputum production persisting throughout the whole of treatment), or only experience a transient response to treatment (the patient gets better at first, but then get worse again, all the while on treatment). It is very uncommon for patients not to respond to TB treatment at all (even transiently), because this implies resistance at base-line to all of the drugs in the regimen. Patients who fail to get any response at all while on treatment should first of all be questioned very closely about whether or not they have been taking their medicines, and perhaps even be admitted to hospital to be observed taking their treatment. Blood or urine samples may be taken to check for malabsorption of TB drugs. 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).