Introduction Although laboratory culture of Mycobacterium tuberculosis (M TB) remains the gold standard for the diagnosis of active tuberculosis (TB), only 66% of pulmonary and 46% of extra-pulmonary cases of TB in the UK are culture confirmed.1 The Health Protection Agency's Strategic Plan aims for at least 70% of pulmonary TB cases to be confirmed by positive culture. The aim of this study was to identify the reasons for not obtaining culture confirmation in a cohort of active TB patients.
Methods A retrospective study of all patients with active tuberculosis in a TB centre between January and December 2009.
Results 69 patients (46 male) with a mean age of 42 years (range 3–83) were diagnosed. 36 (52%) had pulmonary TB with or without extra-pulmonary disease and 33 (48%) had extra-pulmonary TB only. 29 (81%) cases of pulmonary TB were culture positive and 4 (11%) had no growth on culture. 3 (8%) cases had no sample obtained. These were all children aged 3–5 years who had a positive Mantoux test, evidence of TB on chest radiograph and a history of close contact. 17 (52%) cases of extra-pulmonary TB were culture positive. 10 (30%) had samples taken but no growth on culture. 2 (6%) had samples obtained but not sent for culture while 4 (12%) had no sample obtained.
Conclusion The commonest reason for not obtaining culture confirmation was a negative growth (20%). Failure to obtain a microbiological sample (10%) and failure to send the collected sample for mycobacterial culture (3%) were the other reasons. A positive culture of M TB is important because it not only confirms the diagnosis but also provides the drug susceptibility profile of the organism. Our finding that the largest proportion of cases not confirmed by positive culture was due to no growth from a specimen suggests that the current microbiological methods for growing M TB may be inadequate and further research is needed to increase the diagnostic yield. Secondly, there is a need to educate those sending samples, including surgeons and radiologists performing biopsies, so that specimens are sent correctly.
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