MYCOBACTERIA TESTING

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Specimen: Sputum (3-5 separate samples); bronchoscopy brushing, lavage; tissue biopsy (eg lymph node, lung, endometrial curettings, bone); body fluids (eg CSF, pleural, synovial, pericardial fluids); pus (swabs do not provide an adequate sample); urine (3 complete early morning collections); blood culture collected into radiometric culture medium; bone marrow aspirate; faeces; tissue fluid from eg eyebrows, ear lobes.

Method: Acid fast staining (eg Ziehl-Neelsen) and auramine staining for microscopy; gene probe following PCR amplification in appropriate specimens (eg CSF); culture in/on appropriate media (eg Lowenstein-Jensen or Middlebrook). Rapid growers (eg M. fortuitum) may grow in 1-2 weeks, but 6-8 weeks of incubation are required before discarding negative cultures. Identification of mycobacteria and antibiotic susceptibility testing take another 4 weeks using conventional techniques. Radiometric culture combined with gene probe (for identification) considerably reduces the time required for testing.

Application: Suspected tuberculosis or atypical mycobacterial infection , incl unexplained lung infection; “sterile” pyuria; meningitis; diarrhoea or fever in patients who have AIDS; chronic skin ulcers/lesions; subacute/chronic unexplained lymph-adenopathy; infertility (endometrial curettings); suspected leprosy (microscopy of tissue fluid from eyebrows, ear lobes, skin, or biopsy). Biopsy of tissue has higher sensitivity than fluid samples.

Interpretation: The presence of acid fast bacilli in sputum or in normally sterile fluids or tissues is generally sufficient to establish the diagnosis of tuberculosis. However, the sensitivity of microscopy in body fluids and CSF is low (<20%). Use of gene probing after PCR amplification allows more rapid diagnosis; culture confirms the diagnosis. Identification and antibiotic susceptibility testing are required for adjusting antibiotic treatment. M. leprae cannot be grown in vitro and the diagnosis of leprosy is based on the presence of acid fast bacilli in a clinically suspicious lesion.

Reference: Nolte FS and Metchock B. In: Murray PR et al eds. Manual of Clinical Microbiology. 6th ed. ASM Press 1995.

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