18th June 2024

Diagnosis

Imaging

It is not possible here to describe TB’s radiological appearance in every organ system it affects. A plain film chest radiograph, however, is the first investigation in investigating TB. Fibrotic changes can be seen in the lung apices in prior secondary TB, while consolidation with or without cavitation, mediastinal adenopathy or pleural effusion can be seen in secondary or reactivation TB. Primary TB can affect any part of the lung and typically does not feature cavitation.

Microbiological diagnosis

TB is a microbiological diagnosis ultimately; however, obtaining microbiological proof is not always possible. While the gold standard for TB diagnosis is culture (performed on the specialised Löwenstein-Jensen medium), this can take up to 7 weeks. Samples can undergo Ziehl Neelsen staining for “acid fast bacilli”, or auramine staining of samples (i.e. sputum, bronchial washings urine, tissue, CSF, etc.) for quicker results, however yield is lower than culture. In “paucibacillary” sputum samples negative for staining, nucleic acid amplification technique (NAAT) can be used to give a rapid result, as well as detecting genes associated with rifampicin or isoniazid resistance. These tests have the best evidence and best performance in respiratory samples rather than extrapulmonary sites.

Latent TB is diagnosed using a combination of history and immunological testing. Mantoux testing uses an intradermal injection of purified proteins from M.tuberculosis to generate a localised skin reaction, the diameter of the induration of which is measured. Prior BCG vaccination can lead to false positives however, and more-modern tests such as the Interferon-λ-Release Assay (IGRA), which tests blood T-cell responses to several M. tuberculosis-specific proteins for evidence of prior exposure, can be more informative.