Diagnosis

Tuberculosis is is suggested by the clinical and radiological features but requires the identification of the tubercle bacillus:

Pulmonary TB should be suspected in anyone with a cough of more than three weeks' duration for which there is no other explanation, particularly if it is accompanied by the other symptoms (1)

• haemoptysis
• shortness of breath
• loss of appetite
• weight loss
• fever and sweating, especially at night
• fatigue and tiredness
• swollen glands

Such patients should be investigated urgently, initially by chest X-ray or sputum smear examination, for which the result should be available within 24 hours of receipt of the sputum sample at the laboratory. Where these investigations raise the possibility of TB, patients should be urgently referred to the local chest clinic.

Investigations used in the diagnosis of TB include:

• chest radiography:
  • look for the features of primary, post-primary or miliary tuberculosis
  • always compare with previous radiology if possible
    
    
• sputum:
  • Ziehl-Nielsen or auramine staining of a sputum smear may demonstrate the presence of acid-fast bacilli
  • in vitro culture of the sputum may take 4 to 7 weeks to provide a result; a further 3 weeks is required to identify drug sensitivity
  • in 2001, only 55% of cases of pulmonary TB were found to be sputum-smear-positive - therefore, if clinical and radiological features are consistent with active disease, treatment is generally indicated, even if sputum examination is negative (1)
    
    
• biopsy:
  • diagnosis can sometimes be made based on the histological demonstration of a caseating granuloma
    
    
• guinea pig inoculation is no longer routine
  
  
• tuberculin test:
  • hypersensitivity to the tubercle bacillus is developed about 3 weeks afer initial infection
  • the Mantoux test is usually used

Extra-pulmonary TB can be more difficult to confirm bacteriologically, as more invasive tests are needed

As TB is an AIDS-defining illness, current guidelines state that it may be appropriate to offer HIV testing to patients if they are from an area or background with increased risk of HIV co-infection. Note however, the Joint Tuberculosis Committee of the British Thoracic Society is in the process of preparing guidelines that will advise that all TB patients aged between 15 and 64 years should be offered HIV testing (1).

Notes:

• NICE state for the investigation of possible pulmonary or laryngeal TB (1)
  • diagnosing pulmonary (including laryngeal) TB in adults
    • request rapid diagnostic nucleic acid amplification tests for the M. tuberculosis complex (M. tuberculosis, M. bovis, M. africanum) on primary specimens (listed in table 1) if there is clinical suspicion of TB disease, and:
      • the person has HIV or
      • rapid information about mycobacterial species would alter the person's care or
      • the need for a large contact-tracing initiative is being explored
        
        

• NICE have issued guidance concerning the diagnosis of latent TB (1):
• offer Mantoux testing to diagnose latent TB in adults aged 18 to 65 who are close contacts of a person with pulmonary or laryngeal TB If either is positive, assess for active TB; if this assessment is negative, offer them treatment for latent TB infection
  • If the Mantoux test is inconclusive, refer the person to a TB specialist
  • if the Mantoux test is positive (an induration of 5 mm or larger, regardless of BCG history), consider an interferon-gamma release assay

Reference:

1. Tuberculosis. NICE Guideline (January 2016 - last updated September 2019)14(3):9-12.