Diagnosis and treatment of active pulmonary tuberculosis

Causes and risk

Almost all infections with M. tuberculosis are due to inhalation of droplet nuclei 1-5 microns in diameter, aerosolized by coughing, sneezing or talking from a person with pulmonary tuberculosis; they dry while airborne and remain suspended for long periods, and the air in a room occupied by such a person may remain infectious for approximately 30 minutes even after his or her absence. Infection does not generally occur out of doors, because M. tuberculosis is killed by ultraviolet light. Prolonged exposure and multiple aerosol inocula are usually required to establish the infection, and the most important determinants are closeness of contact and infectiousness of the source.

How it is diagnosed

Unexplained cough for more than 2-3 weeks, in regions where TB is prevalent, or typical chest X-ray changes, should prompt further investigation.

Specimens in pulmonary disease are sputum, preferably 3 samples including an early morning sample, induced with nebulised hypertonic saline if the patient is not expectorating; bronchoscopy with washings or bronchoalveolar lavage; and gastric washing, mainly used for children.

• Direct microscopy of sputum is the most important first step, and is typically positive when 5000-10 000 organisms are present. Stains are Ziehl-Neelsen and auramine fluorescence.
• Culture on solid media (Lowenstein-Jensen, Middlebrook) takes 4-6 weeks; in liquid media such as BACTEC or MGIT growth is faster, 1-3 weeks.
• Nucleic acid amplification, such as the Xpert/RIF test, is useful at the initial stage only, since it frequently remains positive despite treatment through detection of dead organisms. It also detects rifampicin resistance.

Baseline blood tests are FBC, CRP, ESR, U&E and LFTs.

It is suspected based on the clinical presentation: a cough of more than 2 to 3 weeks' duration, lymphadenopathy, fevers, night sweats, weight loss; and a history of prior TB infection or disease, known or possible TB exposure, and past or present residence in or travel to an area where TB is endemic.

And before treatment begins, LFT and RFT should be done in all patients.