An active tuberculosis diagnosis is made by obtaining a positive sputum (or other fluid) culture for mycobacterium tuberculosis, or observation of tubercle bacilli and/or a positive TB culture of a pathological specimen. A suggestive chest x-ray with an apical infiltrate and/or cavity as well as an increase in the size of the area involved is also suspicious for TB. A positive AFB smear is most suspicious for TB, but other organisms can give such a positive smear (nocardia, aspergillus). A positive culture for TB is the sine qua non for the diagnosis of active TB.
Latent TB (LTBI)
An important distinction in the field of TB is the difference between a TB infection (latent TB infection - LTBI) and active TB. Active TB is defined as above (positive culture for TB). Patients with LTBI are suspicious for TB due to a positive tuberculin skin test, but lack of a positive TB culture. These patients require a chest x-ray to see if any TB lesions are present and if they are, a sputum TB culture is needed to rule out active TB.
LTBI patients are usually found by testing patients at risk for TB via a tuberculin skin test (PPD – purified protein derivative) or a new in-vitro assay of whole blood for cell-mediated based immunity, based on the release of interferon gamma from T-lymphocytes by stimulation resulting from an antigen ESAT-6 (see reading a TB test).
It is very important to be aware of patients who are infected with TB (LTBI), and who do not have active TB because it is from this group of people that reactivated cases of TB arise. This group of patients require careful follow-up if we are to eradicate TB.