Mycobacterium tuberculosis produces the antigens early secretory antigen target 6 (ESAT-6) and culture filtrate protein 10 (CFP-10). These antigens are not present in non-tuberculous mycobacteria, nor in BCG vaccine.

The blood tests QuantiFERON-TB Gold and T-SPOT.TB use these antigens to detect people with tuberculosis. Lymphocytes from the patient's blood are cultured with the antigens. If the patient has been exposed to tuberculosis before, T lymphocytes produce interferon γ in response. The test then uses ELISA to detect the interferon γ.

QuantiFERON-TB Gold quantifies the amount of interferon γ when whole blood is exposed to the antigens. T-SPOT.TB counts the number of activated T lymphocytes. These tests are called interferon γ tests and are not equivalent.

Guidelines for the use of the FDA approved QuantiFERON-TB Gold were released by the CDC in December 2005.

The enzyme linked immunospot (ELISPOT) blood test is another blood test available in the UK that may replace the skin test for diagnosis. PMID 14586040

Microbiological studies

Distinctive clusters of colorless Mycobacterium tuberculosis form in this culture.

Sputum smears and cultures should be done for acid-fast bacilli if the patient is producing sputum. If no sputum is being produced,examination of gastric juice, a laryngeal swab, bronchoscopy or fine needle aspiration should be considered. Other mycobacteria are also acid-fast. Even if sputum smear is negative, tuberculosis must be considered and is only excluded after negative cultures. Further PCR or gene probe tests can distinguish M. tuberculosis from other mycobacteria. If this is not available, a culture of the AFB can distinguish the various forms of mycobacteria, although results from this may take four to eight weeks for a conclusive answer.

Full blood count

Although a full blood count is never diagnostic, normocytic anemia and lymphopenia are common. Neutrophilia is rarely found.

Urea and electrolytes are usually normal, although hypocalcemia and hyponatremia are possible in tuberculous meningoencephalitis due to SIADHS. In advanced disease, hypoalbuminemia and hyperglobulinemia may be present.

Erythrocyte sedimentation rate is usually raised.