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Latent TB infection

Latent TB is usually diagnosed by a positive TB skin test (TST) and a whole blood test called Interferon-Gamma Release Assay (IGRA). These are used for patients who are at a high risk of contracting the disease and for those who work or live with high-risk patients. The TB skin test may also be done as part of a physical examination prior to starting school or a new job. Positive results may indicate a latent TB infection or previous BCG vaccination and should be followed by a clinical assessment.

In Australia, TST is a Mantoux type skin injection of tuberculin that relies on detecting delayed type hypersensitivity at 48-72 hours.

IGRA requires approximately 3 mL of blood to be collected into three separate tubes which should be promptly transported to a laboratory and incubated at 37°C within 16 hours of collection. IGRA and TST have equal diagnostic sensitivity but IGRA can distinguish between infection with Mycobacterium tuberculosis and BCG vaccination.

Active tuberculosis

To diagnose active TB of the respiratory tract, at least 2-3 sputum specimens are collected for microscopy and culture. The best samples are obtained first thing in the morning when sputum is most likely to contain the most TB bacteria. Serial sputum samples increase the yield. Approximately 85, 10 and 5% of all cases detected by three serial examinations are found on the first, second and third sputum examination, respectively.

If extrapulmonary TB is suspected, samples are collected based upon where in the body the infection is likely to be. Multiple samples of gastric washings/aspirates or urine may be collected and submitted to the laboratory. Sometimes cerebral spinal fluid (CSF), biopsied tissue, or other body fluids are also collected.

A presumptive diagnosis of TB can be made by examining a smear of the patient's specimen under the microscope after it has been stained with a special stain to detect acid fast bacteria or AFB. Although simple and quick, microscopy has an overall sensitivity of only 30-80% and is less useful for TB disease in children or immunocompromised patients and in extrapulmonary TB. Positive microscopy results are often reported in the WHO scale:

Microscopic detection of TB: staining for acid fast bacteria (AFB)
‘+’ indicates presence of AFB
Report (WHO scale) AFB detected in at least 100 high power microscopy fields (HPFs)
Negative No AFB detected
Occasional 1-9 AFB per 100 HPFs
+ 10-99 AFB per 100 HPFs
++ 1-10 AFB per field in at least 50 HPFs
+++ more than 10 AFB per field in at least 20 HPFs.

Positive AFB smears cannot distinguish between the different species of ‘acid-fast’ bacilli and AFB cultures are set up on all samples submitted for microscopy for AFB. Nutrients and incubation provide a supportive environment for the slow growing mycobacteria. The results of cultures are definitive: they can tell your doctor what organisms are present and what drugs are likely to kill them but they take time - usually 2-3 weeks (but up to 8 weeks) for positive samples. 

PCR tests are available for TB and can be performed on a wide range of sample types. PCR tests give a more rapid result than culture, however are not as sensitive and cannot not provide the detailed information available from culture. PCR testing for Mtb performed directly on patient samples is currently used as an adjunct to microscopy, culture and susceptibility testing. 

Once M. tuberculosis has been identified and treatment has begun, AFB smears and cultures are used to monitor the effectiveness of treatment.

Non-laboratory tests

X-rays are often used as a follow-up to positive TB skin tests to look for signs of mycobacteria growth and to help determine whether someone has active tuberculosis or a latent TB infection. Infection with TB can cause a number of characteristic findings on x-rays, including cavities (holes) and calcification in organs such as the lungs and kidneys.

Last Review Date: August 14, 2017