TB Diagnosis

EBRAHIM M. EBRAHIM, MBBS, MPH

Testing for Latent TB: (1) TST

Radiographs were used in the past but with poor sensitivity and specificity (Nelson & Williams, 2013)

Serology tests were extensively investigated but the number of Tubercle Bacilli is small and cannot elicit antibody immunity.

Testing for cellular immunity via inducing delayed-hypersensitivity (Tuberculin Skin Test, TST) was and is still the most widely used method of latent TB diagnosis. (PPD & Mantoux)

When a standardized amount of Tuberculin is injected intracutaneusly usually in the forearm; patients with TB should have a zone of induration. The width of the induration is measured after at least 48-72 hours of the injection and requires a trained observer. Erythema is present but should not confuse as induration which is to be observed.

Mycobacterial infections may produce cross-reactivity and false positive results, similarly does vaccination with BCG, however, the strength of reactivity is less and no longer interferes with TST after 3 to 5 years after.

Requires intact immunity to achieve maximal reactivity and induration.

(CDC, 2010)

(2) Interferon Gamma Release Assays (IGRA) Modern test; identifies the production of specific interferon gamma by T-cells in response to mycobacterial antigens, (interferon gamma maintains the granuloma and confines the boarders for bacilli). Performed in lab with patient’s blood sample.

Quatiferon Gamma Interferon Gold is the most famous commercially available IGRA, recommended for use in developed countries.

TST is more widely available in health clinics and can be used in middle-income and developing countries.

More specific than TST, in

BCG vaccinated patients

(Pai, 2013).

(Lee, Kim, & Lee, 2011)

From the World Health Organization Use of tuberculosis interferon-gamma release assays (IGRAs) in low-and middle-income countries: policy statement (2011).

There is insufficient data and low quality evidence on the performance of IGRAs in low- and middle-income countries, typically those with a high TB and/or HIV burden;

• IGRAs and the TST cannot accurately predict the risk of infected individuals developing active TB disease;

• Neither IGRAs nor the TST should be used for the diagnosis of active TB disease;

• IGRAs are more costly and technically complex to do than the TST. Given comparable performance but increased cost, replacing the TST by IGRAs as a public health intervention in resource-constrained settings is not recommended

Use of TST and IGRA in active TB:

Case definition:

Testing for Active TB: (1) Chest Radiography

Chest X-rays (CXR) are essential in diagnosing active TB and lesions are upper lobe in reactivation and middle or lower lobe in primary pulmonary infection.

Generally good sensitivity in active TB but poor specificity (Pai, 2013).

Chest CT scan can give clearly image specially in fine and smaller lesions, however, not required in ordinary cases.

CXR are not much useful in latent TB, however, a negative smear along with radiologic findings consistent with TB indicate inactive TB.

(2) Microscopy for Acid Fast Bacilli (AFB)

Direct microscopy is the most widely available in primary health care lab facilities. However, inconvenient to patients as multiple visits required to submit sputum specimens for examination.

Uses Zeihl Nelson Stain and was standardly done at least twice over two days.

Nearly 60% of culture positive stain positive in smears. In addition to that some other infections are acid fast and appears relatively similar (e.g. Nocardia).

The WHO in the same day diagnosis of TB by microscopy policy statement recommends submission of two sputum specimens on the same day to avoid patients drop and make it less inconvenient.

Same-day sputum smear microscopy is as accurate as standard smear microscopy. Compared with the standard approach of examination of two smears with ZN light microscopy over 2 days, examination of two smears taken on the same day had much the same sensitivity (64% [95% CI 60 to 69] for standard microscopy vs 63% [58 to 68] for same-day microscopy) and specificity (98% [97 to 99] vs 98% [97 to 99]) (Davis, et al. 2013)

Sputum smear microscopy is commonly used for diagnosing tuberculosis (TB). Although patients with sputum smear–negative TB are less infectious than patients with smear-positive TB, they also contribute to TB transmission.

In some countries, patients with smear-negative, culture-positive TB are responsible for 13% of TB transmission (Tostmann, et al. 2008).

Countries that have ample resources should expand their TB-control efforts to include prevention of transmission from patients with smear-negative, culture-positive pulmonary TB.

AFB Microscopy,… Cont.

(3) Mycobacterium TB Culture

Gold Standard.

Routine culture uses a nonselective egg medium (Lowenstein-Jensen or Middlebrook 7H10) and often requires more than 3-4 weeks because of the 22-hour doubling time of M tuberculosis.

Radiometric broth culture (BACTEC radiometric system) of clinical specimens was found to significantly reduce the time (10-14 days) for mycobacterial recovery.

Blood culture is also used, particularly in HIV patients as prevalence of bacilliema is high.

(4) Nucleic Acid Amplification Tests, e.g. PCR.

Ribosomal RNA probes and DNA PCR assays allow identification within 24 hours.

The DNA probes are approved for direct testing on smear-positive or smear-negative sputa. However, smear-

positive specimens yielded higher sensitivity.

In a restrospective cohort, Mycobacterium tuberculosis direct nucleic acid amplification testing (NAAT) yielded

improved diagnostic accuracy, shortened time to diagnosis, and reduced unnecessary treatment (Marks, et al.

2013).

(5) Xpert MTB/RIF

Called the game-changer in TB diagnostic world.

Fully automated system that allows operator to perform sample DNA amplification, and detection of M.

Tuberculosis and rifampicin resistance in just two hours with very little hands operation.

When used as an initial test replacing smear microscopy (15 studies, 7517 participants), Xpert achieved a

pooled sensitivity of 88% (95% CI: 83%-92%) and pooled specificity of 98% (95% CI: 97%-99%). The pooled

sensitivity was 98% (95% CI: 97%-99%) for smear-positive, culture-positive TB and 68% (95% CI: 59%-75%)

for smear-negative, culture-positive TB (15 studies); the pooled sensitivity was 80% (95% CI: 67%-88%) in

people living with HIV and 89% (95% CI: 81%-94%) in people without HIV infection (four studies).

For rifampicin resistance detection (11 studies, 2340 participants), Xpert achieved a pooled sensitivity of

94% (95% CI: 87%-97%) and pooled specificity of 98% (95% CI: 97%-99%) (Steingart, et al. 2014).

Testing of patient’s contacts:

It is recommended that contact investigation be conducted for house - hold and close contacts when the index case has any of the following characteristics:

• Has sputum smear-positive pulmonary TB,

• Has multi-drug-resistant TB (MDR-TB or extremely-resistant TB (XDR-TB) (proven or suspected),

• Is a PLHIV or

• Is a child < 5 years of age. Strong recommendation, very low-quality evidence.

Tostmann, A., Kik, S. V., Kalisvaart, N. A., Sebek, M. M., Verver, S., Boeree, M. J., & van Soolingen, D. (2008). Tuberculosis transmission by patients with smear-negative pulmonary tuberculosis in a large cohort in the Netherlands. Clinical Infectious Diseases, 47(9), 1135-1142.

Lee, J. E., Kim, H. J., & Lee, S. W. (2011). The clinical utility of tuberculin skin test and interferon-γ release assay in the diagnosis of active tuberculosis among young adults: a prospective observational study. BMC infectious diseases, 11(1), 96.

Davis JL, Cattamanchi A, Cuevas LE, Hopewell PC, Steingart K. Diagnostic accuracy of same-day microscopy versus standard microscopy for pulmonary tuberculosis: a systematic review and meta-analysis. Lancet Infect Dis 2013;13:147-54. .

Marks, S. M., Cronin, W., Venkatappa, T., Maltas, G., Chon, S., Sharnprapai, S., ... & Crosby, C. (2013). The health-system benefits and cost-effectiveness of using Mycobacterium tuberculosis direct nucleic acid amplification testing to diagnose tuberculosis disease in the United States. Clinical infectious diseases, cit336.

Steingart, K. R., Schiller, I., Horne, D. J., Pai, M., Boehme, C. C., & Dendukuri, N. (2014). Xpert® MTB/RIF assay for pulmonary tuberculosis and rifampicin resistance in adults. The Cochrane Library.

World Health Organization. (2011). Same-day diagnosis of tuberculosis by microscopy: policy statement.

World Health Organization. (2011). Use of tuberculosis interferon-gamma release assays (IGRAs) in low-and middle-income countries: policy statement.

World Health Organization. (2012). Recommendations for investigating contacts of persons with infectious tuberculosis in low-and middle-income countries. World Health Organization.

References