Tuberculosis: Likelihood Ratios

Introduction: Tuberculosis is a major health hazard due to its high contagiousness and ability to exist in a latent state in the host.

Note that the tests have all been assessed for patients with active TB (since there is no gold standard for latent TB).

There are two major tests being used currently to detect latent TB infection - the PPD skin test and the T cell activation blood test.

Note that these are also used in the evaluation of active TB, though they are supplanted (for pulmonary TB) by chest x-ray and sputum AFB stain/culture.

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The likelihood ratios of findings for Tuberculosis are listed below. See the left navigation bar to change the display.

Good Positive and Negative Likelihood Ratios

Finding	+LR	-LR	Comments, Study
T-Spot Duplicate	inf	0.1	active TB, HIV-negative adults specificity is vs bCG-vaccinated population Study: Am J Respir Crit Care Med. 2006 Oct 1;174(7):736-42. PMID: 16799073
T-Spot Duplicate	inf	0.1	active TB, HIV-positive adults specificity is vs bCG-vaccinated adults Study: Am J Respir Crit Care Med. 2006 Oct 1;174(7):736-42. PMID: 16799073
Quantiferon	27	0.2	active TB, HIV-negative adults specificity is vs bCG-vaccinated adults Study: Am J Respir Crit Care Med. 2006 Oct 1;174(7):736-42. PMID: 16799073
Sputum AFB Smears	27	0.3	active TB. Three successive smears Study: J Clin Microbiol. 2002 Sep;40(9):3482-4. PMID: 12202598

Good Negative Likelihood Ratio

Purified Protein Derivative (PPD)

1.4

0.3

adults with active TB

>=10mm threshold for induration

specificity in Japanese adults with a high rate of bCG vaccination. Note that non-TB mycobacterium (e.g. avium) also causes false positives: in US health care workers, in one study 50% of indurations 10-14mm were from M avium (PMID 11769770).

Some centers use a 'booster' or serial PPD strategy whereby a second PPD is placed 1-4 weeks after the first. The idea is to generate a stronger immune reaction in the second test, given that the reaction may be blunted if the initial TB exposure was long ago. This has not been evaluated for specificity.

Study: sensitivity from UpToDate http://www.utdol.com/online/content/topic.do?topicKey=tubercul/13154

specificity from Am J Respir Crit Care Med. 2004 Jul 1;170(1):59-64. PMID: 15059788