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Alaska Tuberculosis Program Manual - Epidemiology - State of Alaska

Alaska Tuberculosis Program Manual - Epidemiology - State of Alaska

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Diagnosis <strong>of</strong> Latent <strong>Tuberculosis</strong> Infection<br />

The diagnosis <strong>of</strong> latent tuberculosis infection (LTBI) has traditionally been based upon<br />

results <strong>of</strong> tuberculin skin testing. However, the QuantiFERON®-TB Gold (QFT-G) test<br />

and the QuantiFERON®-TB Gold in-tube (QFT-GIT) test, which are whole-blood<br />

interferon gamma release assays (IGRAs), are now other options for detecting LTBI.<br />

Use the Mantoux tuberculin skin test (TST) or an IGRA to test for Mycobacterium<br />

tuberculosis infection. QFT-GIT or QFT can be used in all circumstances in which the<br />

TST is used, and QFT-GIT or QFT usually can be used in place <strong>of</strong> (and not in addition<br />

to) the TST. 5<br />

Interferon Gamma Release Assays (IGRA) are not widely available in<br />

<strong>Alaska</strong>. Reference laboratories currently provide QuantiFERON®-TB Gold<br />

in-tube (QFT-GIT) testing in select communities such as Anchorage and<br />

Fairbanks.<br />

Mantoux Tuberculin Skin Testing<br />

The Mantoux method <strong>of</strong> tuberculin skin testing (TST) is used to detect infection with<br />

Mycobacterium tuberculosis.<br />

In general, it takes 2 to 10 weeks after infection for a person to develop a delayed-type<br />

immune response to tuberculin measurable with the Mantoux TST. 6 During the test,<br />

tuberculin is injected into the skin. The immune system <strong>of</strong> most persons with<br />

tuberculosis (TB) infection will recognize the tuberculin, causing a reaction in the skin.<br />

Repeated TSTs do not produce hypersensitivity.<br />

The size <strong>of</strong> the measured induration (a hard, dense, raised formation) and the patient's<br />

individual risk factors should determine whether TB infection is diagnosed. 7<br />

Based on the sensitivity and specificity <strong>of</strong> the purified protein derivative (PPD) TST and<br />

the prevalence <strong>of</strong> TB in different groups, three cut-points are used by CDC for defining a<br />

positive tuberculin reaction:<br />

Greater than or equal to 5 mm<br />

Greater than or equal to 10 mm<br />

Greater than or equal to 15 mm <strong>of</strong> induration 8<br />

For more information on cut-points for the TST, see the “Interpretation <strong>of</strong><br />

the Tuberculin Skin Test” topic in this section.<br />

A L A S K A T U B E R C U L O S I S C O N T R O L P R O G R A M M A N U A L Diagnosis <strong>of</strong> Latent Tuber culosis Infection<br />

R e v i s e d N o v e m b e r 2 0 1 2<br />

7 . 7

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