FAQs from CDC: Treating latent TB infection

These questions and answers were excerpted from the 2005 CDC "Guidelines for Preventing the Transmission of TB in Health-Care Settings":

Q: A health care worker who has been vaccinated with BCG is being hired. She states that BCG will make her TST result positive and that she should not have a TST. Should this HCW be exempted from baseline two-step TST?

A: Unless she has documentation of a positive TST result or previously treated LTBI or TB disease, she should receive baseline two-step TST or one BAMT. Some persons who received BCG never have a positive TST result. For others, the positive reaction wanes after five years. U.S. guidelines state that a positive TST result in a person who received BCG should be interpreted as indicating LTBI.

Q: Does BCG affect TST results and interpretations?

A: BCG is the most commonly used vaccine in the world. BCG might cause a positive TST (i.e., false positive) result initially; however, tuberculin reactivity caused by BCG vaccination typically wanes after five years but can be boosted by subsequent TST. No reliable skin test method has been developed to distinguish tuberculin reactions caused by vaccination with BCG from reactions caused by natural mycobacterial infections, although TST reactions of ≥ 20 mm of induration are not usually caused by BCG.

A: What steps should be taken when an HCW has had a recent BCG vaccination?

Q: When should the TST be placed? A TST may be placed anytime after a BCG vaccination, but a positive TST result after a recent BCG vaccination can be a false-positive result. QFTG should be used, because the assay test avoids cross-reactivity with BCG.

Q: Who should be treated for LTBI?

A: Persons with LTBI who are at increased risk for developing TB disease should be offered treatment for LTBI regardless of age, if they have no contraindication to the medicine.

Q: What are contraindications to treatment of LTBI?

A: Active hepatitis and end-stage liver disease are contraindications to the use of INH for treatment of LTBI. Persons who have these conditions might be eligible for rifampin for four months for treatment of LTBI. Because of the substantial and complex drug-drug interactions between rifamycins and HIV protease inhibitors (PI) and non-nucleoside reverse transcriptase inhibitors (NNRTI), clinicians are encouraged to seek expert advice if the concurrent use of these drugs is being considered in persons infected with HIV. Information regarding use of these drugs is available at www.cdc.gov/nchstp/tb/tb_hiv_drugs/toc.htm.

Q: Do persons need to be in a specific age range to be eligible for treatment of LTBI?

A: No age restriction for eligibility of treatment for LTBI currently exists. Targeted TST programs should be conducted for persons at high risk, and these programs are discouraged for persons or settings considered to be low risk. However, for infection control programs that conduct TB screening that includes HCWs who are frequently at low risk, proper medical evaluation needs to be conducted when an HCW with a positive TST result is identified. In this context, age might be a factor in the decision to administer treatment, because older persons are at increased risk for hepatic toxicity caused by INH.

Q: What is the preferred regimen for treatment of LTBI?

A: Nine months of daily INH is the preferred treatment regimen for patients who have LTBI. The six-month regimen of INH or the four-month regimen of rifampin are also acceptable alternatives.

Q: Why is the two-month regimen of rifampin and pyrazinamide (RZ) generally not offered for treatment of LTBI? Although the two-month regimen of RZ was previously recommended as an option for the treatment of LTBI, reports of severe liver injury and death prompted the American Thoracic Society and the CDC to revise recommendations to indicate that this regimen should generally not be offered for treatment of LTBI.