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Outpatient Practice Management Tips Testing for Latent Tuberculosis Asad Ayub, MD, Department of Internal Medicine, Marshfield Clinic, Marshfield, Wisconsin Steven H. Yale, MD, Department of Internal Medicine, Marshfield Clinic, and Clinical Research Center, Marshfield Clinic Research Foundation, Marshfield, Wisconsin Kurt D. Reed, MD, Department of Clinical Pathology, Microbiology Section, Marshfield Laboratories, and Clinical Research Center, Marshfield Clinic Research Foundation, Marshfield, Wisconsin Rana M. Nasser, MD, Department of Infectious Diseases, Marshfield Clinic, Marshfield, Wisconsin Steven R. Gilbert, MD, Pulmonary and Critical Care, Marshfield Clinic, Marshfield, Wisconsin REPRINT REQUESTS :
Steven H. Yale, MD Department of Internal Medicine Marshfield Clinic 1000 North Oak Avenue Marshfield, WI 54449 Telephone: 715-387-5436 Fax: 715-389-3808 Email:
[email protected]
KEYWORDS :
Tuberculosis; Tuberculin test; Mycobacterium tuberculosis; Interferon Type II/diagnostic use; BCG vaccine
RECEIVED :
JUNE 15, 2004 REVISED AND ACCEPTED :
JULY 16,2004
Clinical Medicine & Research Volume 2, Number 3: 191-194 ©2004 Clinical Medicine & Research http://www.mfldclin.edu/clinmedres
Tuberculosis (TB) is a common cause of morbidity and mortality worldwide. The number of new cases continues to increase despite intense efforts to eradicate the disease. According to the World Health Organization, the global incidence rate of TB increased to 8.8 million cases in 2002 with a growth rate of 2.4% per year.1 In the United States, about half of the new cases occur in persons immigrating to this country.2 Treatment of persons with active TB is the number one priority targeted to prevent the spread of infection. A secondary priority, particularly in developed countries where there is a lower incidence of the disease, is the identification and treatment of persons with latent TB infection (LTBI).3 The majority of new cases of TB are derived from a pool of patients with LTBI. Thus, identifying and treating persons with latent infections is an important public health measure, since it not only prevents active TB from occurring in individuals with latent infection, but also prevents the interpersonal spread of the disease. Understanding the available tests and how to correctly interpret and apply the results is the first step in recognizing patients at risk and in providing appropriate treatment. Studies available to diagnose LTBI The goal of testing for LTBI is to identify persons who are at increased risk for developing TB and who would benefit from treatment of the infection.4 This includes healthcare workers and persons who have had recent contact with someone who has active TB as well as those with underlying medical conditions such as diabetes mellitus and lymphomas (tables 1 and 2). Since only persons who would benefit from treatment should be tested, a decision to test presupposes a decision to treat if the test result is positive. There are two tests available to confirm a diagnosis of LTBI: the tuberculin skin test, and the measurement of interferon-γ in whole blood. The tuberculin skin test has been available since the 1900s and is based on the observation by Robert Koch that infection with Mycobacterium tuberculosis caused cutaneous reactivity to tuberculin, the heat-killed, purified protein derivative (PPD) from cultures of M. tuberculosis. The standard tuberculin test consists of an intracutaneous injection of 0.1 ml (5 tuberculin units) of PPD into the volar forearm
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Table 1. Increased risk and who should be tested for latent tuberculosis infection. Risk
Persons at risk
Increased risk due to exposure of infectious cases
Persons with recent close contact with person known to have active TB Children
