Tuberculosis: Laboratory Support of Screening and Diagnosis
Although tuberculosis (TB) is treatable, the disease remains a common cause of morbidity and mortality in many regions of the world, including parts of the United States.1,2 TB may be latent (noninfectious and asymptomatic) or active (infectious and symptomatic) and can progress from latent to active TB.1,2 Notably, latest World Health Organization (WHO) guidelines updated the nomenclature for latent TB to “TB infection” and active TB to “TB disease.”3
Risk factors for TB have been identified (see Sidebar),1 and professional organizations have provided guidelines regarding screening for TB infection.
This article discusses TB, risk factors, and screening guidelines. It also discusses the traditional tuberculin skin test (TST) and more recently developed TB blood tests, as well as the indications for each type of test.
Tuberculosis infection and disease
Tuberculosis is a highly transmittable respiratory infection caused by inhalation of droplets containing the bacterium Mycobacterium tuberculosis.1,2 Approximately 30% of people exposed to M tuberculosis (close contact with a person who has TB disease) develop TB (infection or disease).1,2,4
The prevalence of TB infection in the United States is about 5% overall5 but is much higher in certain populations (eg, 23%-88% among incarcerated persons and 19%-80% among people without homes).5 States with the highest TB case rates (TB disease and TB infection) include Alaska, California, the District of Columbia, Maryland, Texas, New Jersey, and New York.6
Persons with TB infection are asymptomatic and not infectious. Overall, only about 5% to 10% of persons with untreated TB infection will develop TB disease.1 However, the rate is much higher in persons with immunosuppression (eg, due to medications or diseases such as HIV infection).1
TB disease can cause symptoms including cough with or without blood-tinged sputum, general malaise, and intermittent fever.1,2 Although TB infection begins in the lungs, it can spread to the kidneys, spine, and brain. Untreated TB disease can be fatal.1,2,4 Notably, about 80% of TB disease cases progress from TB infection (20% of persons exposed to TB progress directly to TB disease).1
Importantly, treatment of TB infection can prevent or reduce disease progression, transmission, and TB-related morbidity and mortality.1,2 The following sections discuss current screening guidelines for TB infection.