Annals of Long Term Care

Preventing active tuberculosis (TB) in nursing homes is a high priority. Prevention entails identifying newly admitted individuals who have latent TB and testing residents who have had contact with an individual with active TB, as may occur during a hospitalization. Until several years ago, a tuberculin skin test (TST) was the only modality available to screen for latent TB and to test exposed residents (contact investigation). Three blood tests, which work by detecting interferon released by sensitized T-cells, are now commercially available for this purpose, including T-SPOT.TB, QuantiFERON-TB Gold (QFT-G), and QuantiFERON-TB Gold In-Tube (QFT-GIT). We report the case of a nursing home resident who was exposed to an individual with active TB during a hospitalization and discuss the use of TST versus blood tests for contact investigations.

Case Presentation
A frail 83-year-old woman returned to our nursing home, where she has been a resident for 4 years, after a 10-day hospital stay. In the hospital, she was treated for left middle and upper lobe pneumonia. The patient had a history of moderate dementia, seizure disorder, hypertension, and hypothyroidism. Upon admission to the intensive care unit (ICU), she was receiving bilevel positive airway pressure for hypoxia and intravenous vancomycin and piperacillin/tazobactam for healthcare-associated pneumonia. After 6 days in the ICU, she was transferred to a double room, where she stayed until discharge. For the last 4 days of her hospital stay, her roommate was a young woman being treated for community-acquired pneumonia and a lung abscess. Forty-eight hours after the patient returned to the nursing home, we were informed that her roommate had active TB, diagnosed by positive sputum smears and an aspiration specimen from the lung abscess.

Upon being admitted to the nursing home 4 years earlier, the patient underwent a two-step purified protein derivative (PPD) skin test, which was negative. TST testing was repeated 4 days after her return to the nursing home from the current hospital admission and was negative. A QFT-GIT test performed 1 week after her return was also negative. A chest radiograph done 6 weeks after her hospital stay showed complete resolution of her pneumonia. The QFT-GIT test was repeated 2 months later and was negative. A TST was also repeated 8 weeks after her return from the hospital and was negative. The initial sputum cultures on the patient’s roommate subsequently grew Mycobacterium tuberculosis.

Discussion
Nursing home residents should be screened for latent TB because they reside in an environment where there is an increased concentration of susceptible individuals, and the rate of TB reactivation increases with age.1 Before blood tests measuring T-cell interferon release were available, latent TB was defined as “a positive TST result in an asymptomatic person exposed to TB with no clinical or radiographic signs of active TB.”2 TST was the only method for detecting latent TB for 110 years, which was largely due to the false impression that TB ceased being a public health risk following the advent of effective antimicrobial therapy in 1946. As a result, little effort or funding was directed toward improving diagnostic methods. With TB infection rates increasing over the last 20 years, however, interest in developing other testing methods grew.

TB Blood Tests
TB testing methods that have become available over the last several years are based on the ex-vivo measurement of interferon-gamma released by circulating T cells or mononuclear cells in response to specific M. tuberculosis antigens. Development of these testing methods was made possible by advances in the delineation of the M. tuberculosis genome. Proteins encoded by sequences in the genome that are not present in the Bacille Calmette-Guérin (BCG) vaccine or in most environmental mycobacteria are used in the tests.