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After two decades of steady, incremental decline, tuberculosis in the United States has leveled off at some 3 cases per 100,000 people — a rate that will not result in the goal of TB elimination (less than 1 case per million), the CDC reports.1
“Epidemiologic modeling suggests that even if the previously observed annual declines in the United States had been sustained, TB elimination … would not occur by the end of this century,” the CDC stated. “The determinants of this leveling in TB incidence are not yet clear; further evaluation of available data is required to understand the causes of this trend.”
The 1985-1992 resurgence of TB in the U.S. was attributed to HIV coinfection, immigration from countries with higher prevalence, and increased transmission. However, the proportion of TB patients coinfected with HIV has declined substantially in the U.S., and TB incidence among U.S. foreign-born persons — while still high — has continued to decline. In contrast, the stabilization of TB incidence among U.S.-born persons, together with evidence provided by molecular genotyping of TB cases, demonstrates that TB transmission within the U.S. continues to occur, the CDC noted in data released in conjunction with World TB Day March 24.
“The continued occurrence of TB cases among U.S.-born children is further corroboration, because TB disease in a young child is a sentinel event representing recent infection,” the CDC warned. “Substance abuse, incarceration, and homelessness associated with TB outbreaks highlight some of the complicated case management work required on the health department frontlines of TB control.”
The CDC’s key points on TB in the U.S. include the following:
Though the latest TB data underscore the continuing threat of exposures to healthcare workers, there is considerable confusion about the current testing options.
Variations in results between TB skin testing (TST) and gamma-interferon-release assays (IGRA) continue to stump providers, who are unsure which test to believe and how to proceed, says Carol Kemper, MD.
Kemper recently reviewed1 a study that says it all in its title: “Diagnosis of latent tuberculosis infection: Too soon to pull the plug on the tuberculin skin test.”2
It is increasingly recognized that TST and IGRAs are not uniformly correlated, and they measure different “arms” of the immune system response, she notes. To confuse matters further, the three IGRAs currently approved for use in the United States (the QuantiFERON-TB test, the QuantiFERON TB Gold In-Tube test, and the T-Spot.TB test) each employ different antigens, with different interpretative cut-offs, and the interpretative cut-offs used in published studies often differ from those used by the FDA for approval.
In low-risk populations, e.g., healthcare workers (HCWs), discordance between TST and IGRA results occurs at least 17% of the time.
“In part, this is related to the vagaries of IGRA testing in those at lower risk — 52% to 65% of HCWs at low risk for TB who had an initially positive IGRA result ‘reverted’ to a negative ‘result on repeat’ testing,” Kemper noted in her review. “For those with low-level positive test results, 75% to 80% reverted their test on repeat testing. For this reason, in those at low risk with low positive results, it is becoming commonly accepted practice to repeat a positive IGRA test in two to eight weeks. In contrast, documented conversion of a TST is generally accepted as a true positive.”
Given this, Hospital Employee Health asked Kemper if would be preferable to use TST for annual screening or possible exposures rather than IGRA. That ship has sailed, she implied, or in parlance of the study authors, that plug has been pulled.
“Sadly, I think the TST is long past for employee screening,” Kemper tells HEH. “It’s too expensive in people-time to have 2,000 [HCWs] in a hospital get a skin test done and two days later have it read — not to mention those that never show up two days later. So, even more sadly, we will continue to spend extra money on labs and have people go to the lab for an extra blood draw [for IGRA], which is somehow easier, but not really a lower risk, and really provides no additional good information. In the meantime, the employee will be wondering if they really have been exposed to TB — and worrying.”
Financial Disclosure: Senior Staff Writer Gary Evans, Managing Editor Jill Drachenberg, and Consulting Editors/Nurse Planners Kay Ball and MaryAnn Gruden report no consultant, stockholder, speaker’s bureau, research, or other financial relationships with companies having ties to this field of study.