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Pathology & Laboratory Medicine
Healthcare providers have a choice when it comes to tuberculosis testing—they can opt for the century-old tuberculin skin test (TST), which requires 2 visits and subjective judgment, or they can choose a simple, one-step, objective blood test.
“Unfortunately, nearly 54% of the assays are still done using the skin test, even though there are limitations and challenges of that test,” commented Hema Kapoor, MD, SM, D(ABMM), senior medical director of infectious disease and immunology and Global Diagnostics Network at Quest Diagnostics. “We are making progress, but we are not there yet.” Dr Kapoor spoke on a recent edition of Becker’s Healthcare Podcast.
Limitations of the skin test
There are several limitations to the TST, Dr Kapoor explained. The proteins in the test reagent react not only with proteins from Mycobacterium tuberculosis, the causative agent of TB, but also with the anti-TB bacillus Calmette–Guérin (BCG) vaccine, as well as with certain non-TB mycobacteria, which may lead to a false-positive result in vaccinated patients. In addition, administration of the skin test requires a significant level of skill to make sure the proteins are injected between the layers of the skin and that a raised wheal is produced. “If that doesn't happen, the person who is performing the test should repeat the test immediately,” Dr Kapoor said. “And I'm not sure how often that we see that actually happening.”
Finally, she pointed out, the TST requires 2 office visits—the first to administer the test, and the second to read the result. Not only does that put an extra burden on both the patient and healthcare providers, but that second visit must be between 48 and 72 hours after the first. “If you do not read the test in that window, then it can either give you false-negative or false-positive results.”
TB blood tests offer significant advantages
The modern alternative to the TST is the interferon-gamma release assay (IGRA), which is run with a single blood sample, and requires only a single patient visit, which may occur in the healthcare provider’s office or at any one of the over 2000 Quest Patient Service Centers across the country.
The IGRA detects the release of interferon-gamma from white blood cells of a person who has been infected with TB, whether latent or symptomatic. Results are read objectively, unlike the TST.
- Quest offers two types of IGRA for TB:
The T-Spot®.TB offers 95.6% sensitivity and 97.1% specificity, and is approved for immunocompromised patients as well as the general population1
- The QuantiFERON®-TB Gold Plus (QFT-Plus) offers 94% sensitivity and 97.1% specificity, and delivers a more comprehensive evaluation of patients’ immune responses to TB2
Both tests have proscribed collection and transport requirements, which are simple and easily met by any clinical lab. Additionally, each patient sample is run with controls, to maintain a high degree of specificity and sensitivity.
“We encourage physicians, when they make the switch from the TST to the blood test, to be familiar with the principles of the technology, and we are there to help them out with their questions,” Dr Kapoor said.