California Tuberculosis Controllers Association (CTCA): Position Statement
The CTCA strongly advocates the use of QuantiFERONTM (QFT) to diagnose latent TB infection (LTBI) in accordance with the recommendations of the Centers for Disease Control and Prevention (CDC) and encourages laboratories to make this test available to providers.
The tuberculin skin test (TST) has been used to diagnose LTBI for over 50 years and has remained virtually unchanged. The TST has many limitations and a test to replace it has been sought for years. Recognizing the lack of advancements in the area of TB diagnostics, both the Advisory Council for the Elimination of Tuberculosis and the Institute of Medicine (2000report) recommend the development and use of new TB diagnostic tools. The QFT test, approved by the FDA in November 2001, is such a tool.
Operational limitations of the TST are well known by practitioners. It requires two patient visits to obtain a reading, a requirement that takes considerable resources for patients and providers, yet continues to result in many unread TSTs. There is well-researched variability in measuring the size of a skin test even by trained health care providers. Patient factors such as prior BCG vaccination, immunosuppression, and infection with non-TB mycobacteria affect the TST and result in false negative and false positive readings. The TST can boost past sensitization to mycobacterial antigens leading to complex serial testing recommendations. All of these factors make the TST a frustrating test for patients and providers and have led to recommendations that are often poorly followed, accepted, or understood.
The QFT answers many of the limitations of the TST. It requires only one-visit to draw blood. It is an in vitro test with objective, results. It tests multiple antigens simultaneously and does not boost anamnestic immune responses. In a CDC-sponsored multicenter trial, there was84% agreement between TST and QFT results, and QFT demonstrated a specificity of 98%in low risk persons. Moreover, as compared to the TST, QFT appears to more reliably distinguish between BCG and TB sensitization. Limitations of QFT include the need for phlebotomy, processing of the whole blood within 12 hours of collection, lack of laboratories that process the test, and lack of institutional experience with interpreting this new test. Currently, the QFT laboratory test ranges from $10-$35 per specimen depending on the volume of testing being performed. While formal cost comparisons between QFT and the TST have not been performed, the elimination of the second visit for reading the TST, alone, is likely to make QFT competitive in cost-benefit considerations.
Currently, the CDC recommends the use of QFT in specific populations (MMWR January31, 2003, V52, RR02; 15-18). Because QFT has not been fully compared against the TST in many “high risk” groups, CDC has not endorsed its routine use in all situations. However, current studies are addressing these populations and evidence is gathering that QFT may outperform or be comparable to the TST in most groups, including TB suspects, children, older individuals, and contacts. Animal studies, where TB was proven by culture, showed QFT to be the superior test with a sensitivity of 93.6% vs. 65.6% and a specificity of 96-98% (Wood et al, Vet Micro40:125-135, 1994). Preliminary human studies in cases of active, untreated tuberculosis have shown similar results. A second generation QFT is currently being tested using more specific antigens, which promises even better accuracy.
The CTCA supports advances that are scientifically sound and technically superior. The use of an in vitro test for LTBI is now available and recommendations exist for its use. CDC guidelines should be consulted when considering groups for QFT testing (www.cdc.gov/nchstp/tb). Some initial groups in which to strongly consider this test include health care workers, others who will be serially tested, and newly arriving refugees. The QFT test or a similar blood assay is likely to become the test of choice for most or all populations in the future. The use of QFT will necessitate shifts in resources and existing TB testing infrastructure, as well as changes in state and local TB screening regulations. The CTCA promotes and supports these activities.
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