Blood tests may be better than standard skin test for diagnosing latent tuberculosis

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Two blood tests for diagnosing latent tuberculosis (TB) infection can individually produce fewer false-positive results than the standard tuberculin skin test, according to a study in this week's issue of The Lancet.

Eradication of TB in low-prevalence countries is a realistic aim. Diagnosis and treatment of people with latent infection, who are a reservoir for future cases, will be key. However, the standard diagnostic test for latent TB--the century-old tuberculin skin test--is unreliable, often producing false negative results in high-risk groups and false positives in others, such as those who have received BCG vaccine. Two commercially available blood tests, T-SPOT.TB and QuantiFERON-TB Gold, might be more reliable.

In a prospective study, Luca Richeldi (University of Modena and Reggio Emilia, Modena, Italy) and colleagues tested the effectiveness of all three tests in routine clinical practice in around 400 people. The performance of each blood test was compared with the skin test. They found that fewer BCG vaccinated individuals were identified as positive by either blood test than by the skin test.

Dr Richeldi states: "QuantiFERON-TB Gold and T-SPOT.TB show good diagnostic agreement with the skin test, but have higher specificity. As such either in combination with or as a substitute for the skin test, they could increase the diagnostic sensitivity of testing for latent TB infection."

The investigators also found that indeterminate and positive results differed between the two blood tests, especially in immunosuppressed patients and young children; these findings suggest that the new tests might produce different results in routine clinical practice. The authors state that the choice of diagnostic test to use should therefore depend on the population being tested, the purpose of testing, and the resources available.

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Contact: Luca Richeldi, Policlinic University Hospital, University of Modena and Reggio Emilia, Modena (Italy). T): +39-335-6142983 richeldi.luca@unimore.it


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