A new interferon-gamma release assay (IGRA) for diagnosing tuberculosis performs as well as an earlier test, but uses only a third of the blood, researchers have found.
"Childhood TB is a huge problem," Dr. Pernille Ravn from University Hospital Hillerod in Denmark told Reuters Health by email. "IGRAs are an improvement compared to the tuberculin skin test (TST), but the use is still limited by cost, infrastructure, and the lack of ability to discriminate between active and latent TB."
Dr. Ravn and colleagues compared the new Quantiferon Microtube (QFT-MT) assay, a prototype that uses only 0.9 mL blood, with the Quantiferon-TB (QFT-IT) assay, which is commercially available and requires 3 mL of blood. They recruited 152 children with suspected TB and 87 adults with confirmed TB from Muheza designated district hospital in Tanga, Tanzania.
Overall, there were no differences in the results of the QFT-IT and the QFT-MT assays, the researchers report online August 9 in the European Respiratory Journal. And the strength of the agreement between the tests was moderate in children and adults.
Median interferon-gamma levels were significantly higher in QFT-IT tubes than in QFT-MT in all 152 children and in the 59 children with possible TB. The only difference in adults was a higher median interferon-gamma level after antigen stimulation in the QFT-MT version that contained the highest concentration of antigen.
Sensitivities were low, in the range of 18% to 23% even for children with confirmed or highly probable TB, for both the QFT-IT and the QFT-MT, but exceeded 85% for adults with confirmed TB.
For the children, sensitivity of the available TST was even lower (8% positive among children with confirmed or highly probable TB and 8% positive among children with possible TB). TST sensitivity in adults (87%) was similar to the IGRA sensitivity.
"We have tried various methods to improve sensitivity, but only at the expense of specificity," Dr. Ravn said. "We hope that in the end we will contribute to the development of a robust test (i.e. high sensitivity and high specificity) utilising minimum amount of blood at low cost and limited need for infrastructure for use in developing countries."
Cellestis, an Australian biotechnology company, provided the tests and accepted the study protocol, but had no further role in the research.
"The equivalent performance of QFT-MT and QFT-IT in children is the most important conclusion of this research," said Dr. Deborah Lewinsohn, a TB researcher at Oregon Health & Sciences University in Portland.
"The need for <1 ml of blood represents a significant improvement in IGRA testing over QFT-IT," she told Reuters Health by email, adding that the greater volume required by the older test limits the feasibility of IGRA in young children.
But Dr. Lewinsohn also pointed to the insufficient sensitivity and specificity of the IGRAs and TST and called for better tests that can distinguish between TB disease and asymptomatic infections.
"While IGRAs should not be used as stand-alone tests to diagnose TB in young children, IGRAs still represent a useful tool in diagnosing TB in many clinical settings," she concluded.
SOURCE: http://bit.ly/NFCxxb
Eur Resp J 2012.
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