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T-cell assays for tuberculosis infection: deriving cut-offs for conversions using reproducibility data.

Veerapathran A, Joshi R, Goswami K, Dogra S, Moodie EE, Reddy MV, Kalantri S, Schwartzman K, Behr MA, Menzies D, Pai M - PLoS ONE (2008)

Bottom Line: While persons with negative QFT results generally stayed negative, positive results tended to vary over time.Conservatively assuming that long-term variability might be at least twice higher than short-term, we hypothesize that a QFT conversion requires two conditions to be met: 1) change from negative to positive result, and 2) at least 30% increase in the baseline IFN-gamma response.Larger studies are needed to confirm our preliminary findings, and determine the conversion thresholds for IGRAs.

View Article: PubMed Central - PubMed

Affiliation: Department of Biochemistry, Mahatma Gandhi Institute of Medical Sciences, Sevagram, Maharashtra, India.

ABSTRACT

Background: Although interferon-gamma release assays (IGRA) are promising alternatives to the tuberculin skin test, interpretation of repeated testing results is hampered by lack of evidence on optimal cut-offs for conversions and reversions. A logical start is to determine the within-person variability of T-cell responses during serial testing.

Methodology/principal findings: We performed a pilot study in India, to evaluate the short-term reproducibility of QuantiFERON-TB Gold In Tube assay (QFT) among 14 healthcare workers (HCWs) who underwent 4 serial QFT tests on day 0, 3, 9 and 12. QFT ELISA was repeated twice on the same sets of specimens. We assessed two types of reproducibility: 1) test-retest reproducibility (between-test variability), and 2) within-person reproducibility over time. Test-retest reproducibility: with dichotomous test results, extremely high concordance was noticed between two tests performed on the same sets of specimens: of the 56 samples, the test and re-test results agreed for all but 2 individuals (kappa = 0.94). Discordance was noted in subjects who had IFN-gamma values around the cut-off point, with both increases and decreases noted. With continuous IFN-gamma results, re-test results tended to produce higher estimates of IFN-gamma than the original test. Within-person reproducibility: when continuous IFN-gamma data were analyzed, the within-person reproducibility was moderate to high. While persons with negative QFT results generally stayed negative, positive results tended to vary over time. Our data showed that increases of more than 16% in the IFN-gamma levels are statistically improbable in the short-term.

Conclusions: Conservatively assuming that long-term variability might be at least twice higher than short-term, we hypothesize that a QFT conversion requires two conditions to be met: 1) change from negative to positive result, and 2) at least 30% increase in the baseline IFN-gamma response. Larger studies are needed to confirm our preliminary findings, and determine the conversion thresholds for IGRAs.

Show MeSH
Test versus re-test QFT results expressed as IFN-γ (IU/mL).Data for each individual is plotted with different symbols, and the line of equality is shown as the diagonal. Note that more points fall above the line of equality than below, indicating higher IFN-γ levels upon re-testing of blood. The spread of the IFN-γ results from the line of equality increases with increasing IFN-γ results, indicating that a log-transformation was appropriate. QFT: QuantiFERON-TB Gold In Tube®; IFN-γ: interferon-gamma.
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pone-0001850-g001: Test versus re-test QFT results expressed as IFN-γ (IU/mL).Data for each individual is plotted with different symbols, and the line of equality is shown as the diagonal. Note that more points fall above the line of equality than below, indicating higher IFN-γ levels upon re-testing of blood. The spread of the IFN-γ results from the line of equality increases with increasing IFN-γ results, indicating that a log-transformation was appropriate. QFT: QuantiFERON-TB Gold In Tube®; IFN-γ: interferon-gamma.

Mentions: In examining the test-retest reproducibility of continuous IFN-γ results, re-test results tended to produce higher estimates of IFN-γ than the original test (Figure 1). As with many biological measures, the between-test variability increases with the level of IFN-γ in the sample, and so log-transformation was required to normalize the difference in IFN-γ (test–re-test) values so that appropriate limits of agreement [16], [17] could be plotted.


T-cell assays for tuberculosis infection: deriving cut-offs for conversions using reproducibility data.

Veerapathran A, Joshi R, Goswami K, Dogra S, Moodie EE, Reddy MV, Kalantri S, Schwartzman K, Behr MA, Menzies D, Pai M - PLoS ONE (2008)

Test versus re-test QFT results expressed as IFN-γ (IU/mL).Data for each individual is plotted with different symbols, and the line of equality is shown as the diagonal. Note that more points fall above the line of equality than below, indicating higher IFN-γ levels upon re-testing of blood. The spread of the IFN-γ results from the line of equality increases with increasing IFN-γ results, indicating that a log-transformation was appropriate. QFT: QuantiFERON-TB Gold In Tube®; IFN-γ: interferon-gamma.
© Copyright Policy
Related In: Results  -  Collection

Show All Figures
getmorefigures.php?uid=PMC2266993&req=5

pone-0001850-g001: Test versus re-test QFT results expressed as IFN-γ (IU/mL).Data for each individual is plotted with different symbols, and the line of equality is shown as the diagonal. Note that more points fall above the line of equality than below, indicating higher IFN-γ levels upon re-testing of blood. The spread of the IFN-γ results from the line of equality increases with increasing IFN-γ results, indicating that a log-transformation was appropriate. QFT: QuantiFERON-TB Gold In Tube®; IFN-γ: interferon-gamma.
Mentions: In examining the test-retest reproducibility of continuous IFN-γ results, re-test results tended to produce higher estimates of IFN-γ than the original test (Figure 1). As with many biological measures, the between-test variability increases with the level of IFN-γ in the sample, and so log-transformation was required to normalize the difference in IFN-γ (test–re-test) values so that appropriate limits of agreement [16], [17] could be plotted.

Bottom Line: While persons with negative QFT results generally stayed negative, positive results tended to vary over time.Conservatively assuming that long-term variability might be at least twice higher than short-term, we hypothesize that a QFT conversion requires two conditions to be met: 1) change from negative to positive result, and 2) at least 30% increase in the baseline IFN-gamma response.Larger studies are needed to confirm our preliminary findings, and determine the conversion thresholds for IGRAs.

View Article: PubMed Central - PubMed

Affiliation: Department of Biochemistry, Mahatma Gandhi Institute of Medical Sciences, Sevagram, Maharashtra, India.

ABSTRACT

Background: Although interferon-gamma release assays (IGRA) are promising alternatives to the tuberculin skin test, interpretation of repeated testing results is hampered by lack of evidence on optimal cut-offs for conversions and reversions. A logical start is to determine the within-person variability of T-cell responses during serial testing.

Methodology/principal findings: We performed a pilot study in India, to evaluate the short-term reproducibility of QuantiFERON-TB Gold In Tube assay (QFT) among 14 healthcare workers (HCWs) who underwent 4 serial QFT tests on day 0, 3, 9 and 12. QFT ELISA was repeated twice on the same sets of specimens. We assessed two types of reproducibility: 1) test-retest reproducibility (between-test variability), and 2) within-person reproducibility over time. Test-retest reproducibility: with dichotomous test results, extremely high concordance was noticed between two tests performed on the same sets of specimens: of the 56 samples, the test and re-test results agreed for all but 2 individuals (kappa = 0.94). Discordance was noted in subjects who had IFN-gamma values around the cut-off point, with both increases and decreases noted. With continuous IFN-gamma results, re-test results tended to produce higher estimates of IFN-gamma than the original test. Within-person reproducibility: when continuous IFN-gamma data were analyzed, the within-person reproducibility was moderate to high. While persons with negative QFT results generally stayed negative, positive results tended to vary over time. Our data showed that increases of more than 16% in the IFN-gamma levels are statistically improbable in the short-term.

Conclusions: Conservatively assuming that long-term variability might be at least twice higher than short-term, we hypothesize that a QFT conversion requires two conditions to be met: 1) change from negative to positive result, and 2) at least 30% increase in the baseline IFN-gamma response. Larger studies are needed to confirm our preliminary findings, and determine the conversion thresholds for IGRAs.

Show MeSH