Within-Subject Variability and Boosting of T-Cell Interferon- Responses after Tuberculin Skin Testing
Richard N. van Zyl-Smit1,
Madhukar Pai2,
Kwaku Peprah1,
Richard Meldau1,
Jackie Kieck3,
June Juritz4,
Motasim Badri1,
Alimuddin Zumla5,
Leonardo A. Sechi6,
Eric D. Bateman1 and
Keertan Dheda1,5,7
1 Lung Infection and Immunity Unit, Division of Pulmonology and UCT Lung Institute Department of Medicine, University of Cape Town, Cape Town, South Africa; 2 Department of Epidemiology, Biostatistics, and Occupational Health, McGill University, Montreal, Canada; 3 Department of Radiology, University of Cape Town and Groote Schuur Hospital, and 4 Department of Statistical Sciences, University of Cape Town, Cape Town, South Africa; 5 Centre for Infectious Diseases and International Health, Royal Free and University College Medical School, London, United Kingdom; 6 Dipartimento di Scienze Biomediche, University of Sardinia, Sassari, Italy; and 7 Institute of Infectious Diseases and Molecular Medicine, University of Cape Town, Cape Town, South Africa
Correspondence and requests for reprints should be addressed to Keertan Dheda, M.D., Ph.D., Lung Infection and Immunity Unit, Department of Medicine, J Floor, Old Main Building, Groote Schuur Hospital, Cape Town 7935, South Africa. E-mail: Keertan.dheda{at}uct.ac.za
Rationale: The optimal strategy for the diagnosis of latenttuberculosis infection is controversial. Adoption of a two-stepstrategy (tuberculin skin test [TST] followed by an IFN- releaseassay [IGRA], compared with an IGRA alone), may be limited byTST-mediated boosting of subsequent IGRA responses. Assessmentof within-subject IGRA variability will aid in establishingthresholds for conversions and reversions, and interpretationof serial testing results.
Objectives: To determine short-term IGRA variability and theimpact of TST on subsequent IGRA results.
Methods: Within-subject variability and TST-mediated boostingof IGRA responses were evaluated in 26 South African participantswith varying exposure risk. IGRAs (T-SPOT.TB, QuantiFERON-TBGold In-Tube [QuantiFERON-TB-GIT], PPD, and heparin-bindinghemagglutinin) were repeated four times over 21 days pre-TST,and on Days 3, 7, 28, and 84 post-TST administration.
Measurements and Main Results: All participants showed within-subjectIGRA variability. Changes of ±3 spots (T-SPOT.TB) or±80% from the mean IFN- response (QuantiFERON-TB-GIT)over 3 weeks explained 95% of the variability. Spontaneous conversions/reversionsoccurred in 7 of 26 subjects (27%) (6 for T-SPOT.TB and 1 forQuantiFERON-TB-GIT [P = 0.049]) during the within-patient variabilitystudies (pre-TST). After the TST eight subjects (33%) boostedabove the defined baseline variability. By Day 7 post-TST, butnot Day 3, 2 (12.5%) initially IGRA-negative test subjects converted.By contrast, boosting of PPD and heparin-binding hemagglutininoccurred by Day 3 post-TST.
Conclusions: When using a two-step screening strategy it appearssafe to perform a QuantiFERON-TB-GIT or T-SPOT.TB IGRA within3 days of performing the TST. A 3-spot or 80% IFN- responsevariation, on either side of baseline values, explains 95% ofthe short-term variability and may be useful for interpretingconversions and reversions, and values close to the cut-point.
Scientific Knowledge on the Subject
The optimal strategy forthe diagnosis of latent tuberculosis infection is unclear. Thetwo-step strategy (tuberculin skin test [TST] followed by anIFN- release assay [IGRA]), as recommended by some guidelines,may be confounded by within-subject variability and TST-mediatedboosting of subsequent IGRA responses.
What This Study Addsto the Field
The TST potentially boosts IGRA responses, predominantlyin IGRA-positive subjects, but occurs after Day 3 and may thusconfound the interpretation of subsequent T-cell responses.IGRAs should ideally be performed at no more than 3 days aftera TST. On the basis of results of within-subject variability,cut-points for conversions and reversions are proposed, butrequire validation before widespread application in low tuberculosisrisk environments.
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Diagnosing Latent Tuberculosis Infection: Guess Who's Coming to Dinner?
Responses after Tuberculin Skin Testing
release assay [IGRA], compared with an IGRA alone), may be limited by TST-mediated boosting of subsequent IGRA responses. Assessment of within-subject IGRA variability will aid in establishing thresholds for conversions and reversions, and interpretation of serial testing results. 
