© 2008 American Thoracic Society
Predictive Value of the Tuberculin Skin Test and the QuantiFERON-TB Gold In-Tube Assay for the Development of Active Tuberculosis DiseaseFrom the Authors:Dr. Hernández-Garduño, in his subanalysis of data from our article (1), contends that we showed that the tuberculin skin test (TST) had similar predictive power for progression to active tuberculosis (TB) as the QuantiFERON-TB Gold (QFT) test in contacts who were not BCG vaccinated. While our study assessed all contacts in our screening investigation, Dr. Hernández-Garduño initially compared QFT results for all contacts, independent of their BCG status, with TST results from only those who were unvaccinated. This analysis is flawed because it assumes the BCG-vaccinated individuals had the same risk of TB infection as the nonvaccinated, it disregards the limited protection of BCG, and it ignores the various social differences between those who choose to be vaccinated and those who do not. The more reasonable comparison of results in only unvaccinated individuals shows 5 of 23 (21.7%) of those who were QFT positive developing active TB, compared with 4 of 42 (9.5%) for the TST. While the P value for difference between the two tests is greater than 0.5, this is likely due to small numbers rather than the absence of a real effect. The risk ratio for this comparison is 2.28 (95% CI, 0.68 to 7.7), demonstrating that a difference certainly cannot be dismissed as absent. It is incorrect to conclude on this evidence that "TST predictive value is similar to QFT in the unvaccinated close contacts" (2). Determination of BCG vaccination status is problematic for research studies, and highly uncertain in routine practice. Most BCG vaccination is at birth, making subject recollection of their vaccine status impossible. Many people come from countries where revaccination is commonplace, but either do not recall their vaccination or do not distinguish BCG from other vaccines. Observation for a BCG scar is highly inexact. We went to great lengths to establish BCG vaccination status, but freely admit that there may be errors in what we reported. Dr. Hernández-Garduño contends that TST should be the first option for screening unvaccinated contacts due to the low cost and inconvenience of venipuncture. We refer Dr. Hernández-Garduño to publications examining costs and demonstrating that QFT alone is cheaper than the TST alone, and further note the cost of wasted follow-up procedures (3–5). For routine screening, using TST for nonvaccinated contacts and QFT for BCG-vaccinated contacts would introduce procedural costs implicit in two systems of testing and recording. Further, while venipuncture is a common medical procedure, TST operators are specifically trained, and subjects have the inconvenience of having to return 2 to 3 days later for test reading. Those who are falsely positive to TST have the additional inconvenience of follow-up procedures. On cost and convenience alone, we suggest the data demonstrate that QFT is preferable, and we maintain that our data support the conclusions stated in our article (1).
University of Düsseldorf FOOTNOTES
Conflict of Interest Statement: R.D. received REFERENCES
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1,000 for speaking at a conference sponsored by Cellestis and 
assay for the development of active tuberculosis disease after recent infection with Mycobacterium tuberculosis. Am J Respir Crit Care Med 2008;177:1164–1170.