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Villarino ME, Burman W, Wang Y, et al. Comparable Specificity of 2 Commercial Tuberculin Reagents in Persons at Low Risk for Tuberculous Infection. JAMA. 1999;281(2):169–171. doi:10.1001/jama.281.2.169
Author Affiliations: Division of Tuberculosis Elimination, Centers for Disease Control and Prevention, Atlanta, Ga (Drs Villarino and Wang); Department of Public Health, Denver, Colo (Dr Burman); Department of Campus Health Services, the University of Arizona, Tucson (Dr Lundergan); Division of Pulmonary and Critical Care Medicine, the University of California, San Diego (Dr Catanzaro); Division of Infectious Diseases, Emory University, Atlanta (Dr Bock); the Marion County Health Department, Marion County, Indianapolis, Ind (Dr Jones); the Seattle-King County Health Department, Seattle, Wash (Dr Nolan).
Context One or both commercial tuberculin skin test reagents
(Aplisol and Tubersol) may have a high rate of false-positive
Objective To compare the reaction size and specificity of skin
testing with Aplisol, Tubersol, and the standard purified protein
Design Double-blind trial, conducted between May 14, 1997,
and October 28, 1997, in which each individual received 4 tuberculin
skin reagents at sites assigned at random.
Setting Health departments and universities in 6 US cities.
Participants A total of 1555 persons at low risk of latent
Intervention Simultaneous skin tests with Aplisol, Tubersol,
PPD-S1, and either a second PPD-S1 or PPD-S2 (a proposed new standard).
Main Outcome Measure Reaction size at each injection site measured
by 2 investigators blinded to type of reagent.
Results Aplisol produced slightly larger reactions than
Tubersol, but this difference did not significantly change skin test
interpretation. The mean ± SD reaction sizes were 3.4 ± 4.2 mm with
Aplisol, 2.1 ± 3.2 mm with Tubersol, and 2.5 ± 3.6 mm with PPD-S1.
Assuming that all participants were uninfected and using a 10-mm
cutoff, the specificities of the tests were high: Aplisol, 98.2%;
Tubersol, 99.2%; and PPD-S1, 98.9%. Significant variability was not
detected in interobserver, host, and lot-to-lot reagent comparisons.
Conclusion Using a cutoff of at least 10 mm, testing with 3
different PPD reagents resulted in similar numbers of uninfected
persons being correctly classified.
diagnosis of latent tuberculosis infection is the basis of
preventive therapy and a key indicator of tuberculosis transmission.
The tuberculin skin test, which is the intradermal injection of a
purified protein derivative (PPD) from broth culture of
Mycobacterium tuberculosis,1 remains the only
validated method for diagnosing latent tuberculosis. Parkdale
Pharmaceuticals, Rochester, Mich (Aplisol), and Pasteur Mérieux
Connaught USA, Swiftwater, Pa (Tubersol), are the 2 companies that
manufacture tuberculin in the United States. Despite regulations for
standardization of tuberculin manufacturing and testing, clusters of
suspected false-positive results involving both products have been
reported.2-8 We performed a randomized, double-blind study
of the 2 commercial reagents and PPD-S1 (the standard) in 2
populations: (1) persons at low risk for latent tuberculosis infection
and (2) patients with culture-positive tuberculosis.
Subjects were enrolled in Denver, Colo; Marion County,
Indianapolis, Ind; Atlanta, Ga; San Diego, Calif; Seattle, Wash; and
Tucson, Ariz. Eligibility criteria included no risk factors for
tuberculosis exposure (by questionnaire, available on request), no
prior BCG immunization, no known immunodeficiency, age 18 to 50 years,
and birth in the United States or Canada. To confirm the immunogenicity
of the tuberculin skin test reagents used, we also studied patients
having culture-positive tuberculosis diagnosed within 5 years and a
favorable clinical response to 2 months or more of therapy. All
participants gave written informed consent.
Skin test placement and reading were performed by
experienced personnel using a standard protocol. The reagents used were
Tubersol (lot numbers 2443-11 and 2458-11), Aplisol (lot numbers 01206p
and 00417p), PPD-S1, and PPD-S2. All reagents were injected using
insulin syringes (Becton Dickinson, Franklin Lakes,
NJ), and participants returned to the study site for reading of test 48
to 72 hours later. Randomization lists were prepared for each of the 6
study sites using randomized blocks of antigen sequences for groups of
either 3, 6, or 9 patients. Sequences were randomized by antigen and
injection site. Separate randomization schedules were configured for
the low risk and the tuberculosis-patient study groups. Three fourths
of subjects were randomized to receive Aplisol, Tubersol, PPD-S1, and
PPD-S2; one fourth received Aplisol, Tubersol, and 2 injections of
PPD-S1. Injections were placed on 2 sites of the flexor surface of each
forearm, 5 and 10 cm below the elbow. The 2 investigators reading
results, who were blinded to the identity of the test reagent and the
other person's readings, recorded the reactions in millimeters of
induration in the transverse diameter.
If the false-positivity rate of tuberculin skin testing is 4%,
detecting a 2% difference between false-positive rates with 80% power
and 95% certainty requires a sample size of 1146. To account for
losses to follow-up, we planned to enroll 1500 low-risk participants.
We evaluated 3 potential sources of variability. First, we assessed
interobserver variability, the difference between 2 independent
readings of the same skin test, by grouping the results into 3
categories (0-4 mm, 5-9 mm, and ≥10 mm) and evaluating the paired
agreement with the κ statistic.9 Second, we assessed host
variability, the difference between 2 PPD-S1 tests in the same
participant, by comparing differences in skin test interpretation.
Third, we assessed the variability between different reagents and
between different lots of the same reagent. For these comparisons we
evaluated the difference between reaction-size means in subjects who
had at least 1 skin test reading greater than 0 mm, using nonparametric
analyses of variance (Friedman test if repeated measures and Wilcoxon
signed rank tests if nonrepeated measures),10 and pairwise
comparisons. We also compared (using Wilcoxon signed rank tests
adjusted for multiple comparisons10) the mean reaction
sizes by study site, age, sex, and race. The results of testing with
PPD-S2 will be presented separately.
We calculated test specificity in 2 ways. First, all low-risk subjects
were assumed to be uninfected; therefore, specificity equals 1 minus
the rate of reactions measuring 10 or 15 mm or more (false-positive
reactions). Second, subjects having reactions of 10 mm or more to
PPD-S1 were assumed to be infected and eliminated from the specificity
calculations. Among patients with culture-positive tuberculosis, we
compared the mean skin test reaction sizes and the rate of
false-negative reactions (<10 mm).
Between May 14, 1997, and October 28, 1997, we enrolled
1596 low-risk subjects, 41 of whom were excluded from analysis for
various reasons, and 99 persons with histories of culture-positive
tuberculosis. Demographic characteristics of the remaining 1555
low-risk participants are shown in
. There were no clinically significant
adverse reactions to skin testing. Of the 1555 low-risk subjects, 366
(23.5%) received 3 unique antigens and 2 injections of PPD-S1; the
rest received 4 unique antigens. Of the 99 patients with TB, 30
(30.3%) received 3 unique antigens and 2 injections of PPD-S1; the
rest received 4 unique antigens.
Of the 1555 low-risk subjects, 127 (8.2%) had a PPD-S1 reading greater
than 0 mm. Among these 127 subjects, the differences between the 2
readers were small in most cases and only equaled or exceeded 5 mm in
18 instances (14%). There was a 69% probability (κ statistic ×
100) that the agreement between 2 readers of the same PPD-S1 test was
not by chance alone. Thirty-six (9.8%) of 366 persons who received 2
PPD-S1 injections had at least 1 of these tests read as greater than 0
mm. Among these 36 subjects, the differences between the 2 PPD-S1 tests
equaled or exceeded 5 mm in 4 cases (11%). Using a 10-mm cutoff, the
difference in the readings would result in a difference in skin test
interpretation in only 2 subjects (0.5%).
There were no significant differences between the mean reaction sizes
of the 2 lots of each commercial reagent (means for the Aplisol lots:
3.43 and 3.43 mm, P = .95; means for the Tubersol lots: 2.50
and 1.71 mm, P = .19). However, there were differences between
the mean (±SD) reaction sizes of Aplisol (3.4 ± 4.2 mm), Tubersol
(2.1 ± 3.2 mm), and PPD-S1 (2.5 ± 3.6 mm) (P = .001 by
analysis of variance and P<.05 for all 3 pairwise
comparisons). Mean reaction sizes were significantly larger at the San
Diego site compared with other sites; however, most of these reactions
were small (263 [91%] of 288 ranged from 1-9 mm). Excluding
participants from San Diego does not significantly change the results
of this analysis (data not shown). There were no significant
differences in mean reaction sizes by age, sex, or race.
The first scenario assumed that all reactions greater than the
cutoff value (10 or 15 mm) were false-positive. At either cutoff value,
with any of the 3 skin test
reagents, the number of persons with positive
reactions was small and the corresponding specificities were all
greater than 98%
(Table 2). At the
10-mm cutoff there was a significant difference in specificity between
Aplisol and Tubersol, but neither commercial reagent differed from
PPD-S1. In the second scenario, all subjects with reactions of 10 mm or
greater to PPD-S1 were defined as latently infected and eliminated from
the analysis (Table 2). In this scenario, there were no significant
differences between the specificities of Aplisol and Tubersol using
either a 10- or 15-mm cutoff.
The mean (±SD) reaction sizes in the persons with culture-positive
tuberculosis were 16.3 ± 5.6 mm for Aplisol, 14.9 ± 6.0 mm for
Tubersol, and 16.2 ± 6.4 mm for PPD-S1 (P = .006 by analysis
of variance). In pairwise comparison, the differences between Tubersol
and PPD-S1 (P = .008) and between Tubersol and Aplisol
(P<.001) were statistically significant, whereas there was
no difference between Aplisol and PPD-S1 (P = .84). Thirteen
persons (13%) had false-negative test results with either PPD-S1 or
Tubersol, and 11 (11%) had false-negative test results with
This study demonstrates that the results of skin
testing with the 2 commercial reagents, Aplisol and Tubersol, are quite
comparable with that of the standard tuberculin preparation, PPD-S1.
Tubersol produced slightly smaller reactions, and Aplisol slightly
larger reactions, than did PPD-S1, but these differences in reaction
sizes did not result in significant differences in skin test
interpretation; the specificities of both commercial reagents were high
(>98%) and similar to that of PPD-S1.
We explored several potential sources of variability in
tuberculin skin testing. Interobserver agreement was similar than that
previously reported.11 Host variability was quite low;
differences between the reaction sizes of 2 simultaneous PPD-S1
tests would have resulted in a discordance in skin test interpretation
in only 0.5% of those tested. We detected an association between
reaction size and enrollment in San Diego. The readers at this site had
previously evaluated skin tests (other than tuberculin) having expected
reaction sizes of less than 10 mm. We suspect that readers from other
sites were not trained to detect small skin test reactions, leading to
a tendency to record such reactions as 0 mm of induration.
Clusters of unexpected positive tuberculin skin test results have
been previously reported, often in groups of low-risk persons tested
with Aplisol that, on subsequent testing with Tubersol, were believed
to be clusters of false-positive reactions.2-8 None of
these reports involved testing with the 2 commercial products
simultaneously and thus cannot exclude the possibility of
false-negative reactions associated with Tubersol, or another kind of
error associated with tuberculin skin tests not performed under the
same conditions. Our study included simultaneous testing of both
commercial reagents, as well as the standard tuberculin, in a large
sample of well-characterized subjects. A limitation of our study is
that we only evaluated 2 lots of commercial tuberculin manufactured in
the same period. It is possible that variations in manufacturing
processes over time may have produced some of the reported differences
in false-positive rates.
Skin test variation related to human factors can be controlled
only to a finite degree. In clinical practice, these factors cannot be
eliminated completely and should always be recognized as potential
sources of false-positive tuberculin skin test results. However, our
study demonstrates that both Aplisol and Tubersol will correctly
classify comparable numbers of persons not infected with M
tuberculosis and that the choice of product used for skin testing
has little effect on test performance.
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