Tuberculin Skin Testing in a Pediatric Population Exposed to Human Immunodeficiency Virus

Although it has been reported that testing for tuberculosis exposure is often difficult in populations with AIDS given the high rate of anergy,1 virtually no published studies have determined the efficacy of using purified protein derivative (PPD) tuberculin skin testing or cutaneous anergy in HIV-infected children. After an increase in the number of pediatric tuberculosis cases reported in New York State,2 an investigation was conducted at the Children's Medical Center of the State University of New York at Stony Brook to assess: (1) the prevalence of anergy in HIV-exposed children at increased risk for tuberculosis infection; and (2) the differences between the reactivity of HIV-exposed children to 5-tuberculin unit (TU) and 250-TU PPD skin tests based on the recommended guidelines of ≥5 mm induration indicating positive response.3 Methods. Children were diagnosed with tuberculosis infection if they had a positive tuberculosis skin test with induration ≥5 mm in diameter palpable at 48 h.4 The same criterion was applied to the Candida skin test which was used to determine anergy.5 Children classified as normal respondents were those whose PPD tests measured <5 mm of induration and whose Candida skin test measured ≥5 mm of induration. Children classified as anergic were those who were nonreactive to both the PPD and the Candida skin tests. Children were classified as PPD converters if there was ≥5 mm of induration after a PPD skin test. Children were diagnosed to have tuberculosis if they displayed clinical and/or radiographic manifestations caused by Mycobacterium tuberculosis. Forty-nine patients received PPD and Candida skin tests, 48 of whom returned at 48 h for follow-up. One child went to a local county clinic for follow-up. The ethnic composition of the study population was 65% African-American, 18% non-Hispanic white and 17% Hispanic. Ninety percent of the patients were >19 months old at the time of entry. No child younger than 6 months of age was enrolled in the study. Thirty-six (73%) patients were HIV-infected. Children were classified as HIV-infected according to criteria established by the Centers for Disease Control.6 If a patient was either PPD-positive or anergic, chest radiography was performed at the 48-h visit. If chest radiography results were positive gastric aspirates were obtained. Contact investigation was used for further follow-up and confirmation of tuberculosis exposure. T cell subset studies were performed within 3 months of PPD skin testing by flow cytometry.7 PPD testing was performed by using standard Connaught 5-TU (0.1-ml) and 250-TU (0.1-ml) PPD tests. Berkeley Biological allergenic extract of Candida (1:500 dilution; 0.1 ml) was used in measuring Candida reactivity. All skin tests were done by two study nurses trained in skin inoculation and read by a single observer using pen and calipers for reading results. Fisher's exact test was used to determine association between anergy and immunologic status. CD4 counts were compared using Student's t test for anergic and nonanergic children. Alpha ≤ 0.05 was considered statistically significant. Results. Of the 49 children enrolled in the study 49% (n = 24) were nonanergic. A comparison of reactivity to the 250-TU and the 5-TU PPD skin tests among nonanergic children revealed that 20 had skin tests that were negative for tuberculosis infection with both the 250-TU and the 5-TU PPD skin tests. One of these children had <5 mm induration on both PPD skin tests. This child, whose father had tuberculosis, was subsequently diagnosed with hilar adenopathy on chest radiography. Two other children tested positive for PPD conversion on both the 5-TU and the 250-TU PPD skin tests (mean CD4 count, 2360/mm3) but only one was confirmed to have pulmonary tuberculosis. This child had positive chest radiography, and while the child was in foster care he had been in contact with his natural father who had recently been diagnosed with tuberculosis. Another 2 children falsely tested positive for PPD conversion with only the 250-TU PPD test (mean CD4 count, 1835/mm3). Their chest radiographs were negative and there were no known cases of tuberculosis in their households or contacts. Fifty-one percent (n = 25) of the children in this study were anergic given that they mounted either no response to Candida skin testing (88%) or had responses of ≤5 mm (12%). Of all anergic children 18 were HIV-infected. Three of these anergic children were diagnosed with tuberculosis (two had miliary tuberculosis and one had pulmonary tuberculosis), all of whom had positive chest radiographs but none of whom had positive gastric aspirates. Two of these children were siblings. Tuberculosis was confirmed by contact investigation. As presented in Table 1 the mean CD4 count for nonanergic children who tested negative for tuberculosis was 1450/mm3 (range, 210 to 3330) with a mean candida reactivity of 16 mm (range, 5 to 15 mm). In contrast the mean CD4 count for anergic children was 1220/mm3 (range, 4 to 3360). There was no statistically significant difference in mean CD4 counts between the anergic and nonanergic children (P = 0.26). Stratification of CD4 values resulted in different findings based on selected CD4 cutoffs. When CD4 values were categorized into <500 or ≤500 between anergic (n = 7 and n = 18, respectively) and nonanergic children (n = 1 and n = 23, respectively), there was a statistically insignificant difference between the two groups (P = 0.08). However, if the CD4 cutoff value was lowered to <200 or ≥200 to separate children with severely compromised immune systems from others, the results indicated a statistically significant difference between the anergic and nonanergic children (P = 0.01). Of the anergic children who presented with normal chest radiographs (n = 21), the median CD4 count was 970/mm3 (mean CD4, 1210/mm3). In contrast the 4 anergic children with abnormal chest radiographies, all of whom were HIV-infected, had a median CD4 count of 865/mm3 (mean CD4, 702/mm3). Discussion. In our population of 49 HIV-exposed children, 36 (73%) were HIV-infected and 5 (10%) were diagnosed with tuberculosis. The prevalence of anergy was 51% (n = 25) and 18 (72%) of the anergic children were HIV-positive. Candida antigen testing was used to assess anergy as recommended by the CDC,8 and findings support the few studies that have reported HIV seropositivity as a significant factor in lower frequency of tuberculin reactivity.3, 8, 9 Patients in our study who responded to skin testing had a wide range of CD4 cells, from 210 to 3330, with a mean CD4 count of 1533. Anergic children had even a wider range of CD4 cells, from 4 to 3360, with a mean CD4 count of 1220. Overall no statistically significant association was found between mean CD4 counts and ability to mount a response to PPD skin testing. However, as CD4 values diminished there was a tendency towards association between the ability to mount a response to antigen testing and CD4 values, with P = 0.08 if the CD4 cutoff was set at 500 and P = 0.01 if the cutoff was lowered to 200. This finding is not surprising given that children with severely compromised immune functions lack the ability to respond to other antigens.10, 11 The tuberculin skin test is the preferred and least expensive method of detecting M. tuberculosis infection; however, it is neither 100% sensitive nor 100% specific. On average 10 to 25% of patients with active tuberculosis do not react to tuberculin.12, 13 We used both the 5-TU and 250-TU PPD skin tests to detect tuberculosis exposure in our population of HIV-exposed children. A comparison of reactivity to both PPD skin tests among nonanergic children (n = 24) revealed consistent results for 22 patients, of whom 20 had negative PPD skin tests. Two children falsely tested positive for PPD conversion infection with only the 250-TU PPD test. The most plausible explanation for this observation is that the reaction to the 250-TU PPD test is representative of sensitivity to atypical mycobacteria in the environment.12, 13 Although the results of this study add to the current knowledge and understanding of cutaneous anergy and the efficacy of PPD testing in HIV-infected children, its findings should be interpreted cautiously given the small sample size, the lack of randomization and the less than optimal sensitivity of Candida skin testing. Intradermal PPD skin testing is an inexpensive and readily available tool for detecting tuberculosis infection in populations of HIV-infected and seroreverted children. Yearly PPD skin testing and chest radiography accompanied by a thorough physical examination and history (including history of contact and source case identification) in anergic children is necessary. The 250-TU PPD skin test is not a helpful test for determining tuberculosis infection in this population. Future longitudinal studies are needed to explore antigen tests with better sensitivity and specificity for use in HIV-infected children. Acknowledgments. We thank Shawney Fine, R.N., Jeannie Burns, P.N.P., Peggy Melendez, P.N.P., and Frank Szollosy, M.D., for their assistance in data collection and patient care. Sharon A. Nachman, M.D.; Maryam Navaie-Waliser, M.P.H.