An Assessment of Mantoux test in the Diagnosis of Tuberculosis in a BCG-Vaccinated, Tuberculosis-Endemic Area

An Assessment of Mantoux test in the Diagnosis of Tuberculosis in a BCG-Vaccinated, TuberculosisEndemic Area Bushra Jamil * **, Salima Qamruddin*, Arif R Sarwari * and Rumina Hasan ** * Dept. of Medicine, ** Dept of Pathology & Microbiology, The Aga Khan University Hospital, Karachi. Abstract Setting Outpatient clinics at a tertiary care referral center where Mantoux test was done as part of pre-employment medical examination and work-up of suspected tuberculosis. Objective To assess the utility of Mantoux test in the diagnosis of active tuberculosis, in a BCG-vaccinated, TB endemic area. Design A prospective, case-control study, conducted over a period of two years. For statistical analysis, patients with active tuberculosis were categorized on the basis of disease site and confirmatory tests. Results The size of induration on Mantoux test was significantly different between patients with active TB and healthy individuals (p<0.05). Test sensitivity was highest at >5 mm induration; test specificity increased with increasing size of induration and was highest (86%) with induration of >15 mm for both pulmonary and extrapulmonary TB. ESR was also significantly higher (p< 0.05) in TB patients. Older patients without active TB were observed to have higher ESR and larger induration on MT. Conclusions Mantoux test, with or without ESR, provides useful supportive evidence of active TB in patients with pulmonary and extrapulmonary TB in our population. Key words Mantoux test, TB, diagnosis introduction In poor regions of the world, the diagnosis of TB is mostly clinical, often supported by radiological findings and erythrocyte sedimentation rate (ESR); histopathology and culture are only occasionally available or utilized to support or refute the clinical diagnosis of TB in circumstances of limited availability of Corresponding Author: Bushra Jamil, Associate Professor, Depts of Medicine, Pathology and Microbiology, The Aga Khan University Hospital, Karachi, E-mail: bushra.jamil@aku.edu, jamilbushra@yahoo.com 18 . Infectious Diseases Journal of Pakistan diagnostic tests and cost constraints. A positive Mantoux test (MT) or tuberculin skin test (TST) indicates infection with Mycobacterium tuberculosis and the test may thus be used to support the clinical diagnosis of active TB. However, MT may be negative in patients with active TB and positive as a consequence of exposure to mycobacteria other than M. tuberculosis and BCG vaccination and the current indication for MT is solely for detection of latent TB. ESR is a non-specific marker of inflammation and is elevated in a number of infectious and non-infectious conditions. Before using MT (with or without ESR) as a diagnostic test, it is important to assess how the test performs in patients with confirmed diagnosis of tuberculosis and in healthy individuals. In this study, we sought to address this question and studied the utility of Mantoux test in the diagnosis of active tuberculosis in a BCG-vaccinated, TB endemic area. Study Population and Methods All registered adult patients who had a Mantoux test done as part of pre-employment examination or work-up of TB, at the Aga Khan University Hospital from January 1998 to January 2000, were evaluated for inclusion in the study. All study subjects were examined by trained physicians and an infectious diseases consultant. All those individuals with significant co-morbid conditions including malignancy, those on immunosuppressive therapy, including high dose of corticosteroids and those with HIV/AIDS were excluded from the study. The study subjects were followed prospectively to assess presence or absence of active tuberculosis. MT was performed using 0.1 ml of 5 Tuberculin units of purified protein derivative (PPD) administered intradermally on the anterior aspect of the forearm. Results were read at 48 hours. For statistical analysis, a numeric scale was designed for this study, based on different diagnostic tests available for tuberculosis. Groups were defined and according to the numeric scale, scores were assigned to each group according to the extent of confirmation of clinical diagnosis of TB, as shown below. TB excluded Clinical diagnosis of TB without radiological support Clinical diagnosis with radiological support Non-caseating granulomas on biopsy 0 1 2 3 Caseating granulomas on biopsy AFB smear/ culture positive 6 9 Table 2: Differences in age, ESR and MT values according to disease site Variables Category of TB disease LN P EP N = 16 N = 36 N = 43 1: 2.20 1: 1.25 19 ± 15 29 ± 17 4.1 ± 5.4 3.4 ± 3.5 40 ± 28 18 ± 6 44 ± 27 16 ±7 1: 1 33 ±14 2.8 ± 2.3 45 ± 35 13 ± 7 ANOVA P value < 0.05 < 0.05 Study variable was size of induration on MT at 48 hrs. Outcome variables included disease categories of lymphadenopathy, pulmonary and extrapulmonary tuberculosis. Covariates included age, gender, ESR and strength of diagnosis according to our scoring system for this study. Data was analyzed using SPSS. Results Three hundred and eighty one individuals with documented MT results were studied. Mean age of study subjects was 24 years (range 1 – 84). Mean age of TB patients was 30 16 years and that of healthy controls was 22 16 years. While the MT was done mostly on males (M:F= 1:0.75) as part of preemployment medical examination and as part of diagnostic work up of tuberculosis, disease was seen predominantly in females (M:F= 1:1.2). In 286 (75%) individuals, TB was ruled out. Ninety five (25%) patients were diagnosed to have tuberculosis on the basis of clinical findings, radiological investigations and laboratory tests (microscopy, culture and/or histopathology). Sixteen (17%) patients had TB lymphadenitis, 36 (38%) had pulmonary TB and 43 (45%) had extra pulmonary TB at sites other than the lymph nodes. Diagnosis was made on clinical grounds alone in 24 (25%) cases, on radiological evidence in 47 (49%) and on histopathological or microbiological (smear and/or culture) evidence in 24 (26%) cases. There were significant differences regarding age, sex, ESR and MT responses between TB patients and those who did not have TB. TB patients were significantly older, were more likely to be female with a high ESR and had a significantly larger induration on MT (p value<0.05) (table 1). Table 1: Differences in patients with active TB and controls Variable All Patients Tuberculosis No Tuberculosis P value N = 397 N = 95 N = 286 24 ± 17 1 : 0.75 27 ± 28 8±8 30 ± 16 1 : 1.2 44 ± 31 15 ± 7 22 ± 16 1 : 0.63 19 ± 23 5±7 < 0.05 < 0.05 < 0.05 < 0.05 M: F Age (yrs) Strength of Diagnosis ESR (Hr) MT (mm) < 0.05 < 0.05 LN: lymph node P: pulmonary EP: extrapulmonary Sensitivity and specificity of Mantoux test by TB disease category is shown in table 3. Sensitivity was highest at induration of > 5mm and specificity increased from 74% at induration of > 10 mm to a maximum of 86% at induration of >15 mm for all categories TB. Table 3 shows relation between sensitivity and 1-specificity for different categories of TB. Table 3: Sensitivity and specificity of Mantoux Test by TB disease category MT (mm) ≥5 ≥8 ≥ 10 ≥ 12 ≥ 14 ≥ 15 Lymphadenitis Sensit- Speciivity ficity 100% 100% 93% 80% 80% 80% 61% 71% 74% 80% 85% 86% Pulmonary Sensit- Speciivity ficity 94% 92% 86% 81% 72% 61% 61% 71% 74% 80% 85% 86% Extrapulmonary SensiSpecitivity ficity 84% 81% 77% 65% 53% 47% 61% 71% 74% 80% 85% 86% Age (yrs) M: F ESR (/Hr) MT (mm) In 286 individuals, in whom tuberculosis was ruled out, there was a significant positive correlation between age, ESR and MT response. Older individuals were significantly more likely to have higher ESR (p< 0.05) and larger areas of induration on MT (p< 0.05) even if they did not have evidence of active TB. Discussion Mantoux (MT) or tuberculin skin test (TST) is based on cutaneous sensitivity to purified protein derivative (PPD) which develops on exposure to Mycobacterium tuberculosis and other related mycobacteria; a positive result may also be a consequence of BCG vaccination1. Reaction to PPD has been observed to wane with time, with less than 10 percent of recipients retaining tuberculin positivity 10-15 years after vaccination and BCG is not considered a major reason for MT positivity in adults in a population vaccinated in infancy or childhood2,3 . Thus, high Jan-Mar 2008 . 19 Significant differences were also observed among the three categories of lymphadenitis, pulmonary and extrapulmonary TB. Patients with TB lymphadenitis were younger and had larger induration on MT as compared to those with pulmonary and extra pulmonary tuberculosis (table 2). The size of induration, although still greater than 10 mm, was smallest in patients with extrapulmonary tuberculosis. In TB patients, there was no correlation of age with either ESR or size of induration on MT. Volume 17 Issue 01 BCG vaccination coverage in infancy in our population should not limit the usefulness of the MT as a tool for diagnosing TB in adults2. Mantoux test has an excellent ability to distinguish M. tuberculosis-infected from -uninfected persons. The operating characteristics of the tuberculin test are superior to those of nearly all commonly used screening and diagnostic tests4. Interpretation requires consideration of the purpose for testing5 and may be affected by the prevalence of tuberculosis in the population being tested4,5. A positive result indicates the presence of latent or active tuberculosis infection; a negative result does not rule it out6 and some cases with active disease may thus be missed if MT is used for the purpose of diagnosis. In the developed world, where BCG vaccination is not universal and incidence of TB is low, MT is mainly indicated for the diagnosis of latent tuberculosis 7,8. If positive, the patient gets treated with antituberculous drugs to prevent subsequent development of overt disease 9,10. The strategy for identifying latent TB through tuberculin skin test is neither feasible nor practical in a population like ours where incidence and prevalence of TB are high. With a population of 149, 910, 783, the annual incidence of TB cases in Pakistan is 181 per 100,000 persons11. Tuberculosis remains the leading cause of death from a single infectious pathogen and has been declared a 'global emergency' by the World Health Organization12; Pakistan ranks sixth on WHO's list of high-disease burden countries11. Each year, it is estimated that 81/100,000 new smear positive cases are added to the existing TB patient population 11. However, after acquiring infection by inhaling aerosolized M. tuberculosis from such smear positive patients, the majority of healthy individuals develop a cellular immune response, which arrests the growth and propagation of the microorganism, and prevents progression of clinical tuberculosis 13 . Such exposure also results in development of cutaneous sensitivity to PPD and thus up to 47% of endemic population and up to 86% of household contacts of patients with smear positive pulmonary TB may be MT positive14. In this study, we assessed Mantoux test as an adjunctive diagnostic test for active tuberculosis, by comparing the size of induration on MT in healthy, BCG vaccinated subjects with that in patients with tuberculosis of the lymph nodes, lung and other extrapulmonary sites. Additionally, we also looked at ESR values in all groups of TB patients and in healthy controls. For this prospective study, we devised a system for grading of diagnostic confirmation, which was based on clinical diagnosis, radiological and microscopic appearances, and culture of M. tuberculosis. This information was subsequently used for statistical analysis. We observed a significant difference in size of induration on MT and ESR values, between healthy individuals and those with confirmed diagnosis of active tuberculosis; patients with 20 . Infectious Diseases Journal of Pakistan localized disease, e.g. TB lymphadenitis, had significantly larger areas of induration as compared to those with pulmonary and extrapulmonary disease. This could be a reflection of differences in T cell responses when TB affects different tissues of the body. Immunologically, cutaneous response to PPD (and also protective cellular immune responses against Mycobacterium tuberculosis) depends upon CD4+ÊT-cell and the complex interplay of Th1 and Th2 subsets with mononuclear phagocytes 15,16,17. It has been shown that T cell responses differ in TB patients, depending on the site of infection. Sequestration of antigen in different host tissue compartments has been suggested to lead to differential activation of Th1 and Th2 responses and this may lead to differential PPD positivity, producing induration of different sizes on MT in TB affecting different organs14. Patients with lymph node disease have strong Th1 responses; this may also be true for minimal or localized pulmonary disease, while T cell memory response may be impaired in patients with moderate to advanced pulmonary disease but not in patients with lymph node involvement14. Thus patients with TB lymphadenitis invariably show induration of >10 mm on TST 18,19,20 where as the result may be variable in pulmonary TB and may even be negative in extrapulmonary TB involving sites other than the lymph nodes14, 21. Patients with active extrapulmonary tuberculosis had significantly smaller indurations on MT in our study. It has been shown that with disease progression or dissemination, Th1 responses are suppressed and PPD positivity decreases with disease severity 14 . This is probably the consequence of suppressive influence of activated monocytes, which lead to depression of delayed-type hypersensitivity responses during active tuberculosis21. This mechanism may also be responsible for the negative MT, which may be seen in 8 to 20 percent of patients with proven tuberculosis and no apparent immunosuppression 23,24,25,26. MT remains falsely negative in TB patients with immunosuppression caused by factors other than active tuberculosis22 and in those on renal dialysis27. The exact prevalence of extra-pulmonary tuberculosis in Pakistan is unknown, although based on data published from other developing countries, it is likely that a significant proportion of TB cases are extrapulmonary 28,29. This hypothesis is substantiated to some extent by the fact that fifty eight percent of specimens received for M. tuberculosis culture and/or polymerase chain reaction (PCR) at the Aga Khan University Hospital laboratory in Karachi are from extra-pulmonary sites30. Despite many limitations and the non-specific nature of these tests, Mantoux test, along with ESR, are two of the most common investigations used in the work up of patients with suspected extrapulmonary tuberculosis. Whether these tests are sufficiently sensitive and specific for the diagnosis of active extrapulmonary TB is debatable and was one of the objectives of this study. There is data, both for and against use of MT and ESR for this purpose31,32,33. Studies from the developing countries report significantly high MT and ESR values in cases of active TB 19, 32-36 and suggest that positive MT (induration > 10mm) and high ESR values may be used to initiate anti-tuberculous therapy in the presence of relevant clinical presentation37. As is evident from our strength of diagnosis scoring system, most cases of extrapulmonary TB were diagnosed on the basis of clinical suspicion and radiological evidence. Histopathological or culture confirmation was not possible in most cases. It is in such situations that tests like MT and ESR become important. We observed that values for both these tests were significantly higher in TB patients as compared with healthy control, allowing useful discrimination and providing supportive evidence for the presence of extrapulmonary TB. It has been suggested that tuberculin reaction is an all-or-nothing phenomenon. Among patients with increased likelihood of true TB infection because of clinical and/or radiographic abnormalities, the size of the tuberculin test is of no diagnostic utility once the threshold of 5 mm is passed 38. Our findings are in agreement with this observation in that MT sensitivity in our TB patients was highest for induration of >5mm and reached 100% in TB lymphadenitis, where histopathological and/or microbiological confirmation was also available. Similar test sensitivity in TB lymphadenitis has been reported in other populations as well39. Specificity of MT increased with increasing size of induration and was highest (86%) with induration of >15 mm for pulmonary and extrapulmonary tuberculosis and TB lymphadenitis. For endemic population, using a cut off 5 mm induration either for screening or for diagnosis cannot be recommended. Our results underscore the fact that Mantoux test should not be requested unless the indication for doing so is absolutely clear in the requesting physician’s mind. Ordering the test indiscriminately is likely to give misleading information and would invariably lead to over diagnosis of active infection. However, in the presence of compatible clinical presentation, a positive MT, especially if the size of induration is larger than 15 mm is highly suggestive of active TB. Regarding erythrocyte sedimentation rate, significantly higher values were seen in patients with active TB. However, older individuals in this study who did not have evidence of TB also had high ESR (and MT values). Advancing age has been shown to be associated with increases in the TST-positive rate in other studies as well13, emphasizing the fact that MT and ESR may lose their utility as supportive tests of active TB in older individuals. The results of our study underscore the importance of interpretation of MT and ESR values in the clinical context. When used appropriately and in the right clinical context, MT with or without ESR, provides invaluable information and has good sensitivity and specificity for active TB, even in a BCGvaccinated, TB endemic population. Volume 17 Issue 01 Conclusions In our study population, significantly larger areas of induration were observed 48 hours after administration of PPD in patients with active tuberculosis. Indurations were significantly larger in lymph node tuberculosis as compared to pulmonary and extrapulmonary TB at sites other than the lymph nodes. Erythrocyte sedimentation rate provided additional supportive evidence of active TB in younger individuals. We therefore conclude that the Mantoux test with or without ESR does have useful information to offer as a diagnostic tool. 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