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Adjunct to tuberculin skin test recommended to aid TB diagnoses
Using the mycobacteria growth indicator tube as an adjunct to tuberculin skin testing may improve early tuberculosis diagnoses in pediatric patients, according to study results published online.
Tomàs M. Pérez-Porcuna, MD, MSc, of the University of Barcelona, and colleagues evaluated new strategies for early TB diagnosis using an outpatient protocol in children aged 0 to 5 years who had recent household contact with a person with TB.
The investigators looked at data on 102 children, 32 of whom with suspected TB based on a clinical exam. The researchers included patients with recent contact with a symptomatic index case-patient in the previous 6 months, or those patients who met WHO definitions of suspected TB.
Pérez-Porcuna and colleagues said a positive culture with the mycobacteria growth indicator tube was typically associated with a strong tuberculin skin test, “allowing the diagnosis of 33% more cases with little or no symptomatology compared with the exclusive use of the clinical classification.”
The researchers said concordance between microbiologic cultures and clinical scores was low, and only four of seven children who had positive cultures were not classified as having TB. They added that smears of sputum failed to detect any cases.
The researchers concluded that diagnosing TB in young patients is often hindered because of difficulties involved in obtaining good respiratory samples, so backup methods are needed.
Disclosure: Dr. Pérez-Porcuna reports no relevant financial disclosures.
Perspective
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The study by Porcuna and colleagues had a somewhat small population, as is the case for many childhood tuberculosis studies. They started with 102 patients and sent cultures on 32, of whom only 56% had abnormal chest X-rays. The fact that some children with normal radiographs, nonetheless, had positive cultures reflects the sometimes arbitrary distinction between TB disease and infection in young children. It is interesting that radiographic findings did not impact the probability of having a positive polymerase chain reaction or culture. Many prior studies have shown that rates of culture positivity are higher with more extensive pulmonary involvement; the best example would be infants with miliary TB. However, it is difficult to compare studies, as there is no validated chest X-ray scoring system to gauge radiographic severity.
The authors also found an association between tuberculin skin test (TST) positivity and culture positivity. While the TST has been estimated to be 90% to 95% sensitive for TB disease, many children with TB disease are anergic to the TST; this is particularly true for children with TB meningitis and miliary TB. It would have been interesting to see the results of interferon-gamma release assays in this bacille Calmette-Guérin-immunized population.
The investigators appropriately emphasize the importance of the TST in contact investigations and emphasize the importance of getting cultures, as the diagnostic scoring scales showed suboptimal sensitivity. However, physicians should remember that it is not necessary to obtain gastric aspirates in all contacts with positive TSTs, especially when they have negative radiographs (and this population comprised almost one-half of the subjects in this study).