Nasopharyngeal specimens allow pulmonary TB diagnosis in children
When induced sputum and culture are not available, nasopharyngeal specimens may be used for rapid molecular diagnosis of pulmonary tuberculosis in children, a new study shows.
Optimally, the children would provide two induced sputum specimens on sequential days, for Xpert (rapid molecular diagnosis) and liquid culture, lead investigator Dr. Heather J. Zar told Reuters Health by email.
But Dr. Zar, from University of Cape Town and Red Cross War Memorial Children's Hospital in Cape Town, South Africa, added, "if (that's) not feasible then two nasopharyngeal aspirate specimens for Xpert are useful and will detect around two-thirds of children with culture-confirmed disease."
Her study assessed diagnostic yields from nasopharyngeal aspirates and sputum using culture, smear, and Xpert (Cepheid) in a study of 535 children hospitalized with suspected pulmonary TB.
A nasopharyngeal aspirate was suctioned first, after two drops of sterile saline were instilled into each nostril. A second nasopharyngeal aspirate was obtained the next day, or at least four hours after obtaining the first induced sputum sample.
All of the children had at least one paired nasopharyngeal aspirate and sputum sample, and 396 had two sets of paired specimens.
According to a report online July 2 in Clinical Infectious Diseases, 87 children had definite TB. Thirty (5.6% of the total cohort) had a positive smear, 81 (15.1%) had a positive Xpert result, and 87 (16.3%) had a positive culture.
Xpert detected all 30 smear-positive cases and 40 of the 57 (70.2%) smear-negative cases.
Using culture from any specimen as the reference, the sensitivity of two Xpert results was 71.4% for induced sputum samples and 65.1% for nasopharyngeal aspirate specimens.
"In contrast to adult data, two specimens (induced sputum or nasopharyngeal aspirate specimens) obtained on sequential days are needed to achieve acceptable detection rates using Xpert," Dr. Zar said.
Xpert sensitivity did not differ significantly in kids with and without HIV infection, using either induced sputum or nasopharyngeal aspirate specimens.
The specificity of Xpert was 99.1% for induced sputum and 98.2 for nasopharyngeal aspirate specimens.
The sensitivity and specificity of Xpert for diagnosis of rifampicin resistance were 83.3% and 99.1%, respectively, and results were faster (same day) than culture results (15 days).
"Xpert is an important advance in diagnosis, enabling rapid initiation of effective therapy," Dr. Zar said. "However, we still need a better point of care diagnostic test, given that more than 40% of children were diagnosed with possible TB (and were culture and Xpert negative). Amongst this group of possible TB cases, we need a better test to distinguish those children with TB from those without TB, but evaluating such a test will be challenging without a gold standard (culture)."
She and her group are presently evaluating other diagnostics, including urine LAM (lipoarabinomannan), interferon-gamma release assays, serology, and combinations of diagnostics, e.g., LAM and Xpert.
They're also testing Xpert in primary care settings, "where children have less severe disease and where the largest burden of childhood TB exists," rather than in hospital settings, Dr. Zar said.
The study was funded by the National Institutes of Health, the National Health Laboratory Services Research Trust, the Medical Research Council of South Africa, and The Wellcome Trust.
SOURCE: http://bit.ly/MdP3CG
Clin Infect Dis 2012.
Medscape Today