Rapid Molecular Diagnosis of Pulmonary Tuberculosis in Children Using Nasopharyngeal Specimens
Heather J. Zar
()
1
2
3
Lesley Workman
1
2
3
Washiefa Isaacs
1
2
3
Jacinta Munro
1
2
3
Faye Black
1
2
3
Brian Eley
1
2
3
Veronica Allen
0
2
5
Catharina C. Boehme
2
4
Widaad Zemanay
0
2
5
Mark P. Nicol
0
2
5
0
Division of Medical Microbiology
1
Red Cross War Memorial Children's Hospital
2
Received 20 March 2012; accepted 14 June 2012; electronically published 2 July 2012. ICH Bldg, Red Cross War Memorial Children's Hospital
,
Cape Town
,
South Africa
3
Department of Paediatrics and Child Health, University of Cape Town
4
Foundation for Innovative New Diagnostics
,
Geneva
,
Switzerland
5
Institute for Infectious Diseases and Molecular Medicine, University of Cape Town, National Health Laboratory Service, Groote Schuur Hospital
,
Cape Town
,
South Africa
Background. A rapid diagnosis of pediatric pulmonary tuberculosis (PTB) using Xpert MTB/RIF (Mycobacterium tuberculosis/rifampicin) automated testing on induced sputum (IS) is possible, but the capacity for performing IS is limited. The diagnosis using a nasopharyngeal aspirate (NPA), which can be non-invasively obtained, is desirable. Methods. Paired specimens (NPA and IS) were tested using smear, liquid culture and Xpert. The diagnostic accuracy of Xpert and smear was compared with culture for different specimens in children with suspected PTB. Results. There were 535 children [median age 19 months, 117 (219%) HIV-infected] who had one IS and one NPA specimen; 396 had two paired specimens. A positive smear, Xpert test or culture occurred in 30 (5.6%), 81 (15.1%) and 87 children (16.3%), respectively. The culture yield was higher from IS (84/87, 96.6%) vs NPA (61/87, 70.1%, P < .001). Amongst children with two paired specimens, 63 culture-confirmed cases occurred [60 (95.2%) IS vs 48 (76.2%) NPA, P = .002]. The sensitivity of two Xpert tests was similar for IS and NPAs [(45/63) 71% vs (41/63) 65%, P = .444)]; the sensitivity of smear was lower for IS (21/63, 33%) and NPA (16/63, 25%). The incremental yield from a second IS was 9 cases (17.6%) by culture and 9 (25%) by Xpert testing; a second NPA increased the culture yield by 10 (26.3%) and Xpert by 11 (36.7%). Xpert specificity was 99.1% (98.1-100) for IS and 98.2% (96.8-99.6) for NPAs. Xpert testing provided faster results than culture (median 0 vs 15 days, P < .001). Conclusions. Xpert testing on 2 NPAs is useful in children with suspected PTB, particularly in settings where IS and culture are not feasible. A diagnosis of pulmonary tuberculosis (PTB) in children remains challenging due to non-specific clinical or radiological signs and an inability to obtain appropriate specimens [1]. Moreover, as childhood tuberculosis is paucibacillary, a diagnosis using smear microscopy is usually negative. However, recent advances in specimen collection and diagnostic testing have shown that
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a microbiologic diagnosis can be effectively made [2].
Repeated induced sputum (IS) specimens are feasible
and effective for the culture-confirmed diagnosis of
PTB in children [35].
We recently reported that a rapid diagnosis of PTB
in children using the Xpert MTB/RIF automated,
cartridge-based test (Xpert, Cepheid, CA, USA) for
Mycobacterium tuberculosis and resistance to rifampicin
could be effectively obtained using IS specimens [6].
Two Xpert tests detected 76% of culture-confirmed cases,
twice the yield from smear, with a specificity of 99% [6].
Rifampicin resistance was reliably detected, and the
results were faster than liquid culture [6]. These provide
pediatric evidence to support WHO recommendations
that Xpert testing replace smear as the initial diagnostic
test in suspected HIV-associated tuberculosis or
multidrug-resistant tuberculosis and as a follow-on test in
settings where these are less of a concern [7].
The implementation of pediatric Xpert testing is constrained
by the limited capacity to obtain IS. The use of a
nasopharyngeal aspirate (NPA) is attractive, as this can be easily and
non-invasively obtained with a lower risk of nosocomial
transmission. However, a study of Peruvian children comparing 2
NPAs with 2 gastric aspirates (GAs) found that only 50% of
culture-positive cases were detected with NPAs [8]. Moreover,
an in-house polymerase chain reaction (PCR) had insufficient
sensitivity or specificity to detect tuberculosis for clinical
use [8]. However, no comparison with IS was done. A small
Ugandan study reported promising results for culture and PCR
performed on NPAs from children with suspected tuberculosis
[9]. The availability of Xpert now provides a reliable test with a
high sensitivity and specificity for IS samples [6, 10]. We aimed
to investigate the yield from repeated NPAs compared to IS
specimens using culture, smear and Xpert.
MATERIALS AND METHODS
Consecutive children hospitalized with suspected PTB in Cape
Town, South Africa from 1 February 2009 to 30 June 2011 were
enrolled. The reasons for hospitalisation included severe or very
severe pneumonia defined by WHO criteria, the need for
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