Indian Pediatr 2010;47: 90-92
T-Cell Assay as a Diagnostic Tool for
and A Bose†
A Warier, S Gunawathi, Venkatesh*, KR John
From the Department of Child Health, *Low Cost Effective
Care Unit and †Department of
Christian Medical College, Vellore, India.
Correspondence to: Dr Anuradha Bose, Department of
Community Health, Christian Medical College, Vellore 632 002, Tamilnadu,
Received: September 25, 2007;
Initial review: November 19, 2007;
Accepted: October 27, 2008.
Published online: 2009 April 15.
This study aimed to estimate the specificity and
sensitivity of a whole blood IFN-g assay (ELISPOT) test for diagnosis in
childhood tuberculosis. 96 patients, less than 18 years of age,
diagnosed and commenced on anti-tubercular therapy were enrolled and
tested. 47 age and sex matched controls were also tested. 23 tests were
deemed invalid and analysis done on the remainder. The sensitivity was
53.3% in confirmed cases and less in other groups. The specificity was
high at 97.9%. This test can be an useful aid in the diagnosis of
Key words: Children, Diagnosis, Interferon assay,
Tuberculosis in children is paucibacillary
in nature and bacteriological confirmation by culture of Mycobacterium
tuberculosis rarely exceeds 30-40%(1). There is a need for an
effective diagnostic test for childhood tuberculosis. An in vitro
whole blood test that detects M. tuberculosis infection by
measuring IFN-g responses to a number of specific proteins including
culture filtrate protein (CFP-10) and early secreted protein 6 (ESAT-6) is
now available. The two antigens mentioned are part of the M.
tuberculosis genome, absent from genomes of all BCG sub strains and
most non-tuberculous mycobacteria (NTM)(2).
We used one of the available assays, the enzyme linked
immunospot assay (ELISPOT test) (Oxford Immunotec, Oxford, UK). The
ELISPOT has been widely used for screening for latent infection(3-5) and
has been compared with the tuberculin skin tests (TST)(6,7). In this
study, we evaluated the sensitivity and specificity of ELISPOT in the
diagnosis of childhood TB. Specificity was estimated in a group of BCG-vaccinated
children with no known exposure to M. tuberculosis infection.
Sensitivity was determined in three defined groups, confirmed, probable
and possible tuberculosis.
The study was conducted, after obtaining ethical
approval, and with informed consent from the parent or patient between
March 2005 and November 2006 at Christian Medical College, Vellore.
Children and young people below the age of 18 years who were diagnosed and
started on anti-tubercular treatment less than 2 weeks previously, were
prospectively enrolled in the study.
The clinical diagnosis of tuberculosis was made when at
least 2 of the following criteria were positive: positive tuberculin skin
test (TST), defined as induration >10 mm in the horizontal plane; clinical
findings compatible with diagnosis; positive history of contact; culture
positivity; biopsy positivity of skin, lymph node or pleura; and, positive
imaging studies. The cases were further categorized as (i)
Confirmed tuberculosis: Culture or smear positive for M.tuberculosis;
and/or biopsy-pathological features suggestive of tuberculosis; and (ii)
Probable tuberculosis if the chest X-ray was suggestive of active
tuberculosis; presence of cervical lymphadenopathy of more than 2 cm, with
a positive TST; abdominal mass, ascites, or lymphadenopathy on ultrasound;
CSF changes consistent with TB meningitis, or presence of tuberculomas on
neuroimaging, (iii) Possible tuberculosis: was considered with a
history of contact, with either a positive TST or with failure of
nutritional rehabilitation. Controls were matched for age and sex, had a
diagnosis other than tuberculosis, had no history of contact with TB, and
were well apart from the acute clinical condition for which health care
was being accessed. TST (10 units per mL) was only done on controls if
clinically indicated. They were reviewed in six months, to ensure that
they had remained well, and were unlikely to have been harbouring early
infection with tuberculosis.
All assays were performed by one of the two
investigators. ELISPOT assays were done, following protocol, with kits
provided by Oxford Immunotech. Quality control of the Vellore laboratory
was done at the start of the study, and results validated. ELISPOT plates
were read manually by an inde-pendent blinded observer and subsequently
scored in Oxford with an automated ELISPOT counter. Sett-ings for
intensity and spot size were predefined and fixed. Statistical analysis
was done using Epi-Info.
143 tests were done, 96 cases and 47 controls, 32
results were initially declared invalid and 9 subsequently manually
verified and validated. Of the failed samples, 2 were from confirmed, 7
‘probable’ and 14 ‘possible’ cases. All failures were due to ‘positive
control’ failures, ie, less than 20 spots and less than 75% saturated.
There were 41 girls and 32 boys amongst valid cases,
and 16 girls amongst controls. The cases were diverse: bone and joint
involvement (n=6), cough and fever (n=13 and 27), hemoptysis
(n=2), meningitis (n=15), lymph node enlargement (n=17),
and weight loss (n=6). Table I shows the number of
positive results in the different groups of patients. Sensitivity in cases
were 53.3%, 36.8%, and 30% in the confirmed, ‘probable’ and ‘possible’
groups, respectively. The specificity was 97.9%.
Children in Each Group and Positive Results
Validation of a diagnostic tool in tuberculosis is a
challenge. The gold standard of a positive Mycobacterial culture rarely
exceeds 30-40%. Factors such as tests not being done in appropriate
populations, bias in patient selection, bias due to lack of blinding have
been addressed in the selection of patients and controls.
A study from South Africa has shown higher sensitivity,
i.e., two thirds in children with a clinical diagnosis of tuberculosis and
in 83% with culture-confirmed disease(8). Our results showed a lower
sensitivity; 53% in confirmed cases and lower in the other categories,
perhaps because of poor specificity of the diagnostic criteria in the
other groups. The specificity of the test was very high, 97.8% (46/47).
All but 11 of the 143 children had a BCG scar, indicating that the BCG
does not have an effect on the ELISPOT result. The presence in the control
patients of a variety of infectious, inflammatory, and granulomatous
diseases indicates that the assay is not confounded by nonspecific
activation of the cellular immune system.
The ELISPOT assay has some limitations. It is
expensive, requires trained laboratory personnel and sophisticated
laboratory equipment. The blood needs processing within 4 hours of
collection. In our series, there was some failure of the positive control
Dr Ajit Lalvani for facilitating this study, Dr Ian
Durrant for providing the kits, and Dr Jayaprakash Muliyil for his advice
on design of the study. We acknowledge the following co-investigators on
the project: Dr Leni Mathew, Dr Indira Agarwal, Dr Mona Baskar, Dr Joy
Mammen, Dr Maya Thomas.
Contributors: AB designed the study, in
consultation with KRJ. AW and SG were the Research Officers on the
project, identified the patients, assisted in sample collection. All
authors provided critical input into the preparation of the manuscript.
Funding: Oxford Immunotec company.
Competing interests: None stated.
What This Study Adds?
Enzyme linked immunospot assay (ELISPOT) has low sensitivity but
high specificity for the diagnosis of pediatric tuberculosis.
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