TB ELISPOT: A POSSIBLE DIAGNOSTIC TEST IN CHILDHOOD TUBERCULOSIS?
A. Bose*, A. Warier**, M. Baskar***, J. Mammen****
Christian Medical College, Vellore, India. *, Christian Medical College, Vellore, India. **, Christian Medical College, Vellore, India. ***, Christian Medical College, Vellore, India. ****
Introduction: There is a need for a sensitive and specific test for tuberculosis in childhood. Bacterial culture positivity is the gold standard but has a low yield in childhood. Obtaining suitable specimens for culture is challenging. A relatively new blood test, Enzyme Linked Immuno Spot Assay (ELISPOT) detects T cells specific for antigens in Mycobacterium tuberculosis. It has been found to have a sensitivity equal to the Tuberculin Skin Test (TST), and has been used as a screening tool in low endemic areas.

Aims:The aim of this study was to assess the diagnostic value of the TB ELISPOT test in a paediatric population in an endemic area, in Vellore, India.

Methods: Inclusion criterion was clinical diagnosis of tuberculosis by the attending physician. Recruitment was within 2 weeks of starting antituberculous therapy. The cases were grouped into Confirmed, Probable and Possible tuberculosis. The assays were done blinded, by 2 investigators and the results read manually and by an Elispot Reader at Oxford, UK, where the technical quality of the assays were validated.

Results: The coloured spots in each well were counted manually using a magnifying glass. The plates were then sent to Oxford for a reading by the Elispot reader. Only those plates which pass quality control and where there was concurrence on the results were included in the analysis. A total of 125 tests were done, including those done on 20 controls. 32 results were declared invalid at Oxford. 93 samples were valid and the results are shown below.


DEFINITIVE CASES

PROBABLE CASES

POSSIBLE

CONTROLS

Total Number 15 38 20 20
Number positive 8 14 6 1
Percentage positive 53.3% 36.8% 30% 5%


Conclusion: The sensitivity, specificity and the positive predictive value of this test was found to be 41%, 95% and 96.7% respectively. This compares well with bacterial culture but has the advantage of being done on a sample of blood. The low sensitivity is unexpected but a potential advantage in a high endemic country. It could be argued that the sensitivity levels are a result of over diagnosis of tuberculous infection. It may also be a manifestation of the immunological response in a nutrient deficient population.

References :
  1. http://w3.whosea.org/EN/Section10/Section186/Section191.htm A.C.Hesseling, H.S.Gie et al : A critical review of diagnostic approaches used in the diagnosis of childhood tuberculosis: Int J Tuberc Lung Dis 2002; 6(12):1038-1045.
  2. S. Liebeschuetz, Shella Bamber, Katie Ewer, Jonathan Deeks, Ansar A Pathan, Ajit Lalvani. Diagnosis of tuberculosis in South African children with a T-cell-based assay: a prospective cohort study www.thelancet.com Vol 364 December 18/25, 2004.

Acknowledgement : Technical support and resources provided by Dr. Ian Durrant and his team at Oxford Immunotec, UK
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