IX NATIONAL CONFERENCE OF PEDIATRIC INFECTIOUS DISEASES, CHENNAI, OCT 2006
A. Bose, A. Warier, M. Baskar, J. Mammen
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 Immunospot 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 :
- 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.
- 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
