Polymerase chain reaction
With the help of PCR, millions of identical copies of any specific DNA or RNA sequence can be made. This sequence may be a gene, or a part of gene, or simply a stretch of nucleotides.
This is done by using highly specific oligonucleotide primers to amplify the target nucleic acid by repeated rounds of denaturation, primer annealing and amplification. The target DNA is amplified a billion fold within 2-4 hours, and becomes quantitatively enough to be detected by a variety of detecting system.
The principle is used to target the DNA of even very small number of mycobacteria available in the specimen, amplify it and detect the material, now available in a detectable quantity. The whole process takes about 24 hours. It can detect less than 10 organisms in the specimen.
It is a very sensitive and specific test.
However, even a slight contamination can result in a false positive result.
Enzyme linked immunosorbent assay (ELISA)
Due to the highly complex nature of both Mycobacterium tuberculosis and the human immune response to it, there is no known 100 percent diagnostic serological test for confirming the presence of tuebrculous disease in an individual.
ELISA techniques have been in use for some time to now to detect antibodies against various mycobacterial antigens in serum of patients suspected to have TB. The commonly done tests detect antibodies (IgG, IgM,) against cytoplasmic antigen 60 (A60) of mycobacteria.
These tests are sensitive towards infection with Mycobacterium tuberculosis but may not be specific in differentiating tubercular infection from tubercular disease. Although some people have claimed a specifically of upto 90 percent for ELISA testing, there is not enough data to define the right cut off value (which may even vary from population to population) which can differentiate between mere infection with, and disease caused by Mycobacterium tuberculosis. In addition, the test may cross react with other mycobacteria and other bacterial antigens.
ELISA is not recommended at present for sue in the routine work up of a patient suspected to have TB.
