Diagnostic tests
The tuberculin skin test (TST) became the gold standard for clinical diagnosis and identification of individuals with M. tuberculosis (MTB) infection (passive and active) after publication of Robert Koch’s work (1891) with guinea pigs and the tubercle bacilli. Presence of a “wheal” reaction at the site of inoculation indicates a positive test requiring a repeat clinic visit for confirmation. When testing individuals vaccinated with bacille Calmette-Guerin (BCG) vaccine or those who cannot return for reading of the TST, blood tests, such as interferon-gamma (IFN-g) release assays (IGRA), are available and recommended by the Food and Drug Administration (FDA). The IFN-g substance is present in the white blood cells of infected individuals and can be detected by IGRA tests, such as the QuantiFERON®-TB Gold In-Tube test and the T-SPOT® TB test.
Disadvantages of the blood test method are the required laboratory processing within 8 to 30 hours of exposure and the failure of success in children less than five years or in immune-compromised persons. The test is preferred for anyone who received BCG or cancer therapy. BCG vaccine is usually given to children to prevent TB meningitis in developing countries where transmission of TB is high. The vaccine is not recommended in the United States for pulmonary TB because of its questionable effectiveness and interference with the TST, which is used routinely.
Genotyping
For epidemiological purposes, the genotyping of MTB strains has created an international database of information to connect patients, determine outbreaks, distinguish reinfection from reactivation, and identify laboratory contaminants. Programs for TB control in public health laboratories include genotyping, as directed by the CDC.
The molecular technique employed, IS6110-restrictive fragment length polymorphism (RFLP), continues as the standard of reference for typing M. tuberculosis. The sequence, IS6110, present in numbers 0 to >20, is inserted at various loci on the genomes of MTBC isolates and used to compare patterns among laboratories.
Spacer oligonucleotide typing (Spoligotyping), a PCR technique requiring small quantities of DNA, is another typing method. The newer, rapid typing MIRU-VNTR is based on variable numbers of tandem repeats (VNTR) of genetic element classes, or mycobacterial interspersed repetitive units (MIRU). MIRU-VNTR is more manageable but less discriminatory and may replace the current standard, the RFLP analysis.
