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| Diagnostic tool | Method/inference | Merits | Limitation /drawback |
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| Tuberculin skin Test (TST)/Mantoux test | 5 tuberculin units were injected intradermally and read 48-72 hours later. Positive when induration of 5-15 mm is seen | Used as an essential screening diagnostic tool
Helpful in the diagnosis of active TB
More precise than radiographs
Easy to perform | False-positive test results due to cross-reactivity with BCG or non-TB mycobacteria
False-negative results in immunocompromised individuals
Difficult to use in children
Test results are interpreted only after 48-96 hours; thus, a follow-up visit is required |
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| Interferon release assays (IGRAs) | The amount of interferon-gamma (IFN-Y) in response to contact with the TB antigens is measured | Not confounded by previous BCG vaccination
Approved by the Food and Drug Administration (FDA) as a more precise substitute to TST for the diagnosis of TB infection | Expensive, poor predictors for TB progression
Cannot distinguish between LTBI and active TB |
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| TST or IGRAs alone have a suboptimal ability to rule in or negate active TB. Hence, suitable clinical samples for microbiological and molecular assays should be collected from every patient suspected of active TB. IGRAs should always be employed with other investigations (e.g., TST results and chest X-ray findings) to establish an active TB diagnosis |
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Staining
(a) Ziehl-Nelson staining (AFB staining)
(b) Auramine fluorescence | Acid-fast bacilli (AFB) are seen as bright red rods against a blue, green, or yellow background | Simple method, economical, noninvasive | As there is a relative dearth of tubercle bacilli in oral specimens, the ability to affirm acid-fast bacilli in histological samples is quite low (7.8%)
A similar appearance may be seen with saprophytic mycobacteria
Requires expensive equipment
Used as a screening tool, not for final diagnosis |
| Visualizes acid-fast bacilli as bright rods against a dark background using a fluorescent microscope | Contrast bacilli can be readily seen under a high-dry objective
More sensitive
Less tiring, quick results for a large number of slides | Requires expensive equipment
Used as a screening tool, not for final diagnosis |
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Histopathology
Histopathological evaluation is necessary for patients with nonhealing ulcers (of more than 3 weeks) with the absence of constitutional features | Granulomatous disorders may be considered if the histologic examination reveals the presence of granulomas | Gold standard diagnostic aid | Delayed or erroneous histologic diagnosis may be seen as granulomas may not be noticeable in early lesions, or can be absent in immunosuppressed individuals |
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| A combination of acid-fast staining (Ziehl-Neelsen staining) and histopathology can serve as definitive investigative aids for a precise diagnosis |
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| Radiographs of suspected TB cases should be advised for posterior-anterior (PA) and lateral view chest radiographs, even in the absence of constitutional symptoms | Areas of calcifications, cavities, or radiolucency | Easy to perform
Quick interpretation | Exposure to X-rays
Poor sensitivity
Cannot distinguish between active TB and healed TB in case of scar formation |
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Culture
(a) Lowenstein–Jensen media (LJ media)
(b) BACTEC | When grown on LJ media, M. tuberculosis appears as brown granular colonies | Less expensive than BACTEC
Fewer chances of contamination | Takes 4-6 weeks to get visual colonies on media
No differentiation between M. tuberculosis and other Mycobacterium species |
Detects the presence of oxygen in fluorescence by scanning it after every hour
Positive samples may contain 105–106 CFU/ml | Early detection
Differentiates M. tuberculosis from other Mycobacterium species
More sensitive than conventional LJ media | Expensive
More risk of contamination |
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| Polymerized chain reaction (PCR) | Helps in the detection of infectious agents and can differentiate between nonpathogenic and pathogenic strains | Rapid diagnostic aid
Easy amplification of even very small-sized DNA
High sensitivity, virus detection soon after infection and even before the disease onset | Localization within tissues is not possible
Staging of mycobacterial disease is not possible
GeneXpert requires professional training and is expensive |
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