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Investigations of TB
List of investigations
1. Radiology 2. Bacteriology examination 3. Histopathological examination 4. Immunological diagnosis 5. Nucleic acid amplification methods 6. Biochemical markers of diagnosis
Primary TB
Normal radiographic findings may be seen in up to 15% of patients with proved tuberculosis.
Parenchymal disease
Disease manifests as dense, homogeneous parenchymal consolidation in any lobe; however, predominance in the lower and middle lobes is suggestive of the disease, especially in adults
Lymphadenopathy
Lymphadenopathy in a patient with primary tuberculosis
Miliary disease
Pleural effusion
Post primary
Parenchymal disease
The earliest finding in parenchymal disease is patchy, poorly defined consolidation, particularly in the apical and posterior segments of the upper lobes.
Airway involvement Pleural extension
Extra pulmonary
Tuberculous meningitis. Axial contrast-‐enhanced T1-‐weighted MRI shows florid meningeal enhancement that is most pronounc ed within the basal cisterns.
Pott abscess in a patient with TB spondylitits
Bacteriological examination
Specimen
Pulmonary TB Extra pulmonary TB ● Sputum ● Induced sputum ● Laryngeal swab ● Bronchoalveolar lavage ● Lung biopsy
● Renal TB → first-‐morning urine ● Intestinal TB in AIDS patients → Stool ● Disseminated TB → Whole blood and/or bone marrow ● Skeletal TB → Bone ● In tuberculous pleural effusions → pleural biopsy ● Lymph nodes, skin lesion material ● Abscess contents
Smear
1. AFB smear staining o Smear staining is based on the high lipid content of the cell wall of mycobacteria which makes them
resistant to decolorization by acid-‐alcohol after the primary staining. 2. ZN stain
Culture
Culture media: ● Löwenstein-‐Jensen ● Liquid medium ● The BACTEC TB-‐460
“The definitive diagnosis f TB is isolation of MTB in pure culture.”
Bactec culture system
● Results can be obtained after 10 days ● Its principle is based on that the TB bacilli metabolize C14 containing palmetic acid of a broth media,
producing radioactive labeled 14CO2 in the atmosphere that collect above the broth in the bottle. ● The bactec instrument measure the amount of radioactivity called (growth index)
Histopathological examination
● For detection of TB granuloma (also diagnostic for TB). ● Histopathological examination is done for biopsy specimen:
a) Transthoracic CT guided biopsy for pulmonary lesions. b) Pleural biopsy for pleural TB. c) Biopsy specimen from extra-‐pulmonary TB lesions.
Immunological
Serology has advantages in situations when: ● The patient is unable to produce adequate sputum ● Sputum smear results are negative ● TB is extrapulmonary
Tuberculin Test
Intradermal injection of 0.1 purified protein derivative Interpretation after 48-‐72 hours
An induration of 5 or more mm is considered positive in
• HIV infected persons • A recent contact of a person with TB disease • Persons with fibrotic changes on CXR consistent with prior TB • Patients with organ transplants • Persons who are immunosuppressed for other reasons (eg taking the
equivalent of >15mg /day of prednisolone for 1 month or longer, taking TNF-‐alpha antagonist)
An induration of 10 or more mm
is considered positive in
• Recent immigrants (<5years) from high prevalence countries • Injection drug users • Residents and employees of high risk congregate setting • Mycobacteriology laboratory personnel • Persons with clinical conditions that place them at high risk • Children <4 years of age • Infants, children , and adolescents exposed to adults in high risk
categories An induration of 15 or more mm
is considered positive in Any person including person with no known risk factors for TB
What are false negative results ?
• Preallergic phase. • Fulminant cases, especially in miliary TB. • Steroid therapy or immunocompromized hosts. • Technical reasons.
What are false positive results?
• Had recent BCG vaccine before • Infection from non MTB
Interferon-‐gamma release assays
● IGRAs are in vitro blood tests of cell-‐mediated immune response. ● They measure T cell release of interferon-‐gamma (IFN-‐gamma) following stimulation by antigens unique to M.
tuberculosis. ● Not affected by Bacille Calmette-‐Guérin (BCG) vaccination status ● Unaffected by most infections with environmental nontuberculous mycobacteria ● Tyoes of assays
o QuantiFERON-‐TB Gold In-‐Tube (QFT-‐GIT) assay: quantification of IFN-‐gamma o T-‐SPOT.TB assay: number of IFN-‐gamma producing T cells
TST &IGRAs cannot distinguish between latent infection and active TB disease
ELISA
● Using ELISA test: of little use in diagnosis of TB as it has low sensitivity and specificity ● ELISA based methods for the detection of mycobacterial antigen in body fluids. ● Positive test may perhaps “rule in”a diagnosis, but a negative test cannot “rule out”a diagnosis of
tuberculosis. ● Affected by BCG vaccination, previous infection and environmental NTM exposure.
Nucleic acid amplicfication method
PCR
Biochemical markers of diagnosis
● Adenosine deaminase (ADA) ● Bromide partition test ● Gas chromatography of mycobacterial fatty acids (Tuberculostearic acid).