Tuberculosis - Classification and Laboratory Diagnosis (I) Flashcards
Mycobacterium
Family
Gram stain
Aerobic requirement
Mycobacteriaceae
Gram positive bacilli
Obligate aerobe
3 reasons why mycobacterium takes long time to diagnose and is difficult to treat
1) Long generation time, slow growing»_space; need long incubation culture
2) High lipid content in cell wall»_space; Poor penetration by antibiotics
3) Unique antibiotic susceptibility patterns»_space; Limited anti-mycobacterial agents
3 classes and subclasses of mycobacteria ***
1) Mycobacterium tuberculosis COMPLEX
2) Mycobacterium leprae
3) Other: Non-tuberculosis mycobacteria (NTM), MOTT, Atypical mycobacteria
3 dominant species in Mycobacterium tuberculosis COMPLEX
M. tuberculosis (human, captive animals)
M. bovis (Cattle, human, captive animals)
M. bovis BCG (Human, for vaccination)
2 methods to classify NTM
Runyon classification (growth rate and pigmentation)
Molecular techniques
4 classes of NTM based on Runyon classification
Growth rate?
I = Photochromogens II = Scotochromogens III = Non-chromogens IV = Rapid growers
All slow growth except Runyon group IV
Give examples of 4 classes of Runyon classification
I = Photochromogens = M. kansasii, M. marinum II = Scotochromogens = M. gordonae III = Non-chromogens = M. avium complex IV = Rapid growers = M. fortuitum complex, M. abscessus
Compare latent and active TB
- Bacteriological Dx result
- Clinical symptoms and signs?
- Diagnostic methods
- Drug treatment
Active TB = likely +ve bacteriological Dx, Clinical symptoms present
Latent TB Dx by immunological test or past history of TB
Immunological tests cannot tell active vs latent TB
Latent TB Tx: 1 or 2 drug regimen
Active TB Tx: 4 drugs
3 immunological tests for Latent TB Dx/ adjunctive tests
Can they differentiate active and latent TB
Tuberculin skin test (TST)
Interferon gamma release assay (IGRA)
Adenosine deaminase (ADA)
Cannot differentiate active and latent TB, evidence of post-infection
Non-tuberculous mycobacteria.
Types of infections? (5)
- Pulmonary infection (superimposed with existing disease) - M. avium complex, M. kansasii
- Lymphadenitis - M. abscessus
- Skin and soft tissue infections - M. abscessus, M. marinum
- Catheter-related, Nosocomial infections - Rapid growers
- Disseminated infection in immunocompromised - M. haemophilum
Usually all environmental, contamination
5 approaches to TB Dx?
Clinical Radiological Microscopy Culture Nucleic acid amplification test (NAAT) - PCR
Course of action after positive M. tuberculosis culture?
Act asap on treatment
Difference in action after positive culture of NTM from normally sterile sites vs superficial sites
NTM +ve at normally sterile sites (e.g. immunocompromised with IV line infection)»_space; Act asap on treatment after excluding contamination
NTM +ve at superficial site (e.g. sputum, wound swabs)»_space; Check clinical symptoms and radiological changes, contamination
List 3 techniques to identify mycobacterium species.
Is culture still necessary?
- PCR +/- Sequencing ***
- MALDI-TOF Mass spectrometry
- Physiological and biochemical tests
Culture still gold standard: most sensitive, can identify species, Can test antimicrobial resistance
Define MDR-TB
M. tuberculosis
Resistant to at least isoniazid and rifampin
Define XDR - TB
Resistant to at least isoniazid and rifampin
+ any fluoroquinolone
+ at least 1 of Amikacin, Kanamycin, Capreomycin
Define TDR -TB
Total drug resistant
XDR TB plus more
antimicrobial susceptibility testing for each class of mycobacterium?
For M. tuberculosis:
- Phenotypic method = standard
- Genotypic method = Fast, may miss some resistance
For M. leprae:
- Can’t culture, routine testing impossible
For NTM:
- No standardized testing method for most species
What contexts must be accounted for to build clinical suspicion for TB?
- Clinical presentation
- Epidemiology: age group, close contacts
- Host predisposition: Immunocompromised (HIV, Diabetics, Malnutrition), Smoking, Drugs, Cancers…etc
- Radiological: Not only in apical lung, can be basal; cannot tell TB from NTM infection
CSF characteristics for TB infection
Look, pressure, WBC, Protein, glucose, culture rate
Appearance: cloudy, fibrin webs Opening pressure: high White cell count: 10-350, slightly elevated, polymorphs and lymphocytes Protein: High Glucose: Low (<50%) Culture: (35% positive)
List 4 specimens for Bacteriological, pathological or molecular Dx of TB
Sputum Urine (first void)
Blood
Others: CSF
List 2 specimen for immunological adjunctive Dx of TB
Blood for Interferon gamma release assay
Skin - Tuberculin skin test
What determines the yield of lab Dx tests for TB? (3)
Quality and Quantity of specimen at site of infection
- Quality: e.g. BAL, Transbronchial/ transthoracic biopsy for good sputum instead of saliva sample
- Quantity: e.g. volume of CSF, fluids
- Tissue biopsy at exact site of infection
Indication for bronchoscopy to Dx Pulmonary TB. (5)
BAL +/- Transbronchial/ transthoracic biopsy
Always >2 sputum samples
1) Lack of respiratory symptoms
2) No sputum production
3) CRAPPY sample (saliva)
4) Exclusion of concurrent infection/ non-infectious pathologies
5) Certain location of TB e.g. Endobronchial TB
3 advantages and 4 disadvantages of microscopic Dx of TB
Next step if microscopy is negative?
Adv:
- Simple
- Cheap
- Rapid
Disadv:
- Low sensitivity
- Operator dependent
- Cannot differentiate M. tuberculosis from NTM; live from dead bacteria
- Low positive rate (<50% active pulmonary TB have +ve result)
Next step: PCR
Limitations to AFB culture, PCR tests, IGRA assays, Histopathology for TB Dx.
AFB:
Takes 6-8 weeks for positive
PCR:
- Negative PCR does not rule out TB, esp. Pulmonary TB
- Sensitivity rate varies too much between brands
IGRA:
- Cannot tell latent vs active TB
Histopathology:
- Site of biopsy might miss location of TB
Is Adenosine Deaminase test useful for TB? Why is it used?
No
Derived from host lymphocytes and monocytes
Not specific for M. tuberculosis or TB at all
Only specific to show Pleuritis, Peritonitis, Pericarditis, Meningitis
4 Advantages and 5 disadvantages of NAAT test to Dx TB
Adv:
- Highly Specific
- Fast
- Can identify species
- Rapid detect resistance genes (e.g. rifampicin)
Disadv:
- Expensive
- Varied sensitivity depends on specimen
- Inhibitors in specimen
- No definitive information on drug susceptibility
- Can’t easily tell living or dead
Specificity and sensitivity of NAAT tests to Dx TB?
Varied sensitivity: Low in extrapulmonary, CSF, blood, pleural fluid; High in respiratory specimen
Highly specific
2 Advantages and 2 disadvantages of Histopathology and Cytology in Dx of TB ?
Adv:
- Useful when bacilli are few
- Can show typical pathology (caseous necrosis, Langhans giant cells)
Disadv:
- Cannot tell species, only AFB
- Cannot tell drug resistance
Skin biopsy features of TB lesions?
Panniculitis (fat cell inflammation)
Granuloma with epithelial giant cells
Miescher’s granuloma
Lympho-histiocystic infiltration
Fibrinoid necrosis of arterioles
Describe Tuberculin skin test
- What’s injected?
- Response time?
- Interpret positive result?
Purified protein derivative»_space; intradermal injection
Delayed type (IV) Hypersensitivity reaction: 48-72 hours
+ve: Cut-off of 5mm, 10mm and 15mm induration
- Does not mean protective immunity against M. tuberculosis
- BCG vaccine given
- Past TB infection or NTM infection
Standardized methods to record TST result for TB testing?
Induration measured in diameter - transversely to long axis of forearm, at widest diameter
Palpate or Sokal’s ballpoint method to find boundaries
Causes of false positive TST for latent TB testing
False negative?
- False positive:
BCG vaccine (but >20mm is not due to BCG)
NTM exposure/ infection - False negative:
Technical: injected too deep
Biological: Immune response variation
Describe IGRA for TB testing
- 2 examples
- Mechanism
- Limitations (2)
QuantiFERON-TB; T-SPOT TB
Release of interferon gamma in response to MTB-specific antigens
Affected by underlying immunity
Cannot tell latent vs active TB
2 complications from BCG vaccination
Absolute contraindication?
Severe suppurative adenopathy
Inoculation site abscess
Never give to immunocompromised baby»_space; disseminated TB
