Mycobacterium Tuberculosis Flashcards
What is the most common type of infection? How is it transmitted?
Pulmonary TB
Highly contagious, respiratory droplet transmission
How to diagnose?
Test for suspected TB
- Chest X-ray
- Sputum sample
- Microscopy (acid-fast stain)
- Culture and molecular assays (nucleic-acid based testing)
- Drug susceptibility testing
Tuberculin skin test
Interferon gamma release assays (blood tests)
What does tuberculin skin test contain and show?
- Intradermal injection of 100uL of tuberculin (PPD)
- Small pale bump appears and needs to be examined in 72 hours
- Raised, hard area of swelling >5mm: positive
What does false-positive TST result inclulde?
- Received Bacillus Calmette-Guerin vaccine
- Improper administration of test
- Inaccurate interpretation of your test reults
- Infection with nontuberculous mycobacteria
What are the advantages of interferon gamma release assay?
- Single patient visit > blood sample
- Results within 24 hours
- No interpretation bias
- TB specific
- No cross-reactivity with BCG vaccine
What are the disadvantages of interferon gamma release assay?
- Blood samples must be processed within 16 hours
- Limited data on use of QFT-G in children, recently exposed to Tb patients and in immunocompromised patients
- QFT-G cannto determine who is at risk for developing TB
What are the important biochemical assay tests that suggest active TB?
- ADA in pleural fluid (T cell activation)
- Renal TB –> Sterile pyuria
- TB meningitis– > White cell, protein, glucose in CSF
What is the diagnosis of drug resistant TB based on?
- Conventional culture methods: Solid agar methods, liquid broth methods (BACTEC/MGIT)
- Molecular detection methods; molecular probes
- Drug susceptibility testing
Describe TB sputum culture
- Slow
- Expensive
- Gold standard in TB diagnostics
- Sputum culture can take 1 to 8 weeks to provide results
- Sputum cultures higher sensitivity - 82%
What is the 1st-line drug?
Combination of 2/3/4 drugs for 6 months
- First 2 months: Rifampicin, isoniazid, pyrazinamide, ethambutol
- Streptomycin not included but remains highly used
- Second 4 months: Isonizaid, rifampicin
Why and who are 2nd line drugs used for? What are they associated with?
- Due to drug resistance, adverse effect/discontinuation of treatment, DDI
- Non-responsive patients
- Side effects/toxicity are significant
- Often less potent drugs
What are the TB drug resistance types?
- Mono-resistance (first-line anti-TB drug only)
- Poly-resistance (>1 first-line anti-TB drug)
- Multidrug (at least both isoniazid and rifampicin)
- Extensive drug (any fluoroquinolone, at least one of three second-line injectable drugs + MDR)
- Rifampicin-resistant (often used as surrogate marker for MDR)
- Totally drug (XDR isolates which are additionally resistant to 4th and 5th line drugs)
Characteristics of Tb
- Obligate aerobe
- Acid fast
What is the stain used?
Ziehl neelsen stain
What is the pathological feature of small antigen load + high tissue hypersensitivity TB?
- well formed granuloma containining MTB.
- healing with fibrosis. encapsulation and scar formation
- usually this case for immunocompetent ppl
