Tuberculosis Flashcards
Describe the morphological characteristics of Mycobacterium tuberculosis.
Slow-growing, obligate aerobic, acid-fast bacilli with thick, waxy cell membrane that does not produce a typical Gram-stain response
- Makes diagnosis of TB particularly challenging since it takes 4-8 weeks to confirm growth
Explain the pathophysiology of tuberculosis.
98% of TB cases are via airborne transmission.
M. tuberculosis access the lower airways & are consumed by pulmonary macrophages, which usually results in no infection.
However, in some instances, it can replicate in the lungs, resulting in cellular immunity.
Presents as either asymptomatic (i.e. latent TB) or symptomatic TB (i.e. active TB).
What is your recommendation when an individual is presented with an unexplained cough for more than 3 weeks at the community pharmacy?
Refer for evaluation of TB.
What are some risk factors associated with TB?
Latent & Active:
- Residents of nursing homes, prisons, homeless shelters due to frequent contact with healthcare system
- Close contact with pulmonary TB pt
- Co-infection with HIV
- Living in urban areas
Active:
- Children < 2 y/o
- Elderly > 65 y/o
- Malnutrition -> weakened immune system
- Immunosuppression
What are some clinical presentations in a patient with a likely tuberculosis infection?
Primarily pulmonary infection; extrapulmonary TB is possible (identical to pulmonary TB Tx but longer) e.g. bone & joints, spine, CNS etc.
Signs & symptoms include:
Local - Productive cough, hemoptysis
Systemic - Fever, fatigue (generalised; malaise), night sweats, weight loss
Night sweats & weight loss are classical smx of TB.
Radiological findings (classical):
Infiltrates in apical region (obligate aerobe)
- vs pneumonia: usually lower/middle lobes
Cavitary lesions
How do the symptoms of pneumonia compare with those of TB?
TB: Gradual onset (weeks - months); apical infiltrates in CXR
Pneumonia: Acute onset (hours - days); dense infiltrates in middle/lower lobes usually in CXR
What are the conditions in which an individual is indicated for latent TB infection (LTBI) screening?
High-risk group AND intent to treat if positive
High-risk group includes:
- Children w/ recent TB contact (lifelong with increasing risk with age)
- HIV infected individuals
- Pt considered for tumour necrosis factor antagonist Tx
- Transplant pt
- Dialysis pt (frequent encounter w/ hospital settings increase TB risk)
Why is LBTI NOT empirically treated for non-high-risk groups, despite being positive for LTBI?
1) RIPES have multiple side effects which have risks outweighing the benefits of Tx.
2) Reactivation of latent TB happens over time; pt w/ life-limiting diagnosis (e.g. end-stage cancer) may not need LTBI Tx due to significant toxicity.
What are the tests available to diagnose a patient with LTBI?
1) Tuberculin skin test (e.g. Mantoux test, tuberculin purified protein derivative [PPD] test)
- Inject 0.1 mL of PPD intradermally & read after 48-72h by a trained reader/nurse.
- Read diameter of induration (NOT redness)
- Positive PPD >= 10mm since most SG are BCG-vaccinated
2) Interferon-gamma release assay (e.g. QuantiFERON-TB Gold, T-SPOT.TB)
- Blood collection into special tubes
- Measures interferon-gamma released by WBC in response to incubation of M. tuberculosis-specific agent
What are some advantages of using interferon-gamma release assay over tuberculin skin test for the clinical diagnosis of LTBI?
Results available w/in few hours vs 48-72h
No false positives in BCG-vaccinated individuals, unlike tuberculin skin test
Minimal cross-reactivity w/ non-TB mycobacteria, thus no false positives in interferon-gamma release assay
What is a common limitation between the use of interferon-gamma release assay and the tuberculin skin test for the clinical diagnosis of LTBI?
False negatives due to immunosuppression in patients; unable to mount sufficient immune response for significant results.
How is active TB clinically diagnosed?
Suspect based on:
- History
- Risk factors
- Clinical presentation
- Physical exam findings
- CXR findings
If sputum obtained for Ziehl-Neelsen stain for acid-fast bacilli is positive, Tx is initiated before confirmatory microbiological test results are obtained.
- Takes 4-8 weeks to grow & confirm TB
- Takes another 4-6 weeks for Abx susceptibility test results
What are some infection control precautions to be taken when treating pt with LTBI & active TB?
LTBI: No special infection control precautions since asymptomatic
Active TB:
- Airborne precautions in hospital w/ negative pressure rooms & compulsory donning of PPE
- Tx decreases infectiousness; thus, after 2 weeks of effective Tx, can drop airborne precautions
- No need to avoid household members in community, since already exposed; take TB medications, practice cough etiquette, ventilate homes
What are some considerations in the treatment approach of LTBI?
Monotherapy is adequate.
Prior to initiation of LTBI Tx:
1) Should exclude active TB; otherwise, greatly increase risk of developing resistance against TB
2) Weigh risk vs benefit (including SE of RIPES such as hepatotoxicity)
CD is a 38 y/o 70kg male who is required to undergo a purified protein derivative (PPD) test as part of his pre-employment screening. The test was performed on his right forearm which resulted in a 1.5cm induration. CD is otherwise healthy with all labs within normal limits. He is born in Singapore and received all the recommended childhood and adult vaccinations. Recommend the most appropriate treatment to CD for his LTBI.
Isoniazid PO one 300 mg tab (5 mg/kg/day) OD w/ PO pyridoxine 10mg/day for 6 months
- Pyridoxine is co-administered to minimise neuropathy from isoniazid.
Required dose = 5 mg/kg x 70kg = 350mg, but only 150mg & 300mg tablet formulations are available, thus 300mg tablet is given instead.
No HIV, thus Tx duration is 6 months (instead of 9 months for HIV pt).
