TB III Flashcards
What are the risk factors that increase exposure to TB?
Living in a high prevalence area
* Overcrowding
* Poor ventilation
How is TB transmitted?
It is airborne spread.
Inhalation of aerosols containing TB bacilli.
What makes the TB bacilli to remain in the air for a long time?
It is due to the presence of lipid in the bacilli cell wall, making it light weight, which is why it can be suspended in the air for a long time
What is a rare method of transmission for TB?
Ingestion of milk or food containing M. bovis
What is subclinical TB?
In subclinical TB, the patient is asymptomatic, but they have an abnormal CXR and positive TB culture
How can Primary complex be detected?
Enlarged hilar and pretracheal nodes with or without pulmonary infiltrate or nodule
How is a definitive diagnosis of TB made?
It is made when M tuberculosis is found in a clinical specimen
What kinds of specimen can be used to diagnose TB?
It depends on the site of infection.
Sputum - Pulmonary TB
Different microbiological ways of Testing TB
-Microscopy for acid-fast bacilli (AFB)
-Culture of M. tuberculosis
-Molecular diagnosis of M. tuberculosis (e.g.PCR). This also helps with drug susceptibility testing.
On tissue biopsy what would indicate TB?
Histology of the tissue biopsy showing typical caseating granulomatous inflammation with AFB present
What are the advantages of microscopy for acid-fast bacilli?
Simple, rapid, and inexpensive
What are the disadvantages of microscopy for AFB?
Need lots of bacilli to be present to see them
* Therefore sensitivity of microscopy for diagnosis of TB can
be moderate and variable (though it does identify the most infectious patients)
* Particularly poor in people who have paucibacillary disease
(few TB organisms) such as children, HIV +ve persons,
extra-pulmonary infections)
* Doesn’t differentiate between different kinds of mycobacteria
(though in areas with high TB prevalence, most AFB will be M.
tuberculosis)
* Doesn’t differentiate between live or dead mycobacteria
* When used for follow-up of patients on treatment.
How does does a TB culture take?
Slow, 2-6 weeks due to the fact that M. tuberculosis is slow growing.
however, morden liquid cullture is better than the solid agar media
What is an advantage of TB culture?
Growth in culture also proves that organism is alive- important in follow up of patients on
treatment
What does molecular diagnosis of TB work?
It detects M. bacilli DNA and also detects known genes for causing resistance
In SA, what is the first line test for diagnosing TB?
GenXpert MTB/Rif
What are the advantages of molecular testing?
Highly sensitive: can detect as few as 150 organisms/ml of sputum with latest version of GXP
* Directly from clinical specimens though can also be used on culture isolates
* Quick
* Can be done as point-of –care test though not always cost-effective
Disadvantages of molecular tests
- Expensive
- Don’t distinguish alive from dead organisms, so not so good for patients recently treated for TB
What is a tuberculin skin test? What does it do?
Partially purified extract of M tb protein. It tests for a delayed hypersensitivity response to these proteins.
How is tuberculin skin test done?
Intradermal injection measuring the amount of induration (palpable swelling) that occures 2-3 days later.
The reaction is due to the rapid action of T cells due to being previously sensitized to TB
What is the disadvantage of TST
BUT cannot differentiate
between latent infection and active disease
2 examples of tuberculin skin tests
Mantoux test and Haef test
What are the limitations of TST?
- Can get false negative result in severe immunosuppression, overwhelming TB infection and for
other reasons - Can get false positive result after BCG vaccination or exposure to NTMs
Why is a tuberculin skin test ineffective in South African Healthy adults?
In South Africa many/most healthy adults will have a positive TST indicating previous infection/BCG
vaccination. Therefore, test not useful in healthy adults in our setting
* However, a positive TST is useful for detection of latent TB in persons at high risk of developing
active disease e.g., children under 5 or people living with HIV
How is the spread of TB prevented in healthcare facilities?
To keep infected TB patients away from others, e.g. rapid recognition and separation of people
with TB symptoms from others in waiting areas, keeping TB patients in separate rooms with
appropriate ventilation
- Improved ventilation, either natural i.e. opening windows on both sides of room, or
mechanical - Use of UV irradiation, either natural i.e. outside in sunshine,
- Personal protective equipment:
- N95 respiratory masks
How is TB prevented in communities?
-Improve socio-economic status (nutrition, overcrowding, air pollution, smoking
-Health education
Improved HIV control, diabetes, and common conditions predisposing to development of TB
