TB III

Question 1

What are the risk factors that increase exposure to TB?

Answer 1

Living in a high prevalence area
* Overcrowding
* Poor ventilation

Question 2

How is TB transmitted?

Answer 2

It is airborne spread.
Inhalation of aerosols containing TB bacilli.

Question 3

What makes the TB bacilli to remain in the air for a long time?

Answer 3

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

Question 4

What is a rare method of transmission for TB?

Answer 4

Ingestion of milk or food containing M. bovis

Question 5

What is subclinical TB?

Answer 5

In subclinical TB, the patient is asymptomatic, but they have an abnormal CXR and positive TB culture

Question 6

How can Primary complex be detected?

Answer 6

Enlarged hilar and pretracheal nodes with or without pulmonary infiltrate or nodule

Question 7

How is a definitive diagnosis of TB made?

Answer 7

It is made when M tuberculosis is found in a clinical specimen

Question 8

What kinds of specimen can be used to diagnose TB?

Answer 8

It depends on the site of infection.
Sputum - Pulmonary TB

Question 9

Different microbiological ways of Testing TB

Answer 9

-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.

Question 10

On tissue biopsy what would indicate TB?

Answer 10

Histology of the tissue biopsy showing typical caseating granulomatous inflammation with AFB present

Question 11

What are the advantages of microscopy for acid-fast bacilli?

Answer 11

Simple, rapid, and inexpensive

Question 12

What are the disadvantages of microscopy for AFB?

Answer 12

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.

Question 13

How does does a TB culture take?

Answer 13

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

Question 14

What is an advantage of TB culture?

Answer 14

Growth in culture also proves that organism is alive- important in follow up of patients on
treatment

Question 15

What does molecular diagnosis of TB work?

Answer 15

It detects M. bacilli DNA and also detects known genes for causing resistance

Question 16

In SA, what is the first line test for diagnosing TB?

Answer 16

GenXpert MTB/Rif

Question 17

What are the advantages of molecular testing?

Answer 17

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

Question 18

Disadvantages of molecular tests

Answer 18

• Expensive
• Don’t distinguish alive from dead organisms, so not so good for patients recently treated for TB

Question 19

What is a tuberculin skin test? What does it do?

Answer 19

Partially purified extract of M tb protein. It tests for a delayed hypersensitivity response to these proteins.

Question 20

How is tuberculin skin test done?

Answer 20

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

Question 21

What is the disadvantage of TST

Answer 21

BUT cannot differentiate
between latent infection and active disease

Question 22

2 examples of tuberculin skin tests

Answer 22

Mantoux test and Haef test

Question 23

What are the limitations of TST?

Answer 23

• 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

Question 24

Why is a tuberculin skin test ineffective in South African Healthy adults?

Answer 24

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

Question 25

How is the spread of TB prevented in healthcare facilities?

Answer 25

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

Question 26

How is TB prevented in communities?

Answer 26

-Improve socio-economic status (nutrition, overcrowding, air pollution, smoking
-Health education
Improved HIV control, diabetes, and common conditions predisposing to development of TB