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Medical Studies 1 > 8.3 Pathophysiology & Diagnosis of Tuberculosis > Flashcards

8.3 Pathophysiology & Diagnosis of Tuberculosis Flashcards

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USMLE® Step 1 flashcards
1
Q

Can TB only infect the lungs?

A

No. It can also infect:
- Lymph nodes
- Bones
- Joints
- Other organs

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2
Q

What percentage of people have active vs latent TB?

A

Active: 10%
Latent: 90%

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3
Q

What is the estimated proportion of the global population that has latent TB?

A

1 in 3

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4
Q

Is TB more common in men or women?

A

Men

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5
Q

Are richer or poorer countries suffering more from TB?

A

Poorer

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6
Q

What % of TB cases in Australia come from high burden countries?

A

90%

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7
Q

Risk factors for TB (other than being from a high burden country)

A
  • Aboriginal heritage
  • Elderly
  • HIV
  • Contacts with TB-infected people
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8
Q

Is drug-resistant TB more common in Australia or higher burden countries?

A

Higher burden countries

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9
Q

Number/replication status of organisms in TB infection vs TB disease

A

Infection: low number, dormant
Disease: high number, dividing

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10
Q

Are the TB organisms detectable in TB infection vs TB disease?

A

Infection: No
Disease: Yes

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11
Q

Diagnosis of TB infection vs TB disease

A

Infection: Immune response detected
Disease: Detecting MTBC

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12
Q

Are there symptoms in TB infection vs TB disease?

A

Infection: No
Disease: Yes

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13
Q

Are TB infection vs TB disease infectious?

A

Infection: No
Disease: Yes

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14
Q

How is TB spread?

A

By droplet nuclei when a person coughs, speaks, or sings

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15
Q

Give two uncommon examples of how TB can be transmitted

A
  • High risk procedures
  • Ingestion of unpasteurised milk
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16
Q

What are the four possibilities following TB exposure?

A
  • Exposure with no infection/infection is cleared
  • Latent TB infection (dormant)
  • Subclinical (asymptomatic, but with radiological evidence, likely to progress)
  • Symptomatic disease
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17
Q

Describe the early stages of TB infection (up to granuloma formation)

A
  • TB inhaled
  • ALveolar macrophages try to phagocytose all
  • If unsuccessful, the mycobacterium will travel to the parenchyma, causing local inflammation and recruitment of T and B cells from lymph nodes
  • Combination of cells engulf infected alveolar macrophages, forming a granuloma
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18
Q

Does TB remain localised to the lungs?

A
  • No
  • During early infection, the non-specific immune responses cannot prevent dissemination to other areas of the body (e.g. hilar nodes)
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19
Q

True or false: TB can survive in granuloma’s for several decades

A

True

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20
Q

What determines whether TB resolves into a dormant state or progresses to active disease?

A

Balance between host immune response and ‘virulence’ of TB organism

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21
Q

Does TB release toxins into the host?

A

No.

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22
Q

True or false: TB bacilli cannot replicate once inside a granuloma

A
  • False
  • They can; in fact, the granuloma may eventually fail to control the infection, allowing it to progress
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23
Q

What is the characteristic lesion of TB?

A

Granuloma

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24
Q

What is the function of the granuloma in TB infection? What is the downside of this?

A
  • Function: Walls off and controls the infection
  • Downside: provides an environment in which the TB bacillus can persist
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25
Q

What happens to the macrophages in a TB granuloma? What transformation can they undergo?

A

They can transform into epitheliod cells, sometimes forming multi-nucleated giant cells

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26
Q

What can cause central necrosis in tuberculosis granulomas?

A

Release of inflammatory cytokines

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27
Q

Is granuloma unique to TB

A

No

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28
Q

Is the virulence of TB the specific driver of overall virulence?

A

No, it seems that a majority of the tissue damage comes from the host immune response

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29
Q

Describe how changes in the host immune response can cause virulence in TB

A
  • Reduced/delay response to pro-inflammatory cytokines
  • Excess inflammatory responses
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30
Q

How does changing the bacterial burden of TB effect its ability to be transmitted?

A

Higher bacterial load, more efficient transmission

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31
Q

Are all strains of TB equally transmissible throughout all populations

A
  • No
  • For example, the Beijing strain is less transmitted throughout Australia
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32
Q

Describe primary TB infection

A
  • Sufficient immune response to kill most bacteria
  • However, some survive, causing latent TB infection
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33
Q

Can latent TB be “cured”? How?

A
  • Yes
  • With preventative TB therapy (iosoniazid inhibtis cell wall formation)
34
Q

What is broncho-pneumonia?

A

Acute inflammation of the bronchi and alveoli in surrounding areas of the lung

35
Q

What is primary progressive TB? What are the characteristics of a patient who is most at risk of this?

A
  • Immune response fails to control the spread of TB
  • Primary pulmonary focus may invade bronchi or blood vessel, possibly disseminating to other organs
  • More common in patients who are immunosuppressed
36
Q

Primary progressive TB can disseminate into other organs outside the lungs. Give two examples of conditions that can arise as a result of this

A
  • Meningitis
  • Renal disease
37
Q

How soon after primary infection does active TB usually come about?

A

2-5 years

38
Q

Why does TB more often affect the upper lobes of the lungs?

A
  • This is a more oxygen rich environment
  • TB flourishes in oxygen rich environments
39
Q

Other than the lungs, what organs can tuberculosis infect?

A
  • Brain
  • Spine
  • Kidneys
40
Q

Broadly, describe the pathogenicity of tuberculosis

A
  • Infects tissue, forms granuloma
  • If escapes granuloma, spreads throughout lungs (and possibly other organs)
  • Leads to imbalance of inflammatory response, causing tissue destruction and scarring
41
Q

Under what circumstances would you consider that a patient may have TB?

A
  • Cough > 2-3 weeks (+- fever, night sweats, weight loss)
  • Persistent resp infection unresponsive to antibiotics
  • TB exposure risk factors
42
Q

Which people are high risk of TB?

A
  • People who have come in contact with others who have active TB
  • Spending time in high burden countries
43
Q

Which groups of people are at a high risk of developing active TB if already infected?

A
  • Children younger than 5 years old
  • People who have already had TB (especially in the last 2 years)
  • People with HIV/immunosuppressed (e.g. organ transplant)
44
Q

What is the most common form of TB?

A
  • Postprimary
  • Reactivation after latent infection
45
Q

Pulmonary symptoms of TB

A
  • Cough (2-3 weeks or longer)
  • Haemoptysis
  • Chest pain
  • Dyspnoea
46
Q

Systemic symptoms of TB

A
  • Fever
  • Night sweats
  • Weight loss
47
Q

In what percentage of pulmonary TB cases does extra-pulmonary TB occur?

A

~15%

48
Q

List some common sites of extra-pulmonary TB

A
  • Pleura
  • Cervical lymph nodes
  • Kidneys
  • Bones
  • Meninges (uncommon, but still check just in case)
49
Q

List one symptom of TB of the kidney

A
  • Blood in the urine
50
Q

List one symptom of TB of the meninges

A
  • Headache/confusion
51
Q

List one symptom of TB of the spine

A
  • Persistent back pain
52
Q

List one symptom of TB of the larynx

A
  • Hoarseness
53
Q

List one symptom of TB of the peritoneum

A
  • Abdominal discomfort, bloating
54
Q

General inspection signs of tuberculosis

A
  • Pallor
  • Clubbing
  • Wasted appearance
55
Q

Can CXR be used to definitively diagnose TB? Why, or why not?

A
  • No, it cannot
  • Other conditions can cause similar appearances
56
Q

List some x ray findings typical of TB

A
  • Lung cavitation in the posterior apical regions of the upper lobes
  • However, pre-HIV, 30% of findings were atypical
57
Q

What is the most common form of extrapulmonary TB?

A

Lymph node TB

58
Q

What are the main forms of lab testing used in TB diagnosis?

A
  • Smear microscopy for acid fast bacilli
  • Culture & Drug susceptibility test
59
Q

Can acid fast bacilli smearing differentiate between all AFBs and tuberculosis?

A

No

60
Q

How long does it take to get a detectable culture in Tb culturing for a -ve vs +ve case?

A

+ve: average 14 days
-ve: average 21 days

61
Q

How many good sputum specimens are needed for lab testing?

A

2-3

62
Q

Do tissue specimens or body fluids produce higher yield in terms of lab diagnosis?

A

Tissue specimens

63
Q

Does extrapulmonary TB involve a high or low number of TB bacilli? What is this called?

A
  • Low number of bacilli
  • This is called paucibacillary
64
Q

Can Tuberculin skin testing and interferon gamma release assay differentiate between latent and active TB? What are the implications of this?

A
  • No
  • +ve does not guarantee active TB
  • -ve does not exclude active TB
65
Q

How can undiagnosed TB cause haemoptysis?

A

Erosion of blood vessels overlying a lung cavity

66
Q

List some complications of TB

A
  • Haemoptysis
  • Lung fibrosis
  • Pneumothorax
  • bronchiectasis
  • Aspergillus (fungal ball)
67
Q

Are mycobacteria aerobic or anaeobic?

A

Aerobic (think: this is why they congregate near the apices of the lungs, where oxygen is rich)

68
Q

Do mycobacterial cell walls have high or low lipid content?

A

Low

69
Q

What does it mean that tuberculosis is acid-fast?

A

It can resist decolorisation in the presence of acid.

70
Q

A patient is smear positive for TB. Are they necessarily infectious? Are all these mycobacteria necessarily TB?

A
  • Yes
  • No; not necessarily TB
71
Q

Is acid-fast microscopy sensitive? How does this impact specimen size?

A
  • No, it is not sensitive
  • Therefore, a relatively large sample is needed
72
Q

Describe Gene Xpert testing for TB

A
  • Process sputum and place into cartridge, and put cartridge into machine
  • Machine does PCR, tests if it’s TB, and tests resistance to rifampicin
73
Q

Which is cheaper: gene xpert or acid-fast smear microscopy?

A

Acid-fast spear microscopy

74
Q

Does TB grow slower in solid or liquid media?

A

Solid

75
Q

Why would you use solid media TB culturing over liquid media? After all, it’s slower.

A

Because it may pick up on rare TB strains

76
Q

Describe immunochromatographic testing for tuberculosis

A
  • Place sputum sample on pad
  • Runs along test, binding to antigens if present
  • tests for the presence of a secretory protein of M. Tuberculosis
  • Two lines: present. One line: not.
77
Q

Which cultures are tested for drug susceptibility

A
  • All new culture positive cases
  • Those that are still culture-positive at 3 months
78
Q

How does tuberculosis drug resistance testing work?

A
  • Incubate sample with drug
  • If grows: resistant. If not: not.
79
Q

Should you do a bronchoscopy before sending sputum samples?

A

No. This is higher risk; try and do sputum samples first.

80
Q

How do you test for latent TB infection?

A

Tuberculin skin test

81
Q
A

Medical Studies 1 (90 decks)

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