Latent TB Screening

Question 1

Screening of latent TB

Answer 1

Tuberculin skin test

Question 2

Methods of tuberculin test

Answer 2

• Mantoux method
• Heaf method

Question 3

Tuberculin test (Mantoux or Heaf) >> Type of hypersensitivity?

Answer 3

Type-IV (Delayed, Cell-mediated, Antibody-independant) hypersensitivity

Question 4

Time of reaction to tuberculin after inoculation

Answer 4

48 to 72hours

Question 5

Why does it need 2-3days (48-72hours) to react to tuberculin?

Answer 5

Delayed hypersensitivity signifies delayed nature of the response

Question 6

If an individual reacts to tuberculin in less than 24hours >> Dx?

Answer 6

Dx: Allergy to protein

(NOT latent TB)

Latent TB is only considered if the response is delayed

Question 7

What if a patient undergoing tuberculin test had previous BCG immunisation?

Answer 7

He/she will give false positive result

Question 8

What is in BCG vaccine?

Answer 8

Live Attenuated Mycobacterium Bovis strain

Question 9

If a patient shows positive response (reaction) to tuberculin skin test after 48-72hours- Dx?

Answer 9

Dx: Latent TB

But, check any previous history of BCG vaccine; If present >>> consider this can be false positive

Question 10

Newer method of screening latent TB

Answer 10

Interferon Gamma Release Assay (IGRAs)

e.g. Quanti-FERON or T spot

Question 11

Advantage of “IGRA (Interferon Gamma Release Assays): Quanti-FERON or T Spot” over ‘tuberculin test’ for screening of latent TB

Answer 11

• Tuberculin test uses protein that is common to M.tuberculosis and M.bovis. >>> So, if previous BCG vaccination >>> it may result ‘false positive’ (As BCG contains live attenuated M.bovis strain)
• IGRA (Interferon gamma release assay): such as- Quanti-FERON or T spot >>> offers ‘in vitro’ alternative to skin test >>> uses proteins that are unique to M.tuberculosis >>> to stimulate cytokine release