Infectious Diseases | Microbiology / Positive Tuberculin Skin Test

Question 1

What infectious agent causes tuberculosis (TB)?

Answer 1

Mycobacterium tuberculosis

Question 2

How is TB transmitted?

Answer 2

Person to person through droplet nuclei (in general, extended close contact is required for transmission)

Question 3

In general, someone with untreated active TB will infect 10-15 people a year. True or false?

Answer 3

True

Question 4

If a patient does not have drug-resistant tuberculosis and is compliant with appropriate treatment, about how long after the initiation of antibiotics does the patient become noninfectious?

Answer 4

2 weeks

Question 5

What organs does TB affect?

Answer 5

Most often, the lungs; however, it can attack any part of the body

Question 6

What are typical symptoms of active TB lung infection?

Answer 6

• Fatigue
• weight loss
• night sweats, fever
• coughing
• hemoptysis
• chest pain

Question 7

What is latent TB?

Answer 7

Infection with M. tuberculosis without active disease

Question 8

10% of the world’s population is infected with tuberculosis. True or false?

Answer 8

False (estimates are as high as 30%)

Question 9

Even though patients with latent TB do not have symptoms, they are still contagious. True or false?

Answer 9

False. Latent TB is NOT contagious.

Question 10

Why is it important to diagnose and treat latent TB?

Answer 10

• 5%-10% of people with latent TB can progress to active diseas
• active TB can be fatal
• Treatment of people with latent TB is an important step toward the elimination of TB in the community.

Question 11

How often should healthcare and medical lab workers be tested for TB?

Answer 11

Annually

Question 12

Who should be screened for latent TB?

Answer 12

• Healthcare and medical lab workers
• People who have contact with a person known (or suspected) to have active TB
• people with a compromised immune system (HIV infectionorgan transplant, etc)
• recent immigrants from areas where active TB is common
• people living in or who are exposed to people living in crowded conditions (homeless shelters, jails, etc)
• people with symptoms of active TB
• intravenous (IV) drug users

Question 13

What is QuantiFERON-TB Gold?

Answer 13

• A highly specific whole blood test for TB that works by measuring a cell-mediated immune response in individuals infected with TB.
• It identifies both active and latent infection but cannot distinguish between the two.

Question 14

What is the Mantoux tuberculin skin test (TST)?

Answer 14

• An intradermal injection that screens for TB infection by inducing a delayed hypersensitivity reaction.
• Note: a screening blood test also exists, but it is less readily available and not the preferred method of screening.

Question 15

How is a TST administered?

Answer 15

Tuberculin purified protein derivative (PPD) is injected into the volar surface of the forearm.

Question 16

How long after administration should a TST be read?

Answer 16

Between 48 and 72 hours

Question 17

If a patient does not return within 72 hours after administration of a TST, what should be done to screen the patient for TB?

Answer 17

Reschedule another skin test as soon as possible (unless the patient had a severe reaction to the first test).

Question 18

What constitutes a positive TST reading?

Answer 18

An area of induration (not just erythema) at the site of injection. For most patients, the area of induration must be at least 15 mm in diameter. For higher-risk patients (immune compromised patients, recent immigrants, drug users, etc), a smaller area of induration (ie, greater than 5 mm or 10 mm) indicates a positive result (specific tables are available from the CDC).

Question 19

When should retesting occur for patients who have a known recent contact with a person with active TB but whose TST is negative?

Answer 19

12 weeks after contact (Note: in some cases, treatment for latent TB should start prior to the 12-week mark even if the TST is negative)

Question 20

What is the BCG (bacille CalmetteGuérin) vaccine?

Answer 20

• A TB vaccine used in some countries where active TB is common.
• Persons with previous BCG vaccination may have a false-positive TST- however, this should not preclude screening.

Question 21

What is the next step following a positive TST result?

Answer 21

Determine if the patient has active TB by taking a history, examining the patient, and obtaining a chest radiograph

Question 22

What is the preferred drug regimen (per the CDC) for treatment of latent TB?

Answer 22

Isoniazid (INH) for 9 months

Question 23

What is the most severe adverse effect of INH?

Answer 23

Hepatotoxicity

Question 24

What baseline laboratory tests are necessary before the initiation of INH therapy?

Answer 24

For most patients, NO baseline laboratory tests are necessary.

• Liver function (AST, ALT, and bilirubin) should be tested at baseline and then monitored routinely in patients with a history of
  
  • liver disease (or who use alcohol regularly)
  • HIV infection, or
  • pregnancy (up to 3 months postpartum).

Question 25

What is the purpose of monthly clinical monitoring of patients on INH?

Answer 25

• To assess adherence to regimen
• inquire about symptoms of peripheral neuropathy and/or hepatitis, and examine for signs of hepatitis

Question 26

What are some symptoms of hepatitis?

Answer 26

• Gastrointestinal upset (nausea and vomiting, abdominal pain), dark urine, jaundice

Question 27

Patients on INH who are otherwise asymptomatic should discontinue INH if their transaminase levels reach what level?

Answer 27

Five times the upper limit of normal

Question 28

Patients on INH who are symptomatic should discontinue INH if their transaminase levels reach what level?

Answer 28

Three times the upper limit of normal

Question 29

Which medication can prevent INH-induced peripheral neuropathy?

Answer 29

Pyridoxine 25-50 mg PO once daily