Tuberculosis Skin Test

Status: Required for all students

Valid for: A negative baseline test does not expire; routine repeat testing is not needed unless you have a known exposure or your site requires it.

Why this is required

Unlike the other items in this section, this is a screening test, not a vaccine. As a medical student you will work in settings where tuberculosis (TB) can be present, so placement sites establish your TB status before that exposure begins.

• Establishes your baseline. A baseline result recorded before your placement means that if you ever test positive later, it can be recognised as a new conversion and acted on.
• Protects you. A positive result leads to assessment and, where needed, treatment of TB infection before it can progress to active disease.
• Protects your patients. Screening identifies anyone with unrecognised active TB, who could otherwise transmit it to vulnerable patients.

Health Canada's Canadian Tuberculosis Standards recommend baseline tuberculin skin testing for all health care workers, but advise against routine periodic re-testing of workers whose baseline is negative — so this is usually a one-time requirement.

About tuberculosis

TB is caused by the bacterium Mycobacterium tuberculosis and usually affects the lungs. It spreads through the air when a person with active lung disease coughs. There are two distinct states:

• Latent TB infection — the bacteria are contained by your immune system and stay silent. You have no symptoms and are not contagious, but the infection can reactivate into disease later, especially if your immune system is weakened.
• Active TB disease — the infection is causing symptoms (such as a persistent cough, fever, night sweats and weight loss) and, when it affects the lungs, can be contagious.

How the test works

Two tests are used to detect TB infection:

• Tuberculin skin test (TST / Mantoux) — a small amount of purified protein derivative (PPD) is injected just under the skin of the forearm. If you have been infected, a delayed reaction (a raised, firm area called induration) develops and is measured in millimetres 48 to 72 hours later — so you must return to have it read.
• Two-step TST — because people who are tested repeatedly can have a faded reaction that "boosts" on a second test, a two-step test is done at baseline to establish a true starting point (a second TST usually 1 to 4 weeks after the first).
• IGRA blood test — an interferon-gamma release assay measures your immune response in a single blood draw. It uses antigens specific to M. tuberculosis that are not shared with the BCG vaccine, so it is not thrown off by prior BCG vaccination.

What the result means

• A negative baseline means no evidence of TB infection — your starting point is recorded.
• A positive result does not mean active disease. Neither the TST nor the IGRA can tell latent infection apart from active disease, so a positive result leads to further assessment — including a chest x-ray and medical evaluation — and consideration of treatment for TB infection.
• If you have ever tested positive before, or had TB disease, you should not be skin-tested again — you submit your previous result and a chest x-ray/assessment instead.

Safety and side effects

The skin test is safe and cannot cause TB — the PPD is not a live or whole bacterium. The most you will usually notice is mild itching, redness or swelling where the test was placed. Larger reactions can occur in people who are strongly positive.

Staying current

A negative baseline result does not expire, and Health Canada does not recommend routinely repeating the test. Re-testing is generally reserved for after a known TB exposure, though some placement sites set their own intervals — confirm what your program requires. Once you are positive, you are not re-tested; follow-up is based on symptoms and any needed treatment.

What to submit

• Documentation of a two-step TST showing both steps, each with the date and the result in millimetres of induration, or
• A lab report for an IGRA (e.g. QuantiFERON) blood test, or
• If you are previously positive: your documented positive result and a chest x-ray report or medical clearance
• Your full legal name and date of birth on every document

Accepted proof

• Clinic or public health TST record showing both steps, dates and induration in millimetres
• Lab report of an IGRA blood test with the result clearly stated
• Chest x-ray report and medical assessment (for anyone previously positive)
• International record with an English translation

Good to know

• A two-step TST takes time: each step must be read 48–72 hours after it is placed, and the steps are 1–4 weeks apart, so plan for up to four visits.
• If you have had the BCG vaccine, a skin test can read falsely positive — an IGRA blood test is not affected by BCG and is often the better choice.
• If you were ever TST-positive, do not book another skin test; gather your prior result and a chest x-ray instead.

Validity

A negative baseline test does not expire; routine repeat testing is not needed unless you have a known exposure or your site requires it.

Source & references

The medical information on this page is adapted from Health Canada and the Public Health Agency of Canada:

• Health Canada
• Canadian Tuberculosis Standards — Chapter 4: Diagnosis of tuberculosis infection
• Canadian Tuberculosis Standards — Chapter 14: Prevention and control of TB transmission in healthcare settings
• Tuberculosis (TB): For health professionals