A positive TB skin test usually does not mean you have TB disease. It means your immune system has met the bacteria at some point. The next step is a chest x-ray and an examination. That separates a dormant infection — no symptoms, not contagious — from active disease, which is the form that spreads.
What the visit covers
- How the skin test is placed, and the window it must be read in
- Two-step baseline testing, and who asks for it
- Why 8 mm is positive for one person and negative for another
- Why a BCG scar muddies a skin test result
- The blood test BCG does not affect, and who pays for it in Ontario
- What follows a positive result: x-ray, assessment, sometimes treatment
- What Mantoux, PPD, two-step and IGRA mean on a letter
How the reading is done, and why BCG complicates it
The test is 0.1 mL of tuberculin — five tuberculin units — injected into the top layer of skin on the inner forearm. It raises a pale bump 6 to 10 mm across. That bump is not the result. The result is read 48 to 72 hours later. What gets measured is induration, the firm swelling, across the forearm rather than along it, in millimetres. Redness is not recorded. A red or itchy arm with no firm swelling under it is not a positive test: that happens to a small number of people in the first 12 hours and does not mean infection. Miss the 72-hour mark and the test cannot be rescued. It is repeated far enough away on the arm that the two reactions do not overlap, and there is no waiting period before that.
There is no single number that means positive. Canada's standards set the cut-off against your own risk. Five millimetres or more is positive for people living with HIV, close contacts of an infectious case within the past two years, and anyone on drugs that suppress the immune system. Ten millimetres or more is the threshold for everyone else, including people with diabetes, current smokers, and anyone with no particular risk factor. So tell whoever reads your arm about HIV, any recent contact with an active case, diabetes, smoking, and any medication that suppresses your immune system. The number on its own does not settle it.
BCG given in your first year of life rarely explains a positive test years later. Canada's standards report that only 1 per cent of people vaccinated in infancy still read 10 mm or more when tested more than ten years afterwards. BCG given later in childhood is more of a problem. The way around it is an interferon-gamma release assay, or IGRA — a blood test that BCG does not affect. Ontario does not fund it. You buy it from a private laboratory or claim it on private insurance, and not every community has a laboratory that runs it. Live vaccines are the other trap. A live virus vaccine in the four weeks before a skin test is a reason to wait, and measles vaccine is the one shown to push a result falsely negative. A skin test can still go in before or on the same day as a vaccine, as long as it is a different site.
A positive test is a reason for a chest x-ray and an examination, not for staying away from people. A dormant infection — latent TB — means the bacteria are there and quiet: no symptoms, you do not feel sick, and you do not pass it to anyone. Active TB disease is the form that spreads, breathed out from the lungs or airways of someone who has it when they cough, sneeze, sing or talk. Canada's standards for preventive treatment require active disease to be ruled out first. The minimum is a clinical assessment and a chest x-ray. Treatment, when it is offered, is pills over months: either four months of daily rifampin, or three months of once-weekly rifapentine with isoniazid. Ontario supplies those first-line drugs free of charge through its public health system.
What OHIP covers
Who pays depends on who is asking. A test a doctor decides you need is insured care. So is a test for someone named as a contact of an active case: Ontario's TB program guideline says that one costs you nothing. A test a school, college or university requires is also on the insured side, including the volunteer hours a high school student needs to graduate. A test required only to get a job is not, and neither is one an employer repeats on a schedule. Those are billed to you or to the employer. A volunteer agency asking on its own account sits outside those categories. Find out which one your letter falls into before you come. Where a test is uninsured, the fee is quoted to you before it goes in, never after.
Walk in, or book?
Phone before you travel in. Two things decide whether a skin test can be arranged at all. One is whether tuberculin is in the fridge. The other is the calendar: the reading falls 48 to 72 hours after the test goes in, and it has to land inside opening hours. Miss that window and the test has to be done again. Ask about both, ask how long a signed result letter takes, and have your deadline in front of you.
What to bring
- The letter from the employer, school or agency, naming which test it wants
- Written proof of what a school, college or placement requires, if that is the reason
- Any record of an earlier TB skin test, with the millimetre reading
- Whether you had BCG, and roughly what age you were
- The date of any live vaccine in the past month
- Your deadline
Questions people ask
Is this the TB test for my immigration application?
No. An immigration medical exam has to be done by a panel physician on the list IRCC publishes, and IRCC states that your own doctor cannot do the exam. Chest x-rays and laboratory tests are arranged as part of that exam, depending on your age. Find a panel physician through IRCC and book with them directly.
Why does my employer want a two-step test?
Because one test cannot always tell a new infection from an old one. In people tested over and over — hospital and long-term care staff — a first test can wake up a reaction to an infection from years ago. It then looks fresh. So a baseline is done twice, the second test one to four weeks after the first. That is up to four trips, because each test needs its own reading. Once a negative two-step result is documented, every test after it is a single step.
My letter says two-step Mantoux. What is that?
Mantoux, PPD and TST are three names for the same skin test. Two-step means two of them, one to four weeks apart. IGRA, QuantiFERON or T-SPOT means the blood test instead, and that needs a laboratory. Match the words on your letter to those before you phone, because they decide how many visits you are in for.
Is TB common in Ontario?
Not common, and not gone. Ontario's TB program guideline puts the province between 4.3 and 5.1 cases per 100,000 people a year since 2012. Across Canada in 2024 there were 2,508 people newly diagnosed with TB disease, a rate of 6.1 per 100,000. A dormant infection can sit for years and be found and treated long before it becomes disease. That is what a screening test is for.
Still have a question? Call the front desk.
Related
- Flu shots and routine immunizationsFlu shots in season, routine vaccines
- Sick notes, work forms and insurance paperworkNot covered by OHIP; ask about the fee
- Physicals and what Ontario screens forAnnual check-ups and screening by age
