The tuberculin skin test (TST) is given to identify individuals with latent tuberculosis infection (LTBI) who are at risk of developing tuberculosis disease. A TST is not used to diagnose tuberculosis disease.
Identification and treatment of LTBI can substantially reduce the risk of a patient developing TB disease in the future, protecting the health of the patient as well as the public by reducing the number of cases of tuberculosis disease in the community.
Ottawa Public Health (OPH) has developed a step-by-step video and downloadable PDF(pdf - 503 KB, this document is currently not in an accessible format) to assist physicians, nurse practitioners, and other health care professionals delegated to perform this medical act.
When is a TST recommended?
Persons with increased risk of tuberculosis exposure, and for whom treatment of LTBI would be considered, are recommended to receive a TST. Clinical assessment is required, however TST is commonly recommended for:
- Contacts of a person diagnosed with pulmonary tuberculosis
- Individuals born in tuberculosis endemic countries
- Individuals with certain medical comorbidities that increase the risk of developing tuberculosis disease, such as HIV infection, diabetes and chronic renal failure
- Individuals who are homeless or under housed
- People who use injection drugs
- Indigenous peoples from areas with high tuberculosis incidence
- Health care workers, depending on the setting
- Residents of correctional facilities
- Residents of long-term care homes and retirement homes < 65 years of age
- Travellers to countries with high TB incidence
Who should not receive a TST?
- Anyone who has had a previous positive TST (documented) or who has had a severe blistering reaction to a TST in the past, as there is a greater likelihood of an adverse reaction
- Anyone who has had a previous anaphylaxis, or hypersensitivity reaction, to a TST
- Anyone who has been diagnosed with or treated for tuberculosis infection or disease in the past, as the TST is of no clinical use
- Anyone with extensive burns or eczema because of greater likelihood of adverse reactions or severe reactions
- Anyone with a current major viral infection or anyone who has received a live virus immunization within the past 4 weeks, as this has been shown to increase the likelihood of false negative TST results
- TST is generally not recommended for residents of long-term care homes and retirement homes ≥ 65 years of age
- A full list of contraindications and warnings can be found on the Tubersol® product monograph
The following are not contraindications for TST:
- A history of receiving BCG vaccination(s)
- A common cold or other mild viral illness
- Pregnancy or breastfeeding
- Immunization with another vaccine on the same day as receiving the TST. For information on timing of TST related to receipt of COVID-19 vaccines, refer to the current recommendations of the National Advisory Committee on Immunization (NACI).
- A positive TST reaction, other than blistering, that is not documented
The two-step TST
A two-step TST is used to establish a baseline for persons who will be tested at regular intervals, such as health care workers.
To perform a two-step TST, administer and read the first TST. If the result is negative, repeat a second TST one to four weeks later (no sooner than one week and no later than four weeks later.) The result of the second TST is the true baseline.
Why two steps? The first TST may elicit little response yet stimulate an immune response. The second TST is given to elicit a greater immune response. This is called the booster effect and is important to detect at baseline as it could be misinterpreted as a new TB infection during future testing. Although the booster effect can last up to one year, an interval of one to four weeks between TSTs is recommended to avoid confusion in case of actual exposure to infectious tuberculosis disease.
A two-step TST needs to be performed only once if properly administered and documented. Any subsequent TST can be one step, regardless of how long it has been since the last TST.
Resources regarding TST administration
OPH has developed astep-by-step video anddownloadable PDF(pdf - 503 KB, this document is currently not in an accessible format) to assist physicians, nurse practitioners, and other health care professionals delegated to perform this medical act.
More information on TST can be found in the Canadian Tuberculosis Standards.
Resources for reporting and treatment of LTBI
LTBI is reportable to Ottawa Public Health. The reporting form (bilingual, pdf - 333 KB) (This document is currently not in an accessible format) is available on our website, along with information on how to order medications for LTBI(bilingual, pdf - 316 KB) (This document is currently not in an accessible format).
For questions regarding the TST, refer to the Canadian Tuberculosis Standards (Chapter 4) or call Ottawa Public Health at 613-580-2424 ext. 24224.
