HomeFive tuberculin units of Tubersol® PPD-Standard is recommended for use in Canada. New, more specific tuberculin testing products have been developed and recently recommended by the World Health Organization (WHO),16 but as of writing, are not yet available in Canada. The only internationally recommended method of tuberculin skin testing is the Mantoux technique, which consists of intradermal injection of tuberculin material on the inner surface of the forearm.7 An induration is measured on a second visit that occurs 48-72 hours after administration. As there are multiple visits to complete a TST, some persons may be unable or unlikely to return for reading—for these individuals, an IGRA should be considered (see next section on IGRAs).16 Details on TST administration, contraindications, precautions and measurement of induration are contained in Appendix 1, Section A.1 at the end of this chapter. Administration and interpretation of TST requires adequate training and quality assurance measures. Methods and tools of quality assessment for TST placement and induration measurement have been described in mobile TST (mTST) protocols and are supported with accompanying videos.17,18 TST is appropriate for use at any age.
Persons with TB infection may display a wide range of induration sizes.19–21 Setting a smaller induration size for a positive result will increase sensitivity (fewer false negatives) but will reduce specificity (more false positives); setting a larger induration size for a positive result will reduce sensitivity but increase specificity. The size of the induration is associated with risk of developing TB disease.10 Table 1 presents cutoff thresholds for TST interpretation.
Table 1. Interpretation of TST results and cutoff thresholds in various populations.7
| TST Result | Situation in which reaction is considered positive |
|---|---|
| <5 mm | In general, this is considered negative |
| ≥5 mm | People living with HIV |
| Known recent (<2 years) contact with a patient with infectious TB disease | |
| Fibronodular disease on chest x-ray (evidence of healed, untreated TB) | |
| Prior to organ transplantation and receipt of immunosuppressive therapy | |
| Prior to receipt of biologic drugs, such as tumor necrosis factor alpha inhibitors, or disease-modifying antirheumatic drugs | |
| Prior to receipt of other immunosuppressive drugs, such as corticosteroids (equivalent of ≥15 mg per day of prednisone for at least one month) | |
| Stage 4 or 5 chronic kidney disease (with or without dialysis) | |
| ≥10 mm | Recent (<2 years) conversion of TST from negative to positive |
| Diabetes (controlled or uncontrolled) | |
| Malnutrition (<90% of ideal body weight) | |
| Current tobacco smoker (any amount) | |
| Daily consumption of >3 alcoholic drinks | |
| Silicosis | |
| Hematologic malignancies (lymphomas and leukemia) and certain carcinomas (such as cancers of head, neck, lung and/or gastrointestinal tract) | |
| Any population considered at low risk of disease. |
Abbreviations: TST, tuberculin skin test; HIV, human immunodeficiency virus; TB, tuberculosis.
Switch To: Français