TST Conversion refers to the situation where an individual's TST result changes from "negative" (typically 0-4mm diameter induration) to "positive" (typically equal to or >10mm diameter induration) within a 24 month period. For an individual with a baseline TST result between 5-9 mm induration, any increase of at least 6mm diameter induration is considered a TST conversion. If testing is done for an individual with a previous negative TST as a result of a known contact with a potential transmitter a new reaction of 5mm diameter induration is considered a conversion.
TST conversion is considered presumptive evidence of acquired latent tuberculosis infection (LTBI) and potential risk for progression to active tuberculosis disease.
All cases of conversion should be reported to the local Medical Officer of Health and if the conversion was occupationally acquired it should be reported to the Ministry of Labour and WSIB.
Cases of TST conversion should be referred to a physician who is experienced in TB management for assessment. Medical assessment should include a review of symptoms suggestive of possible active TB (persistent cough, hemoptysis, night sweats, unexplained weight loss or fever), risk factors for TB, Chest X-Ray and then if active TB has been excluded and if appropriate, recommendation for therapy for LTBI in accordance with current guidelines.
For students in the Hamilton area referral may be made to:
TB Clinic – Firestone Institute for Respiratory Health
St. Joseph’s Healthcare
50 Charlton Avenue East
Hamilton, Ontario L8N 4A6
Phone: (905)522-1155 X36000
Fax: (905) 523-5864
Printable Copy of TST Conversion Fact Sheet
Canadian Tuberculosis Standards, 5th ed., Public Health Agency of Canada & The Lung Association, 2007
Tuberculosis Surveillance Protocol for Ontario Hospitals, Ontario Hospital Association and the Ontario Medical Association, May 2010
Canadian Immunization Guide, 7th edition, Public Health Agency of Canada 2006