Compendium of Latent Tuberculosis Infection (LTBI) Prevalence rates in Canada - Tuberculosis Prevention and Control - PHAC

Compendium of Latent Tuberculosis Infection (LTBI)
Prevalence rates in Canada

Prepared by
Lilian Yuan, MD, MSc, FRCPC
for
Public Health Agency of Canada
Tuberculosis Prevention and Control
July, 2007

Table of contents

• Chapter 1: Background
  
  • Data Collection
  • Data Considerations
  • Layout of Report
  • Comments and Additions
• Chapter 2: Summary of Tuberculin Screening Studies
• Chapter 3: Description of Tuberculin Screening Studies
• Acknowledgements
  

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Chapter 1 Background

Tuberculosis (TB) is a disease caused by bacteria belonging to the Mycobacterium tuberculosis complex. Most people who become infected with M. tuberculosis are able to contain the organism and prevent it from replicating, thus preventing disease development. M. tuberculosis can survive for years but remain inactive within an individual. The infected person develops no symptoms or detectable signs of infection during this time. This condition is called latent tuberculosis infection (LTBI). Many people with LTBI never develop TB but in some individuals, especially those with impaired immunity, bacteria reactivate and cause TB disease.

The main method to diagnosis LTBI is by tuberculin skin testing (TST). When tuberculin testing material is injected intradermally, usually on the volar aspect of the forearm, individuals infected with M. tuberculosis develop a delayed cell-mediated reaction within 48-72 hours. This manifests as an induration of the skin at the site of injection. False positive and false negative reactions can occur with tuberculin skin testing due to technical or biological causes or both. For a discussion of tuberculin testing and diagnosis of tuberculosis infection, please refer to the current issue of the Canadian Tuberculosis Standards which may be viewed at www.publichealth.gc.ca/tuberculosis.

Targeted tuberculin testing for LTBI is a strategic component of TB control that identifies persons at high risk for developing TB and who would benefit by treatment of LTBI. While tuberculin skin testing is commonly performed, background prevalence of LTBI is seldom available for comparison. This compendium of Canadian LTBI prevalence rates was compiled to address this situation.

Data Collection

Both published and unpublished materials were sought for the compendium. Results of TST were used as a surrogate of LTBI. For inclusion, data must be based on screening of a Canadian population and data from contact investigations were only included if there was no evidence of recent transmission.

PubMed was used to locate peer-reviewed articles of Canadian LTBI data using a variety of search terms related to TB infection, LTBI and Canada. Terms were broadened to include TB related activities where LTBI data may be collected. The challenge of the literature search lies in the fact that LTBI may not be mentioned in the title but such data are present in the text e.g. article on the decline of TB, or tuberculosis infection is in the title but the content only addresses converters. In order to find as many relevant articles as possible, bibliographies were also searched for related publications. No limit was set on the year of study or language of publication.

In order to locate unpublished reports, TB directors and key informants in provinces and territories were contacted to identify individuals/organizations that may have LTBI data. Public health units, hospitals, long-term care facilities, correctional services, occupational health programs etc. were also contacted. Significant efforts were made by some organizations but data were often not usable e.g. results of TST were incomplete, denominator data were not available.
The compendium includes peer-reviewed articles published as of July 1, 2007 and unpublished reports collected as of July 21, 2005.

The compendium includes peer-reviewed articles published as of July 1, 2007 and unpublished reports collected as of July 21, 2005.

Data Considerations

When reviewing LTBI prevalence rates reported here, one needs to bear in mind several factors:

1. Time when the tests were conducted: Results span six decades, some were collected as early as 1940 while others were collected as recently as last year. Since the epidemiology of TB has changed over time, prevalence rates reported decades ago may not apply today.

2. Underlying population studied: As TB incidence rates have fallen in the general population, the disease is increasingly found among risk groups such as persons from TB endemic countries. When comparing prevalence rates of LTBI, it is important to consider the comparability of the underlying population.

3. Test used and definition of a positive test: Two types of tuberculin skin tests were used in the studies reported. The most common was the Mantoux skin test but a few early studies used the Tine test. The size of induration chosen as a cut-off for a positive TST test differed among studies. While this is noted in the compendium, readers need to take this into consideration when comparing prevalence rates between studies. In addition, studies differ in the use of one-step tuberculin versus two-step tuberculin testing to determine prevalence. The latter would detect booster reactions while the former would not.
   
   For a description of tuberculin skin testing technique, one step versus two-step testing, causes of false positive and false negative tests as well as an interpretation of test results, please refer to the current issue of the Canadian Tuberculosis Standards.

4. Quality of data: Readers need to be aware that the quality of information contained in the compendium is variable. Data sources include research studies, public health screening, TB contact investigations, and information contained in existing databases. Participation rates and completeness of data vary; confounders such as BCG vaccination were controlled in some studies but not others.

Layout of Report

The compendium includes two additional chapters besides this one. The next chapter provides summary LTBI prevalence rates by risk category. For further information about each study, the reader is referred to chapter 3. Here, studies are listed alphabetically by author or source name. Complete journal references along with a synopsis of each study are provided. A description of the underlying population, the type of TST test used, definition of a significant result, participation rates, overall prevalence rates of LTBI as well as LTBI prevalence rates by subgroups are included.

Comments and Additions

Readers are encouraged to contact Tuberculosis Prevention and Control with comments about the compendium or to contribute information for future editions:

Tuberculosis Prevention and Control
Public Health Agency of Canada
Telephone (613) 941-0238
Email: TB.1@phac-aspc.gc.ca

Chapter 2
Summary of Tuberculin Screening Studies

This chapter summarizes reported results of tuberculin screening by risk group:

• Aboriginal
• Community
• Foreign-born
• Health care worker
• High risk setting (e.g. prison, homeless shelter)
• Injection drug user
• Long term care facility
• Students

The studies in each risk group are listed chronologically by year of TB screening.

Further information about each study and more detailed findings of tuberculin screening can be found in Chapter 3; studies are listed alphabetically by author or source name.

Chapter 3 Description of Tuberculin Screening Studies

This chapter lists authors and sources alphabetically.

For each entry, complete journal reference and title are provided.
Further information about the study population, description of tuberculin testing and additional TST screening results can also be found.

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