Terry's case: A Youth at Risk

Risk Factors for the Development of Active TB

(Source: Canadian Tuberculosis Standards, 5^th Edition, 2000. The Canadian Lung Association, pages 49-50.)

Following primary tuberculosis infection, the lifetime cumulative risk for the development of active tuberculosis is generally estimated to be 10%. Half of these cases will occur in the first 2 to 3 years after infection. Certain factors increase the risk of tuberculosis reactivation because of diminished local or systemic immunity, as summarized in Table 1.

Table 1
Risk factors for the development of active tuberculosis among persons infected with Mycobacterium tuberculosis

Risk factor	Estimated risk for tuberculosis relative to persons with no known risk factor
HIGH RISK
Acquired immunodeficiency syndrome	170.0
Human immunodeficiency virus infection	113.0
Transplantation	20-74
Silicosis	30
Chronic renal failure/hemodialysis	10.0-25.3
Carcinoma of head and neck	16.0
Recent infection (≤ 2 years)	15.0
Abnormal chest radiograph - fibronodular disease	6.0-19
INCREASED RISK
Diabetes mellitus	2.0-3.6
Underweight (< 90% ideal body weight)	2.0-3.0
Age when infected (≤ 5 years)	2.2-5.0
Abnormal chest radiograph - granuloma	2.0
LOW RISK
Infected person, no known risk factor ("Low risk reactor")	1.0

 

Among persons with TB infection, dual infection with HIV is the most important risk factor for the development of disease. The annual risk of active disease varies from 3% to 13% and is highest when the CD4 count falls below 200 x 106/L. Tuberculosis is often the earliest manifestation of HIV-associated immune deficiency and will occur when CD4 counts fall below 500 x 106/L.

Immune suppression may also occur following treatment with cancer chemotherapeutic agents or corticosteroids (prednisone of at least 15 mg/day or equivalent). Certain tumours, such as T-cell lymphomas, increase the risk of reactivation of dormant tuberculosis infection. Pulmonary silicosis (simple or complicated) will increase the risk of reactivation substantially, but only for pulmonary forms of tuberculosis. Tuberculin reactors whose weight is less than 90% of ideal body weight will have twice the risk of reactivation of tuberculin reactors whose weight is in the ideal range, and four times the risk compared with tuberculin reactors whose weight is more than 110% of ideal.

Table 2
Interpretation of tuberculin test

Tuberculin reaction size, mm induration	Setting in which reaction considered significant (meaning probable TB infection)
0-4	HIV infection AND expected risk of tuberculosis infection is high (e.g.patient is an immigrant from a country where TB is endemic, is a household contact, or has an abnormal radiograph). This reaction size is not normally considered significant, but in the presence of immune suppression may be important.
5-9	HIV infection Close contact of active contagious case Abnormal chest radiograph with fibronodular disease
10	All others

 

Those with significant reactions should be considered to have tuberculous infection. Prescription of isoniazid (INH) or other preventive therapy may or may not be indicated, depending on a consideration of the risks of reactivation of disease versus the risks of therapy (see Chapter II-E: Treatment of Tuberculous Disease and Infection).

Practice a TB Skin Test

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