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Volume: 22S1 • April 1996
Guidelines for Preventing the
Transmission of Tuberculosis in Canadian Health Care
Facilities and Other Institutional Settings
Appendix C
Guidelines for the Use of Isoniazid (INH)
Preventive Therapy
The Canadian Tuberculosis Standards (43) and the
article by the Canadian Thoracic Society (9) provide
further information concerning INH preventive
therapy.
Individuals who are newly infected with M.
tuberculosis have approximately a 5% risk of
developing active TB in the first 2 years after
becoming infected. Estimates of the efficacy of INH
preventive therapy vary, with compliance being a
vitally important issue. An overall protective effect
estimate of 68% (including compliance) appears to be
a prudent "middle of the road" suggestion (94) . INH
preventive therapy for 6 to 9 months, for those who
comply completely with treatment, gives
immunocompetent contacts and new reactors protection
against TB with 90% success. A 12-month course of
preventive therapy is recommended for individuals who
are HIV-positive.
The usual dose of INH for adults is 300 mg daily by
mouth and for children is 10 mg/kg/day by mouth
(43,9) .
Adverse Reactions to INH
Adverse reactions to INH are uncommon. The most
significant adverse reaction to INH is hepatitis.
However, the risk of developing hepatitis is age
dependent, relatively small, and unlikely to result in
permanent sequelae. The risk of hepatitis can be
minimized with careful monitoring of liver enzymes. In
a 1978 study of 13,838 persons taking INH preventive
therapy, the rate of hepatitis was directly related to
age: 0% for those under 20, 0.3% for those 20 to 34,
1.2% for those 35 to 49, and 2.3% for those 50 to 64
years (95) . Approximately 1% of those on INH
prophylaxis will develop peripheral neuropathy, which
is prevented by concurrent administration of vitamin
B6.
INH Distribution
The responsibility for providing INH preventive
therapy varies in different jurisdictions in Canada.
The most effective distribution system is dependent on
the local situation. Providing preventive therapy may
be best performed by health care professionals who work
in a centralized facility and who are familiar with INH
therapy and TB. However, dispensing, monitoring, and
follow-up may be adequately performed by personnel
working in the screening program itself, the local
public health department, the provincial TB control
program, or by a well-advised family physician.
Regardless of who performs this task, the requirements
for appropriate decision making to initiate therapy, to
maintain adequate records, to perform monitoring during
therapy, and to ensure appropriate follow-up are the
same.
INH Compliance Strategies
Compliance is a major issue considering the length
of time medication is to be taken by an asymptomatic
person. Directly observed preventive therapy (DOP), in
which INH is dispensed and consumed by the person on
prophylaxis under direct observation by a health care
professional, may be the best method in certain
circumstances. This is highly dependent on both the
individual and the situation.
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