Interventions for Tuberculosis Control and Elimination 2002

Table 11. Preventive therapy – effectiveness. Effectiveness of preventive therapy
in dependence of risk of tuberculosis, efficacy of treatment regimen, and adherence
to the regimen. All parameters are shown as fractions. Risk of tuberculosis
is allowed to vary from 0.05 (estimated cumulative risk subsequent to the first five
years following infection) to 0.30 (estimated cumulative risk of a person dually infected
with M. tuberculosis and HIV).
Risk Efficacy Adherence Overall Number to treat
of tuberculosis of regimen to treatment effectiveness to prevent 1 case
0.05 0.60 0.30 0.009 111
0.10 0.60 0.30 0.018 56
0.30 0.60 0.30 0.054 19
0.30 0.90 0.30 0.081 12
0.30 0.90 0.50 0.135 7
0.30 0.90 0.80 0.216 5
impact on overall operational effectiveness. The risks of tuberculosis shown
here are for persons with long-standing tuberculous infection, recently
acquired tuberculous infection and concomitant HIV infection (0.05, 0.10,
and 0.30 for the respective risks of tuberculosis). Efficacy examples have
been taken from isoniazid-preventive chemotherapy ranges, and adherence
has been made up to vary as might be expected among different patients
with a condition that is not symptomatic. The overall effectiveness is the
product of these three variables and the number of patients that must be
treated to prevent one case is the reciprocal value of effectiveness. The
example shows that effectiveness will greatly vary depending on the selection
of patients, the type of regimen and the extent to which patients adhere
to treatment. Although reality is not quite as straightforward as in this
example (it assumes that each component proportionally reduces effectiveness),
it may help in deciding under which circumstances preventive
chemotherapy is to be recommended. The specific indication will depend
on the availability of resources, as the overall effectiveness is, under any
circumstance, relatively modest. Not accounted for in this model is the
probability of tuberculous infection actually being present when a “positive”
tuberculin skin test is recorded.
A study in Kampala, Uganda, ascertained the operational feasibility
and effectiveness of preventive chemotherapy in a high-risk population,
apparently motivated to attend voluntary testing sites for HIV. 905 Among
patients who were found to be HIV-positive, only about 60% returned to
obtain their result and to receive counseling (figure 89) and of these only
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