To our knowledge, the present study is the first to study TST practice among HIV-infected persons in Canada. We found a differential approach to TST screening based on risk factors and numerous missed opportunities to prevent active TB. Despite recommendations that all HIV-positive patients be screened for latent TB shortly after diagnosis, as many as 70% of patients failed to receive TSTs. The majority of TB cases within this cohort (nine of 17) occurred among patients who were not screened, resulting in an incidence rate 25 times the Canadian national rate in 2003 (23
Receiving a TST is a multistep process that involves obtaining a prescription from a provider, actually receiving the intradermal tuberculin injection and returning 48 h to 72 h later for a reading. Thus, there are several opportunities for both the patient and the provider to fail to complete screening. Factors identified as determinants of receiving a baseline TST give some indication of how patients may have been selectively screened. Patients who had a greater delay between HIV diagnosis and first clinic visit, and those who were taking ARVs at baseline were less likely to receive a TST. It is possible that these patients were presumed to have already received preventive services and other care elsewhere. Region of birth and primary HIV risk factor (particularly emigrating from an HIV endemic region) were also independently related to receiving a TST at baseline. This raises questions about clinicians’ preferential focus on particular groups perceived to be at higher risk for an opportunistic infection. At least one other study (24
) has found that screening for sexually transmitted infections among HIV-positive patients varied by transmission risk factor and racial groups.
By assuming that patients originating from high-risk regions are in greater need of TB screening than Canadian-born patients or MSM, providers may be missing an opportunity to detect a large proportion of the TB cases that develop within the clinic’s population. Although this assumption is well founded (patients from high-risk countries developed active TB at a rate four times that of patients born in Canada or Europe), the effects are detrimental: 41% of all the TB cases and 56% of the TB cases who were not screened were born in Canada – a clear indication that attention should be given to groups perceived to be at low risk. While relative cost-effectiveness of targeted versus universal screening strategies has been debated, HIV-infected persons have been identified as a targeted group; it is not recommended that they undergo further triaging by attributes such as country of birth or transmission risk factor – an injunction supported by our findings (25
We did not observe that a higher CD4 cell count was associated with the rate of TST testing, suggesting that clinicians were not selectively choosing to avoid testing individuals who may be anergic due to advanced immunodeficiency. However, because higher CD4 cell counts were associated with TST-positivity among those screened, as has been previously reported (11
), offering retesting after CD4 cell recovery following HIV therapy could represent an important strategy to reduce the number of incident TB cases. We found little evidence, however, that clinicians had retested patients (only 12% initially TST-negative were retested).
Of all the stages that presented opportunities for preventing cases of active TB, initiating LTBI treatment proved to be the most crucial. The incidence rate of TB among the TST-positive patients who failed to initiate LTBI treatment was 122 cases per 1000 person-years. This finding reinforces the importance of clinician and patient adherence to recommendations for isoniazid prophylaxis among TST-positive patients (27
). It is noteworthy that while proportions of patients who screened and tested positive for LTBI varied by region of birth, once diagnosed, there were no significant differences in the clinical management of LTBI between these two groups. Chart review revealed that many patients failed to receive LTBI treatment because of potential drug interactions with HAART. Only three patients did not receive LTBI treatment because they failed to return to the clinic for follow-up visits and/or failed to comply with treatment regimens.
Our study offered a unique examination of factors associated with TST screening in a large study population with a high number of foreign-born patients, universal access to care and long duration of follow-up that permitted time-trend analysis. The results of our study parallel those of other studies that have examined TB screening among patients infected with HIV. The absence of ARV treatment (11
), female sex (15
) and foreign birth (14
) have been reported as determinants of receiving a TST in other cohorts. The independent predictors of positive TSTs in our study (CD4 count, country of birth and HIV transmission of risk factor) have similarly been identified as predictors (11
The overall proportion of patients TST screened in the present study was low compared with estimates from other studies, which range from 54% to 69% (11
). The TST screening proportion was also lower than the proportion of patients screened at the clinic at baseline for other infections such as syphilis (40%), toxoplasmosis (41%) and cytomegalovirus (40%) (data not shown). TB screening differs from these other tests because it requires a second visit for the results to be read. Failure to return to the clinic may, in part, explain the lower completed screening rates observed. Based on the chart review, 9% of individuals without a documented TST result had received a TST but failed to return to the clinic for a reading. Ultimately, the use of a reliable blood test to detect latent TB may help to address this drawback of the TST. The results of the current study were presented to the clinic staff with the aim of improving TST testing rate in the clinic population.
Sensitivity analysis revealed under-reporting of TST in the electronic database used. However, characteristics of those misclassified were similar to the remainder of the cohort; thus, it is reasonable to assume that the misclassification was nondifferential and has not affected the validity of other results presented here. We did not collect information on other factors that may be related to decisions to administer TSTs (eg, TB risk factors such as housing status, incarceration and other medical risk factors).
Finally, although we cannot be certain that all cases of active TB that occurred during follow-up were identified, given the nature and severity of the disease and the specific nature of TB treatment regimens, we believe that the vast majority of cases were captured in the database. The MCI is a primary centre for the treatment of TB in Montreal – 37% of all TB cases in Montreal were seen at the MCI and affiliated hospitals in the last five years; therefore, it is unlikely that cases were diagnosed and treated elsewhere without the knowledge of the MCI HIV team. An underestimation of the number of incident TB cases would only enforce our conclusions that low rates of TST screening represent a missed opportunity for active TB prevention among HIV-infected persons.
Our findings support the recommendations of the Centers for Disease Control and Prevention, the Canadian Thoracic Society and the American Thoracic Society to screen all patients infected with HIV on diagnosis (26
). Clinicians should be advised of the important number of cases of TB occurring among HIV-infected patients, both Canadian-born and foreign-born (17
), and should be encouraged to screen all patients regardless of risk profile.