Background: There have been concerns that irreversible immune damage may result if highly active antiretroviral therapy (HAART) is initiated after the CD4 cell count declines to below 350 cells/μL; however, the role of antiretroviral adherence on CD4 cell count responses has not been well evaluated.
Methods: We evaluated CD4 cell count responses of 1522 antiretroviral-naive patients initiating HAART who were stratified by baseline CD4 cell count (<50, 50–199, and ≥200 cells/μL) and adherence.
Results: Among patients starting HAART with <50 cells/μL, during the fifth 15-week period after the initiation of HAART, absolute CD4 cell counts were 200 cells/μL (interquartile range [IQR]: 130–290) for adherent patients versus 60 cells/μL (IQR: 10–130) for nonadherent patients. Similarly, among patients starting HAART with 50 to 199 cells/μL, during the fifth 15-week period after the initiation of HAART, absolute CD4 cell counts were 300 cells/μL (IQR: 180–390) versus 125 cells/μL (IQR: 40–210) for nonadherent patients. In Cox regression analyses, adherence was the strongest independent predictor of the time to a gain of ≥50 cells/μL from baseline (relative hazard [RH] = 2.88, 95% confidence interval [CI]: 2.46–3.37). Among patients with baseline CD4 cell counts <200 cells/μL, adherence was the strongest independent predictor of the time to a CD4 cell count >200 cells/μL (RH = 4.85, 95% CI: 3.15–7.47).
Conclusions: These data demonstrate that substantial CD4 gains are possible among highly advanced adherent patients and should contribute to the ongoing debate over the optimal time to initiate HAART.
The benefits of highly active antiretroviral therapy (HAART) in the management of HIV disease are well established. Through the suppression of plasma HIV-1 RNA, HAART has been shown to improve CD4 cell counts and, in turn, to decrease morbidity and mortality among HIV-infected patients. 1,2 HAART presents major challenges, however, because of the more rapid emergence of drug resistance among nonadherent patients 3 as well as the adverse effects of antiretroviral agents. 4–6 As a result, the optimal time to initiate HAART is one of the most critical questions in the therapeutic management of HIV disease. 7
We have recently evaluated rates of disease progression stratified by baseline CD4 cell count and HIV-1 RNA levels among patients initiating HAART and found that baseline CD4 cell count was the only independent predictor of mortality. 8 In these analyses, we found that only patients initiating therapy when the CD4 cell count had declined below 200 cells/μL were at increased risk of disease progression, a finding that has been independently verified. 9,10 There have been a growing number of studies suggesting that it may not be safe to delay HAART after the CD4 cell count declines below 350 cells/μL, however. 11–14 These results have led to speculation that irreversible immune damage precluding a CD4 cell count response may occur if HAART is delayed below a CD4 cell count of 350 cells/μL. 11–16
A limitation of previous studies 8,10,12–14 has been that they have not been adjusted for patient adherence, 17–19 and there are limited data on the relationship between CD4 cell count responses among adherent and nonadherent patients. This is an area in critical need of data, because the decision regarding when to initiate HAART relies increasingly on CD4 cell count and clinicians are increasingly reliant on CD4 cell count measures when determining the success of therapy. 16 We therefore conducted the present analyses to evaluate CD4 cell count responses among antiretroviral-naive patients initiating HAART and the impact of patient adherence.
From the *British Columbia Centre for Excellence in HIV/AIDS, St. Paul's Hospital, Vancouver, BC, Canada; and Departments of †Health Care and Epidemiology, University of British Colombia, Vancouver, BC, Canada; ‡Pathology and Laboratory Medicine, University of British Colombia, Vancouver, BC, Canada; and §Medicine, University of British Columbia, Vancouver, British Columbia, Canada.
Received for publication September 7, 2003; accepted November 14, 2003.
Supported by the Michael Smith Foundation for Health Research (E. Wood and R. Hogg) and by the Canadian Institutes of Health Research (R. Hogg).
Reprints: Julio S. G. Montaner, Chair, AIDS Research Program University of British Columbia/St. Paul's Hospital, 667-1081 Burrard Street, Vancouver, BC V6Z 1Y6, Canada (e-mail: firstname.lastname@example.org).
The HAART Observational Medical Evaluation and Research (HOMER) study run through the British Columbia Centre for Excellence in HIV/AIDS Drug Treatment Program has been described in detail elsewhere. 8,19 Briefly, the British Columbia Centre for Excellence in HIV/AIDS Drug Treatment Program remains the only free source of antiretroviral medications in the province. In June 1996, the Centre adopted plasma viral load–driven antiretroviral therapy guidelines consistent with those put forward by the International AIDS Society–USA Panel. 20 Consistent with contemporary practice, the Centre guidelines were revised in July 1997 to recommend triple combination therapy for all antiretroviral-naive individuals with plasma HIV-1 RNA levels >5000 copies/mL or CD4 cell counts <500 cells/μL. Plasma viral loads were measured using the Amplicor HIV-1 Monitor (Roche Diagnostics, Branchburg, NJ). For all program participants, a complete prospective profile of antiretroviral therapy is maintained, including the medications prescribed, the dose, the dispensation dates, and the quantity dispensed. The Centre's HIV/AIDS Drug Treatment Program has received ethical approval from the University of British Columbia Ethics Review Committee at its St. Paul's Hospital site, and the program conforms with the province's Freedom of Information and Protection of Privacy Act.
In the present study, analyses were restricted to HIV-infected men and women who were antiretroviral naïve, were first prescribed triple drug antiretroviral therapy between August 1, 1996 and July 31, 2000, and were followed to March 31, 2002. Study subjects were initially prescribed triple drug combination therapy with regimens that included 2 nucleoside reverse transcriptase inhibitors and either a protease inhibitor (PI) or a nonnucleoside reverse transcriptase inhibitor (NNRTI) at the discretion of the enrolling physician. For the purposes of these analyses, we followed the intent-to-treat principle, and subjects were included as they were first dispensed antiretrovirals regardless of whether they later modified their therapeutic regimen.
As an initial analysis, we performed an examination of CD4 cell count responses after the initiation of HAART. Here, patients were pooled into CD4 cell counts of <50, 50 to 199, and ≥200 cells/μL, respectively, 8,19 and as we have done previously, the period of observation was divided into 5 15-week follow-up periods. 21,22 When more than 1 CD4 cell count measure was available during 1 of the 15-week intervals, the measure closest to the end of the interval was used. 21,22
For the purposes of this analysis, patients were further stratified into adherent and nonadherent groups using prescription refill compliance. 23 The definition of adherence was based on the ratio of time that medication dispensed would last as a proportion of follow-up time. This calculation was restricted to each patient's first year on therapy to avoid the reverse causation that could occur among patients who cease antiretroviral therapy after they have become too sick to take medication. We have previously demonstrated how this estimate strongly predicts virologic response 18 and survival 19,24 and how it can adjust for the potentially confounding effect of treatment interruption. 25 For the purposes of this analysis, patients were defined as nonadherent if they picked up less than 75% of their required antiretroviral medications during the first 12 months of therapy, because we wanted to be conservative in our definition of adherence and we have previously found this level of adherence to be associated with elevated mortality. 19,24
In addition, we also modeled factors associated with the time to CD4 cell count responses to discern the relative contribution of adherence and clinical and sociodemographic baseline covariates. Here, we used the same definition that we and others have used previously, 22,26 and we defined a CD4 cell count response as a gain of 50 cells/μL (herein referred to as a CD4 response). Cumulative rates of CD4 cell count response were estimated using Kaplan-Meier methods. Event-free subjects were right-censored as of March 31, 2002 or their last follow-up measure if it occurred before this date.
Cox proportional hazards regression was used to calculate univariate and adjusted relative hazards and 95% confidence intervals (CIs). 27 A number of baseline prognostic variables were examined in this analysis, including gender (male vs. female), age (continuous), adherence (<75% vs. ≥75%), history of injection drug use (yes vs. no), initial HAART regimen (PI-containing vs. NNRTI-containing), baseline clinical diagnosis of AIDS (yes vs. no), CD4 cell count (<50, 50–199, and ≥200 cells/μL), and log10-transformed plasma HIV-1 RNA levels (continuous). The assumption of proportional hazards was validated by inspection of log (−log [survival function]) estimates against log time plots. The multivariate model was fit using the same protocol of adjusting for all variables that were statistically significant (P < 0.05) in univariate analyses as well as baseline CD4 cell count, regardless of statistical significance, because this was a primary variable of interest in these analyses. All statistical analyses were performed using SAS software version 6.0 (SAS, Cary, NC). All tests of significance were 2-sided, with a probability value of less than 0.05 indicating that an association was statistically significant.
Between August 1, 1996 and July 31, 2000, a total of 1683 antiretroviral-naive participants aged 18 years or older began triple combination therapy with either an NNRTI, a single PI, or a double PI regimen. Of these, 161 (10.0%) were excluded from this analysis for not having both baseline CD4 cell count and plasma HIV-1 RNA level measures available within 6 months before the start of antiretroviral therapy. Therefore, the study sample was based on the 1522 (90.4%) subjects who were eligible for the present analyses. No differences in sex, baseline AIDS status, age, or subsequent mortality were observed between the study sample and those excluded. Persons excluded from this analysis were more likely to be taking PIs (P = 0.03), however. Overall, 204 (13.4%), 393 (25.8%), and 925 (60.8%) patients initiated HAART in the <50, 50 to 199, and >200 cells/μL CD4 cell count strata, respectively. A total of 439 (31%) participants started therapy with an NNRTI-based regimen. Among these participants, 413 (94%) used nevirapine, 15 (3%) used efavirenz, and 11 (3%) used delavirdine. Similarly, 983 (69%) initiated therapy with a single PI, and 100 (6.6%) used a double PI-containing regimen. The characteristics of the study population are shown in Table 1.
Figure 1 shows the median (and interquartile range [IQR]) CD4 cell count responses among adherent (A) and nonadherent (B) patients in each of the 3 baseline CD4 cell count strata and at baseline and during the 5 15-week follow-up periods. As shown here, immediate and sustained differences in the CD4 cell count response between adherent and nonadherent patients were observed in each CD4 cell count stratum. Among patients with <50 cells/μL at baseline, during the fifth 15-week period after the initiation of HAART, absolute CD4 cell counts were 200 cells/μL (IQR: 130–290) for adherent patients versus 60 cells/μL (IQR: 10–30) for nonadherent patients (P = 0.009). Similarly, among patients starting HAART with 50 to 199 cells/μL, during the fifth 15-week period after the initiation of HAART, absolute CD4 cell counts were 300 cells/μL (IQR: 180–390) versus 125 cells/μL (IQR: 40–210) for nonadherent patients (P < 0.001). Finally, among patients starting HAART with ≥200 cells/μL, during the fifth 15-week period after the initiation of HAART, absolute CD4 cell counts were 550 cells/μL (IQR: 410–720) versus 300 cells/μL (IQR: 250–505) for nonadherent patients (P < 0.001). Because all statistical comparisons reached conventional statistical significance, the small sample sizes in some groups were not a limitation of this analysis. The number of patients contributing data at each follow-up period is shown in Figure 1.
Figure 2 shows the Kaplan-Meier cumulative CD4 cell count response rate estimates for patients with in the <50, 50 to 199, and ≥200 cells/μL baseline CD4 cell count categories stratified into adherent and nonadherent patients. As shown here, among patients with baseline CD4 cell counts <50 cells/μL (A), the product limit estimate (SE) of the cumulative CD4 response rate at 24 months was 58.7% (12.3%) for nonadherent patients and 91.7% (2.3%) for adherent patients (log-rank:P < 0.001). Among patients with a baseline CD4 cell count of 50 to 199 cells/μL (B), the product limit estimate (SE) of the cumulative CD4 response rate was 56.1% (6.7%) for nonadherent patients and 88.8% (1.9%) at 24 months for adherent patients (log-rank:P < 0.001). Finally, among patients with a baseline CD4 cell count ≥200 cells/μL (C), the product limit estimate (SE) of the cumulative CD4 response rate was 57.2% (3.3%) for nonadherent patients and 91.9% (1.1%) at 24 months for adherent patients (log-rank:P < 0.001).
Results of the univariate and multivariate Cox regression analysis of time to CD4 cell count response are shown in Table 2. As shown here, male gender (relative hazard [RH] = 1.51, 95% CI: 1.29–1.78), age (RH = 1.13, 95% CI: 1.06–1.20 per 10 years older), adherence (RH = 2.93, 95% CI: 2.52–3.43), history of injection drug use (RH = 0.85, 95% CI: 0.75–0.97), PI use (RH = 0.83, 95% CI: 0.74–0.95), and baseline HIV RNA (RH = 1.13, 95% CI: 1.05–1.23 per log increase) were all associated with a CD4 cell count response in univariate analyses. Baseline clinical diagnosis of AIDS and baseline CD4 cell count were not associated with a CD4 cell count response. In a multivariate model adjusting for all variables that were significant at the univariate level as well as baseline CD4 cell count, male gender (RH = 1.37, 95% CI: 1.16–1.62), adherence (RH = 2.88, 95% CI: 2.46–3.37), PI use (RH = 0.83, 95% CI: 0.72–0.94), and baseline HIV RNA (RH = 1.17, 95% CI: 1.07–1.28 per log increase) were all independently associated with a CD4 cell count response. To control for potential confounding resulting from changes in guidelines and availability of newer antiretroviral agents, the final model was also adjusted for year of therapy initiation, which was nonsignificant in both univariate and multivariate analyses. It is noteworthy that a statistically significant gender effect was observed after adjustment for adherence and other covariates. The impact of gender on response to HAART is controversial 28; because of space constraints, this finding will have to be evaluated in the context of a subsequent study.
We also conducted subanalyses in which we evaluated the time to the first of 2 consecutive CD4 cell count measures ≥200 cells/mm3 among patients in the <50 and 50 to 199 cells/mm3 baseline CD4 cell count categories. In this case, adherence was again the strongest predictor of achieving the event. In Kaplan-Meier analyses, among patients with baseline CD4 cell counts <50 cells/mm3, the product limit estimate (SE) of the cumulative rate of achieving the first of 2 consecutive CD4 cell count measures ≥200 cells at 24 months was 10.3% (7.1%) for nonadherent patients and 52.9% (4.3%) for adherent patients (log-rank:P = 0.002). Similarly, among patients with a baseline CD4 cell count 50 to 199 cells/mm3, the product limit estimate (SE) of the cumulative rate of achieving the first of 2 consecutive CD4 cell count measures ≥200 cells at 24 months was 31.5% (6.4%) for nonadherent patients and 75.7% (2.6%) for adherent patients (log-rank:P < 0.001). When a Cox model was fit with the same variables as those shown in the model presented in Table 2, similar factors were associated with the time to the first of 2 CD4 cell count measures ≥200 cells/mm3, with adherence being the strongest predictor of this event (RH = 4.85, 95% CI: 3.15–7.47).
Finally, we sought to evaluate how differing levels of adherence influenced the time to a CD4 cell count response among patients with baseline CD4 cell count levels ≥200 and <200 cells/mm3. Here, we performed a Kaplan-Meier analysis in which we evaluated the time to a CD4 cell count response among patients who were defined as ≥95%, 94% to 75%, and <75% adherent. As shown in Figure 3 (A), deviations from ≥95% adherence had a marked impact on the time to a rise of 50 cells from baseline among patients with a baseline CD4 cell count <200 cells/mm3. The cumulative CD4 response rate at 24 months was 72.8% (2.5%) among patients who were ≥95% adherent, whereas the response was 50.7% (5.4%) at 24 months among patients who were 94% to 75% adherent. As shown in Figure 3 (B), although still statistically significant, the time to a rise of 50 cells from baseline was less marked among patients who initiated HAART when the CD4 cell count was ≥200 cells/mm3. The cumulative CD4 response rate at 24 months was 93.1% (0.9%) among patients who were ≥95% adherent and 84.0% (2.5%) among patients who were 94% to 75% adherent.
In the present study, we found that substantial and sustained gains in CD4 cell count were observed among adherent patients, regardless of baseline CD4 cell count. In addition, the time to a CD4 cell count response was most strongly predicted by patient adherence. In fact, even among highly advanced patients who initiated HAART with CD4 cell counts below 200 cells/mm3, the time to 2 consecutive CD4 measures above 200 cells/mm3 was most strongly predicted by adherence to HAART. These data suggest that previous studies showing that CD4 cell counts <350 cells/mm3 may preclude a CD4 cell count response may have been confounded by patient nonadherence, and the present study should contribute to the ongoing debate over the optimal time to initiate HAART.
In recent therapeutic guidelines, concerns regarding irreversible immune damage if therapy is initiated at a CD4 cell count below 350 cells/mm3 have led to the recommendation that clinicians consider immediate initiation of HAART as soon as the CD4 cell count declines below this level. 15,16 Studies that have been cited as a reason to consider earlier initiation of antiretroviral therapy in recent guidelines 11–16 have not considered the potentially confounding effect of patient nonadherence, however. The data presented here suggest that earlier studies may have been partially confounded by characteristics of patients who present later for treatment, which may include incomplete adherence among others. 29
The present study does not support delaying therapy below 200 cells/mm3, which has been associated with elevated mortality in previous studies. 8,10,19 It is noteworthy, however, that 39% of patients included in the present study initiated HAART after the CD4 cell count had declined below 200 cells/mm3. Similar findings have been reported in other settings, 30 and given the challenges presented by undiagnosed HIV infection, 31 it is likely that a substantial proportion of patients will continue to present for therapy after the CD4 cell count has declined below the levels recommended by therapeutic guidelines. 15,16 As such, the data presented in the present study should be helpful to underscore the importance of adherence for clinicians managing patients who present for HAART with low baseline CD4 cell counts.
It is important to stress that these data arose in a setting where all HIV/AIDS care, antiretrovirals, and laboratory monitoring are available free of charge and where previous studies have shown that virtually all patients acquire antiretrovirals through a single centralized source. 32 As such, we are confident that our results are not influenced by selection biases or issues of access to therapy, which often compromise the interpretation of similar cohort studies. Nevertheless, like all studies of patients treated in observational cohorts, unmeasured differences may exist among study populations; for this reason, caution is warranted. In addition, because a complete prospective record of antiretroviral dispensation was maintained, it was possible to determine precisely each individual's level of prescription refill compliance. Nevertheless, although using refill compliance as a surrogate for adherence has been previously validated, 18,23–25 there is likely a strong conservative bias operating in our study, because patients could have been less than optimally adherent to daily treatment intake despite maintaining a high level of refill compliance during the first year of therapy. In addition, we should note that our primary analyses examined 75% adherence, because we sought to be consistent with previous studies that have demonstrated the clinical significance of this cutoff. 19,24 Nevertheless, although it was not the primary purpose of this study to evaluate the impact of different levels of patient adherence, we should stress that in subanalyses, we demonstrated superior CD4 cell count responses among patients who were >95% adherent. As such, our findings reinforce the notion that patients should strive for 100% adherence, and we should stress that our findings do not indicate that 75% adherence is sufficient to conserve long-term CD4 cell count responses. 16 Finally, we should note that irreversible immune damage and inability to raise CD4 cell count above a specific threshold are not equivalent concepts and future analyses should consider that clonal analysis of the regenerated CD4 pool would be helpful to inform the debate regarding CD4 cell count responses.
In summary, these data demonstrate that large and sustained CD4 gains are possible regardless of baseline CD4 cell count so long as patients are adherent. Our findings do not support the notion that that irreversible immune damage precludes a CD4 response to HAART if the CD4 cell count declines below 350 cells/mm3 and may raise confidence in the 200-cell/mm3 threshold for the initiation of HAART. These results may also be useful for patients weighing the difficult decision regarding the optimal time to initiate HAART as well as for the ongoing development of therapeutic guidelines. In addition, because it is likely that a high proportion of patients will continue to present for treatment after CD4 cell counts have dropped below 200 cells/mm3, these data will help to demonstrate to clinicians the importance of adherence among highly advanced patients.
The authors thank all participants in the HIV/AIDS Treatment Program. They also thank Bonnie Devlin, Peter Vann, Elizabeth Ferris, Chandra Lips, Nada Gataric, Kelly Hsu, and Myrna Reginaldo for their research and administrative assistance.
1. Hammer SM, Squires KE, Hughes MD, et al. A controlled trial of two nucleoside analogues plus indinavir in persons with human immunodeficiency virus infection and CD4 cell counts of 200 per cubic millimeter or less. AIDS Clinical Trials Group 320 Study Team. N Engl J Med. 1997; 337:725–733.
2. Montaner JS, Reiss P, Cooper D, et al. A randomized, double-blind trial comparing combinations of nevirapine, didanosine, and zidovudine for HIV-infected patients: the INCAS Trial. Italy, The Netherlands, Canada and Australia Study. JAMA. 1998; 279:930–937.
3. Gallego O, de Mendoza C, Perez-Elias MJ, et al. Drug resistance in patients experiencing early virological failure under a triple combination including indinavir. AIDS. 2001; 15:1701–1706.
4. Cote HC, Brumme ZL, Craib KJ, et al. Changes in mitochondrial DNA as a marker of nucleoside toxicity in HIV-infected patients. N Engl J Med. 2002; 346:811–820.
5. Heath KV, Hogg RS, Chan K, et al. Lipodystrophy-associated morphological, cholesterol, and triglyceride abnormalities in a population-based HIV/AIDS Treatment Program. AIDS. 2001; 15:231–239.
6. Sterling TR, Chaisson RE, Moore RD. Initiation of highly active antiretroviral therapy at CD4+ T lymphocyte counts of >350 cells/mm3
: disease progression, treatment durability, and drug toxicity. Clin Infect Dis. 2003; 36:812–815.
7. Lane HC, Neaton JD. When to start therapy for HIV infection: a swinging pendulum in search of data. Ann Intern Med. 2003; 138:680–681.
8. Hogg RS, Yip B, Chan KJ, et al. Rates of disease progression by baseline CD4 cell count and viral load after initiating triple-drug therapy. JAMA. 2001; 286:2568–2577.
9. Sterling TR, Chaisson RE, Moore RD. HIV-1 RNA, CD4 T-lymphocytes, and clinical response to highly active antiretroviral therapy. AIDS. 2001; 15:2251–2257.
10. Egger M, May M, Chene G, et al. Prognosis of HIV-1-infected patients starting highly active antiretroviral therapy: a collaborative analysis of prospective studies. Lancet. 2002; 360:119–129.
11. Palella F, Knoll M, Chmiel J, et al. Lower mortality in ambulatory HIV-infected patients who initiate antiretroviral therapy at higher CD4 cell counts. Presented at the Ninth Conference on Retroviral and Opportunistic Infections, Seattle, February 2002.
12. Phair JP, Mellors JW, Detels R, et al. Virologic and immunologic values allowing safe deferral of antiretroviral therapy. AIDS. 2002; 16:2455–2459.
13. Schackman BR, Freedberg KA, Weinstein MC, et al. Cost-effectiveness implications of the timing of antiretroviral therapy in HIV-infected adults. Arch Intern Med. 2002; 162:2478–2486.
14. Ferrer E, Santamarina E, Santin M, et al. Progression to AIDS/death is higher in patients initiating antiretroviral therapy with CD4 cell counts <350 cells/μL. Presented at the 10th Conference on Retroviral and Opportunistic Infections, Boston, February 2003.
15. Dybul M, Fauci AS, Bartlett JG, et al. Guidelines for using antiretroviral agents among HIV-infected adults and adolescents. Ann Intern Med. 2002; 137:381–433.
16. Yeni PG, Hammer SM, Carpenter CC, et al. Antiretroviral treatment for adult HIV infection in 2002: updated recommendations of the International AIDS Society-USA Panel. JAMA. 2002; 288:222–235.
17. Bangsberg DR, Perry S, Charlebois ED, et al. Non-adherence to highly active antiretroviral therapy predicts progression to AIDS. AIDS. 2001; 15:1181–1183.
18. Low-Beer S, Yip B, O'Shaughnessy MV, et al. Adherence to triple therapy and viral load response. J Acquir Immune Defic Syndr. 2000; 23:360–361.
19. Wood E, Hogg RS, Yip B, et al. Is there a baseline CD4 cell count that precludes a survival response to modern antiretroviral therapy? AIDS. 2003; 17:711–720.
20. Carpenter CC, Fischl MA, Hammer SM, et al. Antiretroviral therapy for HIV infection in 1996. Recommendations of an international panel. International AIDS Society-USA. JAMA. July 10, 1996; 276( 2):146–154.
21. Wood E, Yip B, Hogg RS, et al. Full suppression of viral load is needed to achieve an optimal CD4 cell count response among patients on triple drug antiretroviral therapy. AIDS. 2000; 14:1955–1960.
22. Wood E, Hogg RS, Yip B, et al. ”Discordant” increases in CD4 cell count relative to plasma viral load in a closely followed cohort of patients initiating antiretroviral therapy. J Acquir Immune Defic Syndr. 2002; 30:159–166.
23. Steiner JF, Prochazka AV. The assessment of refill compliance using pharmacy records: methods, validity, and applications. J Clin Epidemiol. 1997; 50:105–116.
24. Hogg RS, Heath KV, Bangsberg D, et al. Intermittent use of triple combination therapy is predictive of mortality at baseline and after one year of follow-up. AIDS. 2002; 16:1051–1058.
25. Palepu A, Yip B, Miller C, et al. Factors associated with the response to antiretroviral therapy among HIV-infected patients with and without a history of injection drug use. AIDS. 2001; 15:423–424.
26. Ledergerber B, Egger M, Opravil M, et al. Clinical progression and virological failure on highly active antiretroviral therapy in HIV-1 patients: a prospective cohort study. Swiss HIV Cohort Study. Lancet. 1999; 353:863–868.
27. Cox DR. Regression models and life tables. J Royal Stat Soc B. 1972; 34:187–202.
28. Moore AL, Kirk O, Johnson AM, et al. Virologic, immunologic, and clinical response to highly active antiretroviral therapy: the gender issue revisited. J Acquir Immune Defic Syndr. 2003; 32:452–461.
29. Celentano DD, Galai N, Sethi AK, et al. Time to initiating highly active antiretroviral therapy among HIV-infected injection drug users. AIDS. 2001; 15:1707–1715.
30. Gupta SB, Gilbert RL, Brady AR, et al. CD4 cell counts in adults with newly diagnosed HIV infection: results of surveillance in England and Wales, 1990-1998. CD4 Surveillance Scheme Advisory Group. AIDS. 2000; 14:853–861.
31. Wood E, Montaner JS, Bangsberg D, et al. Extending access to HIV antiretroviral therapy to marginalized populations in the developed world. AIDS. 2003; 17( 17):2419–2427.
32. Strathdee SA, Palepu A, Cornelisse PG, et al. Barriers to use of free antiretroviral therapy in injection drug users. JAMA. 1998; 280:547–549.