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Marginal structural models for estimating the effect of highly active antiretroviral therapy initiation on CD4 cell count

American Journal of Epidemiology, ISSN: 0002-9262, Vol: 162, Issue: 5, Page: 471-478
2005
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Article Description

The effect of highly active antiretroviral therapy (HAART) on the evolution of CD4-positive T-lymphocyte (CD4 cell) count among human immunodeficiency virus (HIV)-positive participants was estimated using inverse probability-of-treatment-and-censoring (IPTC)-weighted estimation of a marginal structural model. Of 1,763 eligible participants from two US cohort studies followed between 1996 and 2002, 60 percent initiated HAART. The IPTC-weighted estimate of the difference in mean CD4 cell count at 1 year among participants continuously treated versus those never treated was 71 cells/mm (95% confidence interval: 47.5, 94.6), which agrees with the reported results of randomized experiments. The corresponding estimate from a standard generalized estimating equations regression model that included baseline and most recent CD4 cell count and HIV type 1 RNA viral load as regressors was 26 cells/mm (95% confidence interval: 17.7, 34.3). These results indicate that nonrandomized studies of HIV treatment need to be analyzed with methods (e.g., IPTC-weighted estimation) that, in contrast to standard methods, appropriately adjust for time-varying covariates that are simultaneously confounders and intermediate variables. The 1-year estimate of 71 cells/mm was followed by an estimated continued increase of 29 cells/mm per year (estimated effect at 6 years: 216 cells/mm), providing evidence that the large short-term effect found in randomized experiments persists and continues to improve over 6 years. Copyright © 2005 by the Johns Hopkins Bloomberg School of Public Health. All rights reserved.

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