TABLE 5.

Estimated effect of each additional month until switching therapy, given current CD4 T-cell count (CD4(j)) and elapsed months since failure occurred (j), among subjects with virologic failure, sampled from a cohort of HIV^*-infected persons in San Francisco, California, 2000–2004

Term†	Estimate (cells/μl) per month of waiting to switch therapy‡	95% confidence interval§
β1	−9.2	−18.1, −0.3
β4	0.05	0.02, 0.08
β5	1.5	−0.4, 3.3
β7	−0.01	−0.02, −0.002

^*HIV, human immunodeficiency virus.

^†Based on the model E(Y_{Ā(j−1),c(j)}(j + 8)|A(j − 1) = 1, Supp(j) = 0, CD4(j)) = β₀ + β₁c(j)+ β₂CD4(j) + β₃j + β₄c(j) × CD4(j)+ β₅c(j) × j + β₆j × CD4(j) + β₇c(j) × CD4(j) × j, where c(j)denotes the counterfactual switching time in number of months after time j.

^‡Coefficient from a history-adjusted marginal structural model for the first nine time points (j = 0, …, 8).

^§Based on robust standard error estimates using generalized estimating equations.