Objective: To describe the relationship between absolute CD4 count and CD4%, and the influence on this of gender, risk group, age, a diagnosis of AIDS, use of zidovudine (ZDV) therapy and PCP prophylaxis.
Methods: 9203 paired serial measurements of CD4 count and CD4% on 1017 initially AIDS-free and ZDV-naive HIV positive patients from a London-based cohort were available for analysis. Multi-level regression procedures were used on log-transformed data to relate values of CD4 count to a given level of CD4%. We estimated the effect of selected covariates on this relationship from the exponent of the covariate coefficient.
Results: A strong linear relationship was found between log CD4 and log CD4%, CD4 = e 1.78(CD4%)1.26 or 5.93 (CD4%)1.26 (excluding covariates). Based on this model, a CD4% of 5%, 15%, and 30% corresponded to an estimated CD4 count (95% confidence interval [CI]) of 45 cells/mm3 (17-117 cells/mm3), 182 cells/mm3 (64-499 cells/mm3) and 438 cells/mm3 (132-1395 cells/mm3), respectively. However, after adjustment for selected covariates, the predicted CD4 count for a given CD4% was found to be lower among heterosexuals and injecting drug users as compared with homosexual men by 30% and 17% respectively; following an AIDS diagnosis by 21%; and after initiation of ZDV therapy and PCP prophylaxis by 19% and 10%, respectively.
Conclusion: This analysis should be useful to clinicians and researchers in relating values of CD4 count to CD4%, although we have demonstrated that this is not a simple relationship. The wide CI observed in the estimated CD4 count particularly at high CD4% values, and the adjustments necessary according to risk group, following an AIDS diagnosis and use of ZDV and PCP therapy limit its application in the clinical setting.