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International Journal of Epidemiology, Vol 26, 1367-1372, Copyright © 1997 by International Epidemiological Association

ARTICLES

Relationship between CD4 count and CD4% in HIV-infected people

LM Yu, PJ Easterbrook and T Marshall
HIV Epidemiology Unit, Chelsea and Westminster Hospital, London, UK.

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.
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