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Reliability of CD4 Quantitation in Human Immunodeficiency Virus-Positive Children: Implications for Definition of Immunologic Response to Highly Active Antiretroviral Therapy

Center for Biostatistics in AIDS Research, Harvard University School of Public Health, Channing Laboratory, Brigham and Women's Hospital, 181 Longwood Ave., Boston, Massachusetts 02115,¹ University of Miami School of Medicine, Room 712, Batchelor Children's Research Institute, 1580 NW 10th Avenue, Miami, Florida 33136,² Departments of Pediatrics and Medicine, University of Colorado Health Sciences Center, 4200 E 9th Ave., Denver, Colorado 80262³

Received 24 November 2004/ Returned for modification 14 January 2005/ Accepted 23 February 2005

Our objective was to develop data-based algorithms for definition of immunologic response to AIDS therapies in pediatric patients, taking account of T-cell subset measurement errors. The study design involved cross-protocol analysis of 2,148 enrollees in six completed Pediatric AIDS Clinical Trials Group trials. We used standard quantitation of T-cell subsets; linear modeling with mean-dependent measurement error variance was used to develop 95% tolerance limits for change in CD4%. For individuals with a CD4% of approximately 25%, the measurement error-based 95% tolerance interval ranges from 15% to 35%, whereas for individuals with a CD4% of approximately 5%, the tolerance interval ranges from 3% to 7%. When pairs of CD4% measures taken within a time interval of less than 30 days are averaged to estimate steady-state CD4%, tolerance interval width decreases by approximately 30%. A simple graphical tool that provides a data-based criterion for immunologic response over and above variation ascribable to T-cell measurement error is provided. Variability in CD4% due to measurement error is substantial, increases with level of CD4%, and complicates assessment of immunologic response to therapy. Replicates of CD4% measures could be used to improve precision of interpretation of CD4% measures.