Measurement of CD4+ Percentage

To the Editor.  —In his Editorial,¹ Dr Volberding notes the stronger correlation between clinical stage and CD4⁺ percentage than with the absolute CD4⁺ cell count. He recommends that practitioners switch to use of this more stable marker of progression. To the array of arguments² previously advanced for this change should be added simplicity and thrift.Many clinicians may not be aware of the redundant procedural burden introduced with the process of derivation of absolute CD4⁺ cell count from CD4⁺ percentage. Flow cytometry initially determines CD4⁺ percentage on a blood specimen. A separate white blood cell count and a differential are required to calculate the total CD4⁺ cell count. High coefficients of variation may be introduced by this additional step,³ which also increases laboratory costs. As CD4⁺ evaluation is now suggested every 3 months for patients infected with the human immunodeficiency virus, the US health care system could