Chapter 26 HIV Infection: Diagnosis and Disease Progression Evaluation

This chapter discusses the applications of flow cytometry in human immunodeficiency virus (HIV) disease. The primary application of flow cytometry in HIV disease is to determine the CD4⁺ T cell number (CD4 count). It is this measurement that has the greatest utility in clinical practice. Most analysis routines set a gate around the lymphocytes, and values are usually provided by the flow cytometer as a percentage of the lymphocytes with the immunophenotype of interest. The percentage can be multiplied by the peripheral blood absolute lymphocyte count to determine the absolute number of each lymphocyte subset per volume of blood (mm3 or liter). These values, especially the CD4⁺ cell number (CD4 count), are used to stage disease, predict outcome, select subjects for trials of therapeutic vaccines, and decide on the timing of treatment with anti-retroviral drugs or drugs for prophylaxis against opportunistic infections. Flow cytometry immunophenotyping is also invaluable for investigations of the underlying basis of lymphocyte dysfunction and activation in HIV disease. Several alternative technologies have been developed that allow absolute CD4 counts to be measured directly on whole blood at a reduced cost for instrumentation.