Antiretroviral monitoring relay on %CD4+ T-cell values particularly in pediatrics. In Ethiopia despite extensive expansion of care and treatment and progress in ART monitoring of HIV infected pediatrics HIV/AIDS management remains severely limited. The limitation of the existing double tube SP FACSCount system, for not providing %CD4+ T-cell count, was one of the challenges in pediatric HIV monitoring in resource limited settings. As a result, clinicians in Ethiopia have been using the back-calculation method to obtain %CD4+ T-cell values although the clinical reference of the method has not been documented. A single tube FACSCount system that gives %CD4+ T-cell value has been introduced which requires evaluation. For the utilization of back-calculated %CD4+ T-cell and for the utilization of the new single tube SP FACSCount system in Ethiopia there is a need to compare it against the gold standard flow cytometry which is currently in use in Ethiopia. This book will provide information regarding the use of back-calculation and the new single tube FACSCount system for the determination of %CD4+ T-cell in HIV infected pediatric patients during monitoring.