Reference intervals are critically important for the correct interpretation of diagnostic tests. The full blood count and CD4 count are two frequently performed tests in HIV-endemic settings such as Gauteng province, South Africa. Haemoglobin concentration is an independent prognostic indicator in HIV, while CD4 count determines initiation and response to antiretroviral (ARV) treatment. (1) The South African National Health Laboratory Services (NHLS) establishes reference ranges with the implementation of new tests, and periodically re-assesses reference intervals to accommodate changes in population demographics as well as technological advancements (ISO 15189:2003 guidelines Point 5.5.5). (2) As differences in altitude, endemic disease and ethnic differences influence reference intervals, other reference intervals are not necessarily pertinent to the Gauteng region.