CD4 and AIDS: In our study, we used pre-treatment CD4 depletion rates to back-calculate the year of seroconversion, while the previous local study was parametrized by the lengths of incubation period, settings of diagnosis (routine testing and symptom-related testing) and the annual number of HIV diagnoses and AIDS diagnoses.[6] Elsewhere, back-calculation of the seroconversion year with CD4 depletion rate has been applied,[12, 13, 15] though most had used the depletion rate of seroconverters as reference group.