d. Differentiate between the difference in the diagnoses of HIV Infection and AIDS and describe the clinical implications. The effect of HIV on the immune system is monitored by measuring the CD4 (helper) lymphocyte count in the blood. A normal CD4 count (between approximately 600 and 1,200 cells/µL) indicates that the immune system has not undergone sufficient damage to put the individual at risk for opportunistic illness. The U.S. Centers for Disease Control and Prevention (CDC) definition of AIDS, initially published in 1986 and revised in 1993, is based on certain clinical conditions, infections, and malignancies associated with HIV infection. Additionally, AIDS may be defined by a CD4 count of <200 cells/µL or <14% of all lymphocytes, even in the absence of the listed conditions. Primary HIV infection is defined as the time period from initial infection with HIV to the development of an antibody response detectable by standard tests. During primary HIV infection, HIV-specific CD8 cells undergo a marked clonal expansion and express high levels of activation markers such as CD38 and human leukocyte antigen. CD4 counts and CD4 function may decline during primary HIV infection, occasionally to levels that allow OIs to develop. Absolute CD4 count often rebounds after the primary infection, but may not return to a normal baseline. After the period of acute HIV infection--during which CD4 counts and viral load change dramatically--a relative equilibrium between viral replication and the host immune response is reached, and individuals may have little or no clinical manifestations of HIV infection. This time between initial infection and the development of AIDS may be long, averaging 10 years, even in the absence of treatment. Despite the relative clinical latency of this stage of HIV infection, viral replication and CD4 cell turnover remain active, with millions of CD4 cells and billions of virions produced and destroyed each day. On average, CD4 counts will drop by 50-90 cells/µL per year in asymptomatic individuals, usually with an acceleration of this rate over time. The rate of progression of infection may vary considerably. In adults, progression from infection to clinical AIDS is rare in the first 2 years of infection; however, reports describe rapid disease progression in infants infected by blood transfusion. According to CDC criteria AIDS is defined by either diagnosis of one of the AIDS-defining conditions, or by measurement of CD4 levels <200 cells/µL. Progression to AIDS from time of infection occurs, on average, 2 years earlier when defined by laboratory criteria (CD4 levels <200 cells/µL) compared to clinical criteria (development of an opportunistic illness). Survival time from the development of AIDS varies according to the AIDS-defining event. In the Multicenter Hemophilia Cohort Study, median survival after a single AIDS-defining condition ranged from 3 to 51 months for the 10 most common conditions. The mean survival time after diagnosis of AIDS in the United States prior to the availability of antiretroviral treatment was 10- 12 months.
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