Immunological Testing


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Immunological Testing

CD4 Counts

The CD4 molecule is expressed on the surface of helper T-lymphocytes. CD4 interacts with HLA class II molecules on the surface of antigen-presenting cells to help stabilize the interaction between the antigen-specific T-cell receptor on the T-helper cell and the antigen-HLA class II complex on the antigen presenting cell. T-helper cell function can be significantly impaired by blocking the CD4-HLA class II interaction. CD4 also serves as the primary receptor for HIV-1 and HIV-2. It is the specific affinity of the HIV envelope glycoprotein (gp120) for CD4 that targets HIV to helper T cells and macrophages.

Even prior to the identification of HIV as the cause of AIDS, the progressive loss of T helper cells was noted to be a characteristic finding in patients with this disease. On average, there is a loss of 30-60 CD4+ cells per year, although in many patients, CD4+ T-lymphocyte counts may remain stable for several years followed by a period of rapid decline (66; 67). Natural history studies and clinical trials have demonstrated that the CD4+ lymphocyte count is an independent risk factor for progression to AIDS and death. The CD4 count provides an estimate of the immunologic status of the patient, and therefore, is an excellent marker of the immediate risk of opportunistic infection. Such complications are rare in patients with CD4 counts above 500 cells/mm3. As the CD4 count drops below 500 cells/mm3 patients may begin to experience recurrent minor infections such as herpes simplex virus or oral candidiasis. The risk of more serious opportunistic infections increases significantly as the CD4 count falls below 200 cells/mm3.

The CD4 count increases promptly in response to antiretroviral therapy. The rise in CD4+ cell count is related to the extent to which virus replication is suppressed. Even patients who do not achieve complete virologic suppression may show significant increases in CD4 counts. Cohort studies have shown that patients who achieve a significant CD4+ cell increase in response to potent antiretroviral therapy have a substantially lower risk of disease progression, whereas those patients who achieve viral suppression but do not have an increase in their CD4 counts remain at increased risk of developing an AIDS-related opportunistic infection (68). Thus, monitoring the CD4+ cell count is an essential component of patient evaluation.

The CD4 count is determined by "staining" patient blood cells with antibodies to various cell surface markers, including CD4. The antibodies are conjugated to fluorescent tags that emit light of a certain frequency when excited by a laser beam. In flow cytometry the stained cells are fed in a stream past the laser, and the proportion of cells that emit light at the right wavelength is determined. The number of lymphocytes circulating in the blood (determined by a blood count) is multiplied by the percent of cells staining positive for CD4 in order to calculate the number of helper T cells.

CD4 counts are subject to considerable inter-assay variation. Most of this variation is due to fluctuations in the total lymphocyte count, rather than any inherent inaccuracy of the flow cytometry portion of the test. CD4 counts are subject to diurnal variation (that is, counts are higher in the morning than in the evening), and may change as a result of an acute illness. The percentage of lymphocytes that are CD4+ (the "% CD4") is less variable than the absolute CD4 count. The % CD4 cells is comparable to the absolute count in predicting the risk of disease progression or in assessing the response to treatment (66).

Guidelines suggest that CD4 + T cell counts be measured at the time of diagnosis and generally every 3-6 months thereafter (65). CD4 counts can show considerable day-to-day variation. For this reason, any large unexpected change in the CD4 count should be confirmed by repeat testing a few days apart. The CD4 count trajectory over several follow-up visits may be particularly helpful.

4/15/01