CD4 count is a laboratory test that measures CD-4 T lymphocytes (T cells) via flow cytometry. This test is an important parameter in HIV management and is used to guide clinical treatment. The CD4 count is a reliable indicator of a patient’s immunologic status and is used to determine the necessity for initiation of prophylactic treatment against opportunistic infections.
CD4 T cells are human helper T cells that express cluster determinant 4 (CD4) molecules. The CD4 molecule is a member of the immunoglobulin family and primarily mediates adhesion to major histocompatibility complex molecules. CD4 T cells are selectively targeted and infected by HIV. HIV proliferates rapidly during acute infection leading to high levels of viremia and rapid impairment and death of CD4 T cells.
Flow cytometry usually determines the CD4 count. The specimen is collected through a standard blood draw and is processed within 18 hours of specimen collection.
Laboratory measurement of CD4 count through flow cytometry is reported as CD4 percentages. The absolute CD4 count is obtained by multiplying this percentage with the total measured white blood cell (WBC) count. The absolute CD4 count usually corresponds to the CD4 percentages. A CD4 percentage of greater than 29% corresponds to an absolute count of more than 500 cells/microliters, 14% to 28% corresponds to an absolute count between 200 and 500 cells/microliters, and less than 14% corresponds to an absolute count of fewer than 200 cells/microliters. It is, however, important to note that an increase in WBC will cause an increase in the absolute CD4 count while the CD4 percentage remains the same. CD4 count measurements may also have inter-assay or inter-laboratory variability.
CD4 count monitoring is primarily used to assess when to initiate prophylaxis against several opportunistic infections. Although it is also obtained in monitoring response to ART, CD4 count is, by itself, insufficient in evaluating response to therapy. Viral load monitoring remains the most reliable indicator of treatment response. The CD4 count must be obtained at baseline 3 months after initiation of ART. It is subsequently monitored every 3 to 6 months during therapy. According to treatment guidelines, less frequent monitoring (every 12 months) may be done after 2 years of ART in patients who have a stable CD4 count of greater than 300 cells/microliters and consistently undetectable viral load.
CD4 count, although an essential part of HIV management, is not primarily used for the diagnosis of HIV infection. However, an absolute CD4 count less than 200 cells/microliter is one of the criteria utilized by the Center for Disease Control and Prevention (CDC) in the definition of AIDS, a clinical stage that indicates high susceptibility to opportunistic infections.
The normal CD4 count range is between 500 and 1400 cells/microliters., taking into consideration laboratory variations.
There is significant intraindividual variability in CD4 count measurement. In a pre-ART (antiretroviral therapy) study, the intraindividual coefficient variation in 1020 untreated HIV patients was found to be at 25% on average. This variability was found even in healthy adult subjects. Several factors contribute to this variability. Any factors that lead to increase or decrease in WBC count may cause changes to the measured absolute CD4 count. These factors include infection, medications, or other chronic conditions. Leukocytosis may increase the absolute CD4 count while leukopenia may result in a decreased count. Patients with a history of splenectomy tend to have higher absolute CD4 cell counts, while patients with the advanced liver disease and splenomegaly tend to have lower counts which may be related to splenic sequestration. In these cases, the CD4 percentage usually has less variation. Therefore, any discordant changes between the absolute CD4 count and CD4 cell percentage should lead the clinician to investigate for possible factors that may be causing the changes, and caution should be used in CD4 count interpretation since it may not truly reflect the patient’s immunologic status.
Certain viral and bacterial infections have been associated with a mild decrease in CD4 cell count. These have been documented in viral infections such as Ebstein Barr Virus (EBV), cytomegalovirus (CMV), and HTLV-1 and bacterial infections such as tuberculosis. Patients with Hepatitis C virus (HCV) cirrhosis tend to have lower CD4 cell counts and HIV patients with HCV co-infection seem to have reduced immune recovery upon initiation of antiretroviral therapy. Medications such as corticosteroids may also affect WBC counts which in turn lead to changes in CD4 cell counts. Alcohol abuse has also been associated with a drop in the absolute CD4 count. A rare syndrome called idiopathic CD4 lymphocytopenia has been described which is defined as a low CD4 count in the absence of HIV infection or any other medical conditions that may explain the disease.
CD4 count depletion is a consequence of HIV infection and leads to devastating opportunistic infections when left untreated. It affects both CD4 helper T cells in the lymphoid tissue as well as T cells circulating in the peripheral blood. In the natural history of HIV infection, there is an abrupt decline in the CD4 count during acute HIV infection that is usually followed by a rebound as a result of CD8 lymphocyte response to viral replication. Without antiretroviral treatment, the CD4 count will then decline over the next several years.
CD4 cell counts are used to monitor immunologic response to ART. With effective viral suppression, CD4 count is expected to increase by at least 50 cells/microliter after 4 to 8 weeks of treatment and by approximately 100 to 150 cells/microliters increase from baseline at one year. This is then followed by an expected increase of 50 to 100 cells/microliter per year. Several factors such as older age, lower CD4 baseline, and severe immunocompromised status have been associated with a less than expected improvement in the CD4 count while on treatment.
CD4 count is not a reliable indicator of virologic suppression and medication adherence. In patients who develop virologic resistance while on ART, it may take months for the CD4 count to decline and it may even initially increase. Several factors may falsely increase or decrease CD4 cell counts that may not necessarily reflect a patient’s true immunologic status. Inter-laboratory variability must also be taken into consideration. Thus, any significant but unexpected difference between two CD4 count measurements, which is defined as a 30% change in absolute CD4 count or 3% change in CD4 percentage, must be confirmed with a repeat testing.
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