(Study type: Blood collected in a green-top [heparin] tube; related body system: Circulatory/hematopoietic and immune systems.)

Enumeration of lymphocytes, identification of cell lineage, and identification of cellular stage of development are used to diagnose and classify malignant myeloproliferative diseases and to plan treatment. T-cell enumeration is also useful in the evaluation and management of immunodeficiency and autoimmune disease. The CD4 count is a reflection of immune status. It is used to make decisions regarding initiation of antiretroviral therapy (ART) and is also an excellent predictor of imminent opportunistic infection. CD4 counts greater than 500 cells/microL are associated with asymptomatic HIV infection; CD4 counts less than 200 cells/microL are associated with a high risk of developing HIV infection. A sufficient response for patients receiving ART is defined as an increase of CD4 of 50 to 150 (cells/microL) per year with rapid response during the first 3 mo of treatment followed by an annual increase of 50 to 100 (cells/microL) until stabilization is achieved.

HIV viral load is another important test used to establish a baseline for viral activity when a person is first diagnosed with HIV and then afterward to monitor response to ART. Viral load testing, also called plasma HIV RNA, is performed on plasma from a whole blood sample. The viral load demonstrates how actively the virus is reproducing and helps determine whether treatment is necessary. Optimal viral load is considered to be less than 20 to 75 copies/mL or below the level of detection, but the actual level of detection varies somewhat by test method. Methods commonly used to perform viral load testing include nucleic acid amplification (NAAT) and polymerase chain reaction (PCR).

Public health guidelines recommend CD4 counts and viral load testing upon initiation of care for HIV; 3 to 4 mo before commencement of ART; every 3 to 4 mo, but no later than 6 mo, thereafter; and if treatment failure is suspected or otherwise when clinically indicated. Additionally, viral load testing should be requested 2 to 4 wk, but no later than 8 wk, after initiation of ART to verify success of therapy. In clinically stable patients, CD4 testing may be recommended every 6 to 12 mo rather than every 3 to 6 mo. Guidelines also state that treatment of asymptomatic patients should begin when CD4 count is less than 350 cells/microL; treatment is recommended when the patient is symptomatic regardless of test results or when the patient is asymptomatic and CD4 count is between 350 and 500 cells/microL. Failure to respond to therapy is defined as a viral load greater than 200 copies/mL. Increased viral load may be indicative of viral sequence variations, drug resistance, or nonadherence to the therapeutic regimen.

Testing for drug resistance is recommended if viral load is greater than 1,000 copies/mL. Genotyping for viral tropism should be used to determine eligibility before initiating treatment with a CCR5 coreceptor antagonist. Tropism testing should also be used to select alternate drugs once resistance to current drug therapy has been identified. An HIV tropism refers to the cell coreceptor used by the HIV-1 virus to penetrate the host cell and begin the infection. The discovery of two coreceptors used by the virus, either CCR5 or CXCR4 or both, to enter target CD4 cells led to development of CCR5 antagonists that block the virus from entering the cells. Genotyping for the presence of the HLA-B*5701 sequence variation should be performed before initiation of abacavir therapy as the sequence variation predisposes an individual to hypersensitivity reactions to abacavir or any abacavir-containing combination drug.