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. A sufficient response for patients receiving ART is defined as an increase of 50 to 150 cells/mm³ per year with rapid response during the first 3 mo of treatment followed by an annual increase of 50 to 100 cells/mm³ 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 branched DNA (bDNA) or reverse transcriptase polymerase chain reaction (RT-PCR). Results are not interchangeable from method to method. Therefore, it is important to use the same viral load method for serial testing. 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/mm³; 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/mm³. Failure to respond to therapy is defined as a viral load greater than 200 copies/mL. Increased viral load may be indicative of viral mutations, drug resistance, or noncompliance to the therapeutic regimen. Testing for drug resistance is recommended if viral load is greater than 1,000 copies/mL.