Type of test Blood
Normal findings
Test explanation and related physiology
This test is used to detect the progressive depletion of CD4 T-lymphocytes, which is associated with an increased likelihood of clinical complications from acquired immunodeficiency syndrome (AIDS). Test results can indicate if a patient with AIDS is at risk for developing opportunistic infections. It is also used to confirm the diagnosis of acute myelocytic leukemia (AML) and to differentiate AML from acute lymphocytic leukemia (ALL).
CD4 (T-helper cells) and CD8 (T-suppressor cells) are examples of T-lymphocytes. T-lymphocytes, and especially CD4 counts, when combined with HIV viral load testing (p. 506) are, used to determine the time to initiate antiviral therapy. They also can be used to monitor antiviral therapy. Successful antiviral therapy is associated with an increase in CD4 counts. Worsening of disease or unsuccessful therapy is associated with decreasing T-lymphocyte counts.
There are three related measurements of CD4 T-lymphocytes. The first measurement is the total CD4 count. This is measured in whole blood and is the product of the white blood cell (WBC) count, the lymphocyte differential count, and the percentage of lymphocytes that are CD4 T-cells. The second measurement, the CD4 percentage, is a more accurate prognostic marker. The third prognostic marker, which is also more reliable than the total CD4 count, is the CD4-to-CD8 ratio.
Of the three T-cell measurements, the total CD4 count is the most variable. There is substantial diurnal variation in this count. Because it is a calculated measurement, the combination of possible laboratory error and personal fluctuation can result in wide variations in test results. With the CD4 percentage and the CD4 to-CD8 ratio, very little diurnal variation and laboratory error exist. The Multicenter AIDS Cohort Study suggests that the latter two measurements are more accurate than the total CD4 count. However, because the total CD4 count was originally thought to be the best marker, this test was used in many of the studies that now form the basis for practice recommendations.
CD4 measurement is a prognostic marker that can indicate whether a patient infected with HIV is at risk for developing opportunistic infections. The measurement of CD4 levels is used to decide whether to initiate Pneumocystis jirovecii pneumonia prophylaxis and antiviral therapy and for determining the prognosis of patients with HIV.
Both immunodeficiency and the dosage of immunosuppressive medications used after organ transplant are also monitored with the use of this cell surface immunophenotyping. Lymphomas and other lymphoproliferative diseases are now classified and treated according to the predominant lymphocyte type identified.
Contraindications
• Patients who are not emotionally prepared for the prognosis that the results may indicate Interfering factors
• Diurnal variation occurs.
• A recent viral illness can decrease total T-lymphocyte counts.
• Nicotine and very strenuous exercise have been shown to decrease lymphocyte counts.
* Steroids can increase lymphocyte counts.
* Immunosuppressive drugs will decrease lymphocyte counts.
Procedure and patient care
• See inside front cover for Routine Blood Testing.
• Fasting: no
• Blood tube commonly used: green or purple
• Never recap needles. Dispose of needles and syringes in a puncture-proof container.
• Keep the specimen at room temperature. Do not refrigerate.
• The specimen must be evaluated within 24 hours.
* Instruct the patient to observe the venipuncture site for infection. Patients with leukemia or AIDS are immunocompromised and susceptible to infection.
* Encourage the patient to discuss his or her concerns regarding the prognostic information obtained by these results.
• Do not give test results over the phone. Decreasing CD4 counts can have devastating consequences.
Abnormal findings
Increased counts
- Leukemias
- Lymphoma
Decreased counts
- Organ transplant patients
- Immunodeficiency diseases
