Introduction

For decades, the CD4 cell count measurement has been used to understand the progression of the human immunodeficiency virus (HIV) disease. HIV is a fatal infection, characterized by the targeting and destruction of CD4 T lymphocytes in the peripheral blood. CD4 T lymphocytes are a part of the human T-lymphocyte cells that are produced in the bone marrow and eventually mature in the thymus. They circulate the body to fight against bacteria, viruses, and other organisms. If HIV goes untreated, the virus enters the cell and replicates, which eventually causes CD4 cells to die. The remaining infected cells release virions, which infect other cells, leading to the progression of the disease. The loss of CD4 T lymphocytes will result in the inability to have a proper immune response.[1][2]

CD4 cell count is a laboratory test that measures the number of CD4 T-cells. The normal range is between 500 to 1500 cells/mm^3. Clinicians use this test to monitor the destruction of CD4 cells, and it also monitors the effectiveness of the antiretroviral treatment (ART). For a physician, the CD4 cell count has become the best indicator of disease progression and is used to stage disease and guide medical therapy. Per the Center for Disease Control and Prevention (CDC), one of the indications for the diagnosis of AIDS is when CD4 cell count drops below 200 cells/mm^3. The decline of CD4 T cells can lead to opportunistic infections, and it increases mortality.

Specimen Requirements and Procedure

The blood specimen required to obtain the CD4 count is collected by standard blood drawn. The blood specimens must undergo processing within 18 hours. Flow cytometry usually gives the results.

Testing Procedures

Several techniques may be useful to calculate the absolute number of CD4 T lymphocytes. The gold standard is immunofluorescence analysis by flow cytometry. Flow cytometry uses fluorochrome labels probes that bind to specific components of the cells. For CD4 cell count, the CD4 T lymphocytes are stained with fluorescent labels that are detectable in the flow cytometer. Monoclonal antibodies bind to the CD4 receptor on the surface of T cells. The relative percentage of cells expressing the receptor on its surface is obtained from the flow cytometer. In other words, the reporting of the results is as CD4 percentages, and the absolute number is obtained by multiplying the percentage, and the total white cell count. For example, a CD4 percentage greater than 29% is equivalent to an absolute CD4 cell count greater than 500 cells/mm^3, and a percentage lower than 14% is equivalent to an absolute CD4 count lower than 200 cells/mm^3.[3][4]

There are alternative systems manufactured by various companies that are options. It is essential to know which technique is in use because absolute count numbers may differ. When using different methods, the analysis of the results requires caution.

Interfering Factors

Physicians and patients should be aware that certain factors may cause a change in CD4 cell count. Any changes in the total white blood cells (WBCs) can lead to changes in the absolute value. Many factors, including circadian variation, influence the absolute count of lymphocytes. Studies have shown that CD4 count is usually lower in the morning and increases throughout the day. Acute infection, for example, influenza, pneumonia, hepatitis B, cytomegalovirus, and chemotherapy, may lead to a decrease in CD4 cell count. Stress and fatigue can also interfere with results. Of interest, corticosteroids may either increase or decrease the levels of CD4 cells. Single doses of steroids may lead to a decline in the absolute value. On the other hand, chronic steroid use may lead to an elevated count. Other factors that should merit consideration are alcohol, nicotine, and pregnancy, as they also may alter the absolute CD4 count.

The factors interfering with CD4 T lymphocytes may cause the percentage and absolute CD4 cells count to vary. The CD4 cell percentage has less variation than the absolute number, and for this reason, it is sometimes preferable. If the results are significantly different, and the patient may require medical intervention, results require confirmation within 3 to 6 months.

Results, Reporting, and Critical Findings

The CD4 count normal range is 500 to 1500 cell/mm^3. If a patient is left untreated, levels can drop below 200 cells/mm^3, which is one indication for the diagnosis of AIDS. The broad range in the normal value is the product of three variables: the white blood cell count, the percentage of lymphocytes, and the percentage of lymphocytes that bears the CD4 receptor.

Clinical Significance

As mentioned, the CD4 count is used to evaluate the progression of HIV. CD4 counts should be done to all patients first diagnosed with HIV disease. The Public Health Service recommends that all HIV-positive patients be tested every 3 to 6 months. Results can offer insight into the possible diagnosis of AIDS and the risk of opportunistic infections. Additionally, the test is an indicator of treatment failure. Antiretroviral therapy (ART) should commence before CD4 levels are below 200 cells/mm^3, as complications are higher in this population of patients. Levels should be followed up every 3 to 6 months after starting ART to check for response to therapy. If the response is appropriate, the CD4 count can be rechecked every 6 to 12 months. Successful treatment is associated with an increase in CD4 count and adherence to therapy. With the use of ART, levels may increase 100 to 150 cells/mm3 at the one-year mark.[5][6]

Quality Control and Lab Safety

The measurement of CD4+ counts requires a blood sample and laboratory analysis. Proper precaution on the collection and handling of the blood sample is essential. First, the tube should be appropriately labeled with the date and time of collection and with a patient identifier. Age and gender must be recorded, as well as the collector's name and initials. Then, a blood sample is collected by a venipuncture as per the standard procedure. The Transportation of Dangerous Goods Act and the International Air Transport Association (IATA) dangerous goods regulations regulate the shipping of containing infected agents, including HIV infected blood. A patient does not require to be fasting for the test. It is vital before doing the test to discuss with the patient possible concerns regarding the results that may be provided by the test. The patient should be emotionally prepared to receive the information given by the results of the test.

Enhancing Healthcare Team Outcomes

The CD4 count is a very useful laboratory parameter when evaluating immunosuppressed patients, especially those with HIV. Nurse practitioners, primary care providers, and other physicians should have some idea about the normal levels of CD4. CD4 counts should be done in all patients first diagnosed with HIV disease. The Public Health Service recommends that all HIV-positive patients be tested every 3 to 6 months. Results can offer insight into the possible diagnosis of AIDS and the risk of opportunistic infections. Also, the test can be an indicator of treatment failure.