CD4 and CD8

If you have been diagnosed with HIV, a CD4 count is used to help evaluate and track the progression of HIV infection and disease. CD4 cells are the main target of HIV and the number of CD4 cells will decrease as HIV progresses. Since CD4 cells are usually destroyed more rapidly than other types of lymphocytes and because absolute counts can vary from day to day, it is sometimes useful to look at the number of CD4 cells compared to other types of lymphocytes. Sometimes a CD4 cell count is compared to the total lymphocyte count and the result is expressed as a percentage, or the CD4 cell count may be compared to the CD8 cell count, and the result is expressed as a ratio. However the absolute CD4 T-cell count is the most important value.

The CD4 count and CD4/CD8 ratio can tell your health care team how strong your immune system is and can help predict the risk of complications and debilitating infections. These tests are most useful when they are compared with results obtained from earlier CD4 counts. They are used in combination with the HIV viral load test, which measures the level of HIV in the blood, to determine the progression and outlook of HIV disease and to monitor the effectiveness of treatment.

A normal CD4 count does not exclude HIV and an abnormal CD4 count can be found for many other reasons, so a CD4 count should not be performed as a 'surrogate' test for HIV.

Sometimes these tests may be used to help diagnose or monitor other conditions such as lymphoma, primary immune deficiency, and in patients receiving immune suppression (e.g. for transplantation). (See FAQ #4).

A CD4 count, or sometimes a CD4/CD8 ratio is ordered with a viral load test when you are first diagnosed with HIV as part of a baseline measurement. The tests should be repeated about two to eight weeks after starting or changing anti-HIV therapy. If treatment is maintained, they should be performed about every three to four months thereafter.

In patients with severe, recurrent, atypical or invasive infection who are known to be HIV negative a CD4 and CD8 count may be requested as part of an assessment of immune function.

Patients receiving immune suppression or being treated for blood cell cancers e.g. lymphoma may have the CD4 and CD8 count measured as part of assessment of the immune system and treatment.

A CD4 count may be interpreted as an absolute level, as a ratio to CD8 or as a percent of total lymphocytes. In adults, the CD4 absolute value or count is used to guide treatment decisions. In children, the normal ranges change significantly from birth to teenage years and the percent CD4 is usually used as a guide to treatment need or response.

In general, the CD4 count and percentage goes down as HIV disease progresses. Any single CD4 count value may differ from the last one even though your health status has not changed. Your doctor will take several CD4 test results into account rather than a single value and will evaluate the pattern of CD4 counts over time.

If your CD4 count declines over several months, your health care team may recommend beginning or changing anti-retroviral treatment and/or starting prophylactic treatment for opportunistic infections such as Pneumocystis pneumonia (PCP) or Mycobacterum avium infection. Your CD4 count should increase or stabilize in response to effective combination anti-HIV therapy.

According to the BHIVA 2015 (interim 2016 update) guidelines antiretroviral therapy should be started in all adult patients diagnosed with HIV irrespective of CD4 count. For Children the PENTA guidelines (2015) recommend all children under 1 year of age and thereafter a CD4 percent and absolute based guidance if there is no clinical evidence of disease. The adult guidelines have changed significantly in the last 5 years and the paediatric guidance is likely to continue to be updated.

The CD4 count tends to be lower in the morning and higher in the evening. Acute illnesses, such as pneumonia, influenza, or herpes simplex virus infection can cause the CD4 count to decline temporarily. Cancer chemotherapy can dramatically lower the CD4 count.

A CD4 count does not always reflect how someone with HIV disease feels and functions. For example, some people with higher counts are ill and have frequent complications, and some people with lower counts have few medical complications and function well.

HIV is usually screened for and diagnosed with an HIV antibody test about 2-8 weeks after exposure to the virus. If exposure is more recent, a p24 antigen test or an HIV viral load test may be used to detect the virus in your blood.