Lymphocyte subsets (CD3/CD4/CD8)

This test measures the numbers and proportions of certain white blood cells called T lymphocytes, which are a key part of your immune system. Using markers on the surface of these cells (CD3, CD4, and CD8), it identifies the main T cell types and how they are balanced, giving insight into the health and function of your immune system.

What this test measures

• CD3: identifies all mature T lymphocytes, giving the total T cell count
• CD4: identifies helper T cells, which coordinate and direct the immune response
• CD8: identifies cytotoxic T cells, which help destroy infected or abnormal cells
• CD4/CD8 ratio: the balance between these two cell types, which can be a useful indicator of immune status

What it can help assess

• Immune system health: giving an overview of whether the main immune cell populations are present in normal numbers
• Monitoring immune conditions: it is widely used to monitor the immune status of people with HIV, where the CD4 count is a key measure
• Investigating immune deficiency: helping to assess suspected problems with the immune system
• Monitoring certain treatments: where a therapy affects the immune system

An important note on interpretation

Lymphocyte subset results are interpreted alongside your clinical history, symptoms, and often other tests, as the numbers can be affected by infections, medications, stress, and other temporary factors. An abnormal result does not diagnose a specific condition on its own but indicates that further assessment may be needed. Any abnormal result should be discussed with your GP or a specialist, who can interpret it in context and advise on next steps.

Why it matters

The balance and numbers of your T lymphocytes provide a valuable window into how your immune system is functioning. This test is particularly useful for assessing and monitoring immune health, most notably in the context of HIV and suspected immune deficiency, supporting appropriate care and monitoring.