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Immunology Laboratory

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Lymphocyte surface marker analysis: CD4 count; TBNK

Also known as: T cell count; CD4 count; lymphocyte subsets; TBNK; CD4/CD8 count; B cell count; CD19 count; CD20 count

This test measures percentages and absolute numbers of lymphocytes, CD3 T cells, CD4 T cells and CD8 T cells and in lymphocyte subsets also B cells and NK cells.

Clinical indication for T cell/CD4 count:

  • CD4 counts and CD8 counts are routinely used in the monitoring of HIV infection.
  • However, the use of CD4 count as a surrogate marker for the diagnosis of HIV infection is unhelpful and constitutes poor medical practice. A low CD4 count is not diagnostic of HIV infection, occurring in a wide variety of other conditions such as primary immunodeficiency, viral and bacterial infection, lupus and steroid therapy.

Clinical indications for TBNK: lymphocyte subsets (T cells (CD3/4 CD3/8), B cells (CD19) and NK cells (CD16/56):

  • Children with suspected or known severe combined immunodeficiency (SCID). SCID is a profound deficiency of T and B cells and though rare, is fatal if untreated. Patients usually present between 2-12 months. They often have prolonged typical infections or severe or atypical infections, including recurrent bacterial infections, persistent viral infections, Candida, PCP. Clinical features include failure to thrive, skin rash, diarrhoea and hepatosplenomegaly.
  • Monitoring of B cell numbers in patients treated with rituximab/ anti-CD20 monoclonal antibody.
  • Patients with DiGeorge syndrome/ 22q deletion.
  • Patients with suspected B cell immunodeficiency/antibody deficiency.

Specimen requirements

1 mL EDTA blood must be received in the Immunology Laboratory within 39 hours of venepuncture. Samples must not have been refrigerated or centrifuged, and must arrive before 5.00pm on Fridays.


Price on application

Laboratory turnaround time

3 working days

Laboratory method

AQUIOS flow cytometry

Reference range/units


Associated tests


Sample stability

39 hours post venepuncture (do not refrigerate or centrifuge)

Shipping and storage

Ambient temperature and first class post. Hays DX users please enquire via email below.

Samples must be received in the Immunology Laboratory within 39 hours of venepuncture. Samples must not have been refrigerated or centrifuged and must arrive before 5.00pm on Fridays.

Further information and contact details

For further information, email

Last reviewed:13 September 2022