High lymphocytes on a blood test: what it means

Lymphocytes are white blood cells that make up a key part of the immune system. They are counted in the full blood count (FBC), which the NHS describes as measuring the number of blood cells in your blood. Lymphocytosis means that the count is above the reference range used by the laboratory. The range is printed on your result, and it can differ between laboratories and with age.

Many lymphocyte rises are short-lived. The immune system expands its lymphocyte numbers when fighting an infection, and the count usually comes back down once the infection has cleared. For that reason, the first question a doctor asks is usually about recent illness.

The York pathway lists these causes of lymphocytosis: viral infection, bacterial infection (especially tuberculosis and whooping cough), hyposplenism (an absent or poorly working spleen), smoking, autoimmune disorders, a stress response such as after a heart attack, and a lymphoproliferative disorder.

Glandular fever is a typical viral cause in teenagers and young adults. The NHS says it is most common in people aged 15 to 24, with symptoms including a high temperature, swollen glands, sore throat, rash, headache and tiredness. A GP may order a blood test to confirm it and rule out other illnesses, and the blood test looks for the Epstein-Barr virus. The NHS adds that most people get better without problems and that antibiotics will not work because it is a virus.

In older people, particularly when there has been no recent infection, a persistently raised count may relate to a lymphoproliferative disorder such as chronic lymphocytic leukaemia. The NHS describes this as a rare type of cancer that affects the blood and bone marrow. It is important to place this in context: the York pathway notes that many such patients never develop symptoms and that there is no advantage to early treatment in those who are well.

The York pathway says to review older blood tests, ask about recent infections, weight loss, sweating and smoking, consider autoimmune conditions, and examine for enlarged lymph nodes, liver and spleen. Investigations include a blood film and, if Epstein-Barr virus is suspected, a Monospot test and, if negative, EBV or CMV blood tests.

If the lymphocyte count is below 10 with an otherwise normal full blood count and no suspicion of a lymphoproliferative disorder, the pathway suggests repeating the FBC in four to six weeks to look for resolution, and not routinely sending additional specialist tests. If the count is above 10, or there is clinical suspicion, the sample may be sent for immunophenotyping, and a referral to haematology outpatients may follow, urgently if there is clinical concern or other abnormalities.

These are one hospital trust's pathways. Practice varies between areas, so your own GP may follow a slightly different approach.

Mention drenching night sweats, unexplained weight loss, itching all over without a rash, lumps in the neck, armpit or groin, or feeling persistently exhausted. These are the features the pathway lists as raising suspicion of a haematological disorder.

If you have glandular fever symptoms, the NHS advises calling 111 for difficulty breathing or painful swallowing, and calling 999 if you cannot swallow even your saliva, have severe difficulty breathing or have severe stomach pain, as a ruptured spleen is a recognised, if uncommon, complication.

Waiting for a repeat test can be unsettling. A raised lymphocyte count on its own, with no symptoms and a normal rest of the blood count, is usually followed up quietly with a repeat test. If it persists, the next steps are laid out by your GP or haematology team. The NHS advises asking a healthcare professional to explain any result you do not understand.

Via the NHS, the FBC and any follow-up tests are arranged by your GP. Private health check providers may report the same test; if a private result is flagged, a repeat through your GP gives a consistent comparison.