Viral - a modest and temporary (< 2 months) rise in lymphocytes

If the patient is unwell, request a blood film and consider testing for EBV, CMV, HIV.

Nb. The EBV screen (Monospot or Paul Bunnell) gives both false positive and false negative results and requesting serology for IgG and IgM antibody is preferable and specific.


Some bacterial (eg pertussis) and protozoal (eg toxoplasmosis) infections also cause a lymphocytosis.

Smoking is a common cause of low level and persistent lymphocytosis.

Less common causes of lymphocytosis include auto-immune disease, medication and stress (extreme exercise, cardiac or trauma)



Lymphocytes may rise most commonly with chronic lymphocytic leukaemia (CLL) and similar low grade lymphoproliferative disorders (LPD).

A variety of lymphomas may also ‘spill’ into the peripheral blood.

Acute lymphoblastic leukaemia (ALL) is rare and  the patient would likely be unwell with other blood abnormalities. The blood film would show blast cells. Refer ALL immediately via medical admissions for  inpatient care.



Asymptomatic CLL does not benefit from early treatment. Lymphocytosis without symptoms can reasonably be monitored in the community with a second FBC after 2 months and then annually.


When to refer CLL:

  • If the lymphocyte count is rising rapidly (> 10 x 109 / l and doubling within a 3 month period)
  • the patient is systemically unwell (involuntary weight loss, night sweats)
  • significant and persistent lymphadenopathy
  • splenomegaly
  • other components of the blood count are abnormal (anaemia, thrombocytopenia)


Investigation lymphocytosis:

Examine for lymphadenopathy and splenomegaly

Seek reactive causes


Refer to routine Haematology if lymphocytosis &:

Significant and persistent (> 2 weeks) lymphadenopathy in the absence of a secondary cause


Systemic symptoms

Anaemia or thrombocytopenia

Lymphocytes  > 10 x 109 / l and double within a 3 month period

(2ww may be appropriate if significantly unwell)

Useful patient resources:







Dr Richard Noble, Consultant Haematologist, RCHT

Dr Bridgitte Wesson, GP & Kernow RMS Haematology guideline lead


Review date: 13/07/2020

Next Review due 13/07/2021