Keywords: Mononucleosis syndromes

Key Information

Appropriate Tests


Review clinical findings, Full blood count, White cell count differential, Blood film.

Table 4.

Assessment of peripheral blood lymphocyte morphology assists in determining the aetiology of the lymphocytosis.

If lymphocytosis is persistent (>3 months) and the aetiology is uncertain, peripheral blood immunophenotyping by flow cytometry may be useful.

Mononucleosis syndromes

  • Infectious mononucleosis
  • Cytomegalovirus infection
  • HIV infection
  • Toxoplasmosis

Syndromes characterised by lymphoid cells with atypical morphology (ie activated lymphocytes).

Other infections, especially

  • Pertussis
  • Viral infection

Characteristically lymphocytes are morphologically normal or show "reactive features".

Hyposplenism / Splenic atrophy or absence.

Associated features of hyposplenism in the peripheral blood film (Howell-Jolly bodies, target cells, acanthocytes, spherocytes, thrombocytosis).

See Splenic atrophy/absence

Reactive, especially

  • Viral infection


Transient lymphocytosis associated with physiological stress


Lymphoproliferative disorders, especially

  • Chronic lymphocytic leukaemia
  • Other B-, T-, and NK-cell lymphoproliferative disorders

Blood film morphology may be suggestive. Immunophenotyping of peripheral blood lymphoid  cells has a role in sub-typing the lymphoid lineage, confirmation of clonality in B cell disorders and may identify a characteristic immunophenotype in some lymphoproliferative disorders.


Bain BJ. Blood cells: a practical guide. Fourth edition. Oxford: Blackwell Publishing, 2007

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