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Differential Diagnosis: Lymphocytosis

Julie Allen, BVMS, MS, MRCVS, DACVIM (SAIM), DACVP, Cornell University

Internal Medicine

|September 2020|Peer Reviewed

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Differential Diagnosis: Lymphocytosis

Following are differential diagnoses for patients presented with lymphocytosis.

  • Age-related cause (eg, dogs and cats <6 months of age often have mild lymphocytosis due to vaccination or exposure to novel antigens)
  • Antigenic stimulation
    • Immune-mediated disease (rare; eg, immune-mediated hemolytic anemia in cats) 
    • Infection (most commonly, Ehrlichia canis; rarely, protozoal [eg, Leishmania infantum], Spirocerca lupi, FIV)
  • Endocrine disease
    • Hyperthyroidism (cats; usually mild; can be seen prior to diagnosis [possibly epinephrine-related] or secondary to methimazole treatment)
    • Hypoadrenocorticism (primarily dogs; lack of a stress leukogram in a sick patient can indicate disease)
  • Lymphoid neoplasia
    • Acute lymphoblastic leukemia
    • Chronic lymphocytic leukemia (± small cell lymphoma)
  • Nonlymphoid neoplasia (eg, thymoma)
  • Physiologic (eg, epinephrine-induced) response (primarily cats)


For global readers, a calculator to convert laboratory values, dosages, and other measurements to SI units can be found here.

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