Rare, often indicative of a myeloproliferative disease (e.g., chronic myelogenous leukemia)
Chronic infections (e.g., tuberculosis), bacterial endocarditis, rickettsiosis and malaria;
collagen vascular diseases (e.g., systemic lupus erythematosus)
inflammatory bowel diseases (e.g., ulcerative colitis)
Accompanies monocytosis in many disorders associated with chronic immunologic stimulation (e.g., tuberculosis, brucellosis);
viral infections (e.g.,hepatitis A, cytomegalovirus, Epstein-Barr virus);
Bordetella pertussis infection.