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Monocytes are agranulocytes, the largest cells of normal blood, and the body's second line of defense against infection. Histiocytes, which are large macrophagic phagocytes, are classified as monocytes in a differential leukocyte count. Histiocytes and monocytes are capable of reversible transformation from one to the other.


These phagocytic cells of varying size and mobility remove injured and dead cells, microorganisms, and insoluble particles from the circulating blood. Monocytes escaping from the upper and lower respiratory tracts and the gastrointestinal and genitourinary organs perform a scavenger function, clearing the body of debris. These phagocytic cells produce the antiviral agent called interferon. This test counts monocytes, which circulate in certain specific disorders such as tuberculosis, infective endocarditis, and the recovery phase of acute infections.


Normal reference values in adults


* Absolute monocyte count: 100 to 500/mm3 or 0.1 to 0.5 x 109/L


* Differential: 3% to 7% of total white blood cell (WBC) count, or 0.03 to 0.07 of total WBC count.


Possible causes of monocytosis

(increased monocyte count) >500 cells/mm3 or >0.5 x 109/L or >10%


The most common causes are:


* bacterial infections


* tuberculosis


* infective endocarditis


* syphilis.



Other causes include:


* monocytic leukemia and myeloproliferative disorders


* gastric, ovarian, or breast cancer


* Hodgkin disease and other lymphomas


* recovery state of neutropenia (favorable sign)


* lipid storage diseases (Gaucher disease)


* some parasitic, mycotic, and rickettsial diseases


* surgical trauma


* ulcerative colitis, enteritis, and celiac disease


* collagen diseases and sarcoidosis


* tetrachloroethane poisoning.



Phagocytic monocytes (macrophages) may be found in small numbers in the blood in many disorders, including:


* sepsis


* systemic lupus erythematosus


* hemolytic anemias.



Possible causes of monocytopenia

(decreased monocyte count) <100 cells/mm3 or <0.1 x 109/L


* prednisone therapy


* hairy cell leukemia


* overwhelming infection that also causes neutropenia


* HIV infection


* aplastic anemia.



Interfering factors

False-positive values may be caused by:


* alprazolam


* ampicillin


* carbenicillin


* chlorpromazine


* granulocyte-colony stimulating factor (G-CSF)


* griseofulvin


* haloperidol


* lomefloxacin


* methsuximide


* paroxetine


* penicillamine


* piperacillin


* prednisone


* propylthiouracil


* quazepam.



False-negative values may be caused by:


* alprazolam




* glucocorticoids (transient)


* triazolam.



Source: Fischbach FT, Fischbach MA. A Manual of Laboratory and Diagnostic Tests. 10th ed. Philadelphia, PA: Wolters Kluwer; 2018.