[4] Carcinocythemia typically occurs secondary to infiltration of the bone marrow by metastatic cancer[6] and carries a very poor prognosis.

[3][9] Features that aid in distinguishing tumour cells from other cells include their very large size, mature nuclear chromatin pattern, vacuolated cytoplasm, and their tendency to appear in clumps or clusters, although some of these characteristics are shared by megakaryoblasts and monoblasts.

Tumour cells are often found at the edge of the blood smear due to their large size, so this area should be examined thoroughly if carcinocythemia is suspected.

[4] When immunophenotyped by flow cytometry, the cells are generally CD45 negative and may express CD56, a profile that is non-specific but unusual for hematologic malignancies.

[3][4] Carcinocythemia must be distinguished from the following conditions:[3][4][8] The presence of tumour cells in the peripheral blood of a cancer patient was first described in an 1869 case report in the Medical Journal of Australia.