Formes frustes in myeloproliferative disorders. Identification by the growth of an endogenous erythroid clone in vitro in patients with arterial vascular disease.

The myeloproliferative disorders, including polycythaemia rubra vera, arise as a result of a single-cell mutation. A characteristic of the abnormal haemopoietic clone is that it can form erythroid colonies in vitro in the absence of added erythropoietin. Such endogenous erythroid clones were consistently found in two of seven patients with peripheral vascular disease. These two patients had mean platelet counts of 600 X 10(9)/l and 630 X 10(9)/l. Culture of blood and bone-marrow cells from patients with raised platelet counts secondary to a variety of other disorders failed to yield such colonies. The presence of endogenous erythroid clones provides early evidence of a myeloproliferative disorder.