PRIMARY BENIGN ERYTHROCYTOSIS WITH HIGH ERYTHROPOIETIN LEVELS AND AN EARLY ERYTHROPOIETIN-SENSITIVE POPULATION IN THE PERIPHERAL-BLOOD

Primary erythrocytosis diagnosed in a 10-month-old female and followed for 12 years is described. The erythrocytosis was associated with an abnormally elevated set point of erythropoietin production in which the sensitivity fluctuated independently, but corresponded to the alterations in the oxygen-carrying capacity of the blood, when the hematocrit was lowered by phlebotomies. Extensive work for secondary erythrocytoses failed to demonstrate a recognizable cause for this abnormal erythropoietin production. Erythroid cell cultures from peripheral blood mononuclear cells showed the existence of at least two populations: one consistent with dramatic expansion of the erythron in keeping with enhanced sensitivity to endogenous erythropoietin, and the other consistent with the features of typical colonies derived from burst-forming units-erythroid (BFU-Es), seen in normal peripheral blood on days 12 to 14 of culture. The expanded population was characterized by the appearance of single colonies on days 4 to 6 and enormous response to the increasing amounts of erythropoietin, which enhanced their number, size, and maturation. The combination of clinical and in vitro data as well as the absence of any abnormality in the erythropoiesis of the parents and sibling suggest that the erythrocytosis in this child represents a new form with a benign course.