1. We studied 20 patients with chronic airflow obstruction, 10 patients without polycythaemia and 10 patients with compensatory polycythaemia having respectively mean red cell mass 24.7 (sd 4.2) and 47.8 (sd 7.5) ml/kg, mean daytime Pao2 7.6 and 6.9 kPa, mean FEV, 0.85 and 0.82 1. Groups were matched for severity of daytime arterial hypoxaemia but nocturnal arterial oxygen desaturation was more severe in the patients with polycythaemia than in those without. We also studied six additional patients with chronic airflow obstruction and polycythaemia and 19 normal controls.

2. Estimates of serum immunoreactive erythropoietin (siEp) in those without polycythaemia were 19 m-i.u./ml (geometric mean) with 95% confidence range 11–35 m-i.u./ml and stable during 3 months. In those with polycythaemia they were similar and consistent in five and, in the other five, higher on at least one occasion. There was no significant difference between siEp in daytime (12.00 hours to 16.00 hours) and morning (07.00 hours) samples but geometric mean estimates of erythropoietin in paired daytime and morning samples were higher and more variable in patients with polycythaemia than in those without. The geometric mean estimate of siEp in all patients with chronic airflow obstruction and polycythaemia was greater than in normal subjects but, despite secondary polycythaemia, siEp could be in the range for normal subjects.

3. In the patients with polycythaemia we were unable to predict the finding of normal or elevated siEp.

4. Changes in siEp after erythrapheresis (10–26% reduction in packed cell volume) were observed in the 10 patients with polycythaemia and in one without. One month after erythrapheresis, packed cell volume remained below and siEp was above initial pretreatment levels, implying an erythropoietin secretory response and that the development of secondary polycythaemia had induced a fall in siEp.

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