Rationale: Little is known about the effect of a hemoglobin-shift on patients, anaemic, non-anaemic or even polycythaemic. It has been established that severe anaemia has deteriorating effects on the patient, which can be (partially) reversed by treatments like transfusion, erythropoietin stimulating agents or iron/vitamin supplements, depending of the etiology of the anaemia. However, the optimal haemoglobin target and threshold for such treatments has yet to be determined. It is therefore necessary to evaluate what the exact effect of various haemoglobin levels, and a shift therein, is on the physiology of patients. Only then can the benefits be weighed properly against the risks for individual patients when considering treatment for anaemia, safety of blood donation, or expected effect of phlebotomy for polycythaemia.~Primary Objective:~- Compare the per individual and per group effects of a reduction in Hb mass on physical functional outcomes (heart rate; activity parameters; QoL) in patients with polycythaemia.~Secondary Objectives:~- Compare the primary outcomes to data from the REMOTE-2 and FAINT-study (similar measurements in transfusion dependent patients and whole blood donors)~Study design: Within-subject design. Patients will be remotely monitored one week before phlebotomy until one week after.~Study population: Adult patients with a polycythaemia requiring phlebotomies on a regular basis.~Main study parameters: heart rate, blood pressure, activity parameters and quality of life.
Single-centre, observational within-subject design: patients that undergo phlebotomies regularly for polycythemia will be measured continuously by wearables one week before until one week after a phlebotomy, for three phlebotomies.