My haematologist, Dr Francis at Sheffield Hallamshire Hospital, was thoughtful enough to point this out to me and gave me the reference. He told me that, based on the guidelines, he wanted to increase my hydroxy rather than send me for my usual venesection. I understand this is because I am now over 65 and therefore classed as a higher risk. I think next time round I might tell him I prefer a mix as I would prefer not to keep increasing my hydroxy dose. However, we are not doctors and should follow advice if we trust the doctor.
See onlinelibrary.wiley.com/doi...
"Randomised trial data supports that venesection of 200–500 ml blood at intervals suitable for patient size/tolerability should be used to achieve and maintain a Hct of <0·45 (Marchioli et al, 2013). In low risk patients this is usually adequate to maintain target Hct. Where frequent venesection is needed to achieve this target then an alternative approach using a cytoreductive agent may need to be considered. High levels of venesection requirement have been reported to have an association with higher thrombosis risk in patients on HC, specifically in those patients requiring 3 or more venesections per year (Alvarez‐Larrán et al, 2017). "
Painkillers while on Hydroxycarbamide for treatment of Polycythaemia Vera 
Update on my polycythemia vera diagnosis 
Variations in Patient Response to interferons
Slightly disconcerting review
