The web says high RBC can by caused by low oxygen levels.
Altitude - I’m close to sea level.
Congenital heart disease, heart failure - I just got a clean cardio cert.
Nicotine dependence - nonsmoker.
COPD/sleep apnea - my GP says I’ve improved so much over the last few years I no longer meet the standards for CPOD, and while I still use the CPAP machine, the datalog says I only have a few seconds of apnea every couple of days; I probably don’t need the machine any more.
Other low oxygen level - I log SpO2 several times per day. It averages 96 or 97, by day or month. The respiratory guy said it was fine.
Also:
Anabolic steroids and some shady bodybuilder stuff - just the standard 200mg/mo TRT dose.
Leukemia, bone cancer, some types of kidney cancers or tumors.
Liver disease.
Emphysema, pulmonary fibrosis - had a respiratory lab done last year with spirometry; everything was OK then.
Dehydration - I hadn’t drunk anything since the previous evening. But that’s usual for when I have an appointment; it minimizes potential bathroom problems. RBC showed OK then.