What causes an elevated haematocrit level

Patient: Hi Dr. I recently got a CBC, because of suspected Mono, but the mono test was NEG. I had other out of range results though: RBC: 5.28 (Range: 3.8 – 4.8) Hematocriten: .45 (Range: .35-.43) All other test where withing range, including the WBC differential. I am 25 years old, 5’7″, 220 lbs, dont smoke, drink rarely. I had really swollen glands and felt like something is stuck in my throat. The feeling is less now than a week ago. I had a strep throat infection about 2 months ago, but it did get better with penicillin. Is this something i need to be concerned about? I also had my period when the test was done, not sure if that has any effect. I dont have medical insurance so costs me $100 just to visit the doctor for 10min, so would like to avoid that if possible. Thank you for your help.

Doctor: Your haematocrit (HCT) level may be elevated if you have a reduced circulating blood volume (e.g. dehydration) or an inc reased red blood cell mass (e.g. Polycythaemia rubra vera, smoking, chronic lung disease, living at high altitude). The HCT is expressed as the percent of a blood sample occupied by intact red blood cells. Polycythemia is suspected when the HCT is >0.48 in women. In the presence of a normal haemoglobin level, the slightly elevated haematocrit and red blood cell count, most likely represents reduced blood volume/dehydration from your acute illness rather than the latter conditions. It may be advisable to have a repeat test on resolution of your current infection, to ensure the haematocrit has returned to normal.