Don't laugh at this! Drink 1 to 2 glasses of wine per day (i prefer whiskey) BUT my point here is this...small amounts of alchohol per day will/has been documented to "thin" one's blood! Also try to intake around 4 to 8 grams of good fatty acids/day (Omega 3, 6, and 9) by way of flax, fish, CLA and/or primrose oil...all standards. J.
Its not any alcohol, its just red wine

, and the dosage would be around 90ml a day (one glass), but it is very controversial since studies don't seem to show a pattern which is good and straight enough to make it a satisfactory test. I wouldn't rely on that

ASA (aspirin), 100mg/day won't "dilute" blood, it would just avoid coagulation which is colloquially known as "thinning blood", but it has absolutely no effect on HC/Hemoglobin levels. It is true running test. increases the probability of a thrombosis disease and ASA could help with that, but not with "thick blood".
How high is your "high-hemoglobin"? I mean, range of "normality" its quite big. I live at 2,600 meters over see level, so having a HB of 18 and HC of 50% is not uncommon. I doubt it, but have you ever been a smoker? What kind of work you do mntbikedude? Cause some professions are more linked to higher HC/HB without this being an issue...
Anyway, yu can drink as much water as you want until 8 hours prior to the exams....
