No, hydration will not effect the cholesterol measurements.
The blood volume is
tightly controlled within very tight perimeters by your endocrine system, the instantaneous effects of osmosis between the intravascular and extravascular spaces and the kidneys.
Secondly, fasting has NO effect on the laboratory measurement of cholesterol levels. Cholesterol measurements, including HDL (good cholesterol carrier) can be perform during the non-fasting state and will reproduce the same number as if it was done fasting.
The reason for "fasting" is to calculate accurately the LDL (bad cholesterol carrier) which is performed via directly measuring the serum's fasting triglyercide level. Post-prandial (after eating) measurements done prior to 10-12 hours of fasting creates a transient elevation in the triglyceride which makes the LDL calculation impossible (immediately after eating) or calculation inaccurate.
Summary:
Cholesterol levels (and HDL) are not effected by hydration or fasting status.
LDL is calculated from the "fasting" triglyceride level and is very dependent on fasting.
For your physician to intepret the meaning of your Lipid Status / Cardiac risk, he/she needs the full picture of 5 components. Cholesterol, HDL, LDL (sometimes VLDL), triglycerides and Family History.
Hope this ease your concern about hydration and the reasoning behind why we request that patients "Fast" for this test.
