Medical Question (cholesterol measurement)

[Dave N]

For a fasting blood cholesterol/triglyceride test, you are allowed to drink water before. If cholesterol is measured in mg/dL of blood, then wouldn't drinking water before the test dilute your blood and give a lower reading?

I had mine tested a while back and drank a few glasses of water before the test so I would hydrated and they wouldn't have trouble getting the blood out. I've just been wondering about this ever since.

[Spilchy]

Yes, you can drink water before the test. It will not affect the results. However, do not drink any other beverages, do not take any vitamins (particulary Vitamin D) and certain medications like MAO inhibitors, steroids or certain antiobiotics. Are you on any oral contraception?! They affect it too.

Basically, fast for 12 hours, drink water if you want and ingest nothing else, and you'll have an accurate reading.

Good luck.

[ScubaStevo]

Yup, definatly don't eat anything. That will give a false reading.

On the other hand, since water doesn't have anything to do with the blood's %'age of cholestoral, it should be fine.

[Dave N]

I think you guys are missing the point of what I'm asking...

Blood cholesterol level is measured in mg/dL (milligrams of cholesterol per deciliter of blood) Now if you have just drank a bunch of water, then the water goes into your bloodstream, total blood volume goes up while the total amount of cholesterol stays the same, and the cholesterol (along with everything else) gets diluted. Unless the cholesterol reading is somehow normalized, then the reading would be affected by your blood volume at the time of the test. I'm basically wondering if the lab normalizes the cholesterol readings to factor out differences in people's blood volumes.

[blkZ28Conv]

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.

[Spilchy]

Originally Posted by blkZ28Conv

For your physician to intepret the meaning of your Lipid Status, he/she needs the full picture of 4 components. Cholesterol, HDL, LDL (sometimes VLDL) and triglycerides.

Bingo! On a side note to Dave N's question, the break down of the HDL / LDL ratio is what's important, not just the number. When I had my cholesterol taken, my number was low, triglycerides were right in line, however, the ratio was "vanilla" as my doctor put it - meaning he said, get my a$$ in gear and get some excercise, not just a good diet.

Unfortunately, heart disease runs in my family and we've been long time watchers of cholesterol, long before it became popular to do so (as I am munching now on a meatball sandwich drowning in melted mozerella!!)

[blkZ28Conv]

Originally Posted by Spilchy

Bingo! On a side note to Dave N's question, the break down of the HDL / LDL ratio is what's important, not just the number. When I had my cholesterol taken, my number was low, triglycerides were right in line, however, the ratio was "vanilla" as my doctor put it - meaning he said, get my a$$ in gear and get some excercise, not just a good diet.

Unfortunately, heart disease runs in my family and we've been long time watchers of cholesterol, long before it became popular to do so (as I am munching now on a meatball sandwich drowning in melted mozerella!!)

Darn Seth,
I had just edit my post to include Family Hx. Very, very important.

[Dave N]

Thanks for the comments and explanations, guys.

Originally Posted by blkZ28Conv

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.

So is it a medical urban myth that drinking a few glasses of water before a blood test will make it easier to hit a vein and/or draw blood?

[Spilchy]

Originally Posted by Dave N

So is it a medical urban myth that drinking a few glasses of water before a blood test will make it easier to hit a vein and/or draw blood?

A quick Google search seems to support your urban myth, however none of my findings were of a scientific or professional nature.

[blkZ28Conv]

Originally Posted by Dave N

Thanks for the comments and explanations, guys.


So is it a medical urban myth that drinking a few glasses of water before a blood test will make it easier to hit a vein and/or draw blood?

Yes and no Dave, rehydration or pre-hydration before a venipuncture is useful "if" ones total blood volume is on the edge. Meaning that the body will equilibrate or distribute the available blood volume to what it determines is the vital organs (brain, GI, coronary and other core organs). Arms veins are low priority.
There are many other factors that will increase the accessibility of extremity veins. A good example is good muscle tone (forearms and bicep/triceps) that forces more flow thru surface vasculature and % of body fat.
Ever notice the veins of an body builder?

[mtwedt]

Dave,
In 25 years of practice, I have never even heard of anyone trying to up their circulating volume with water prior ot a blood draw to supposedly make it easier to obatin a blood sample. For the average healthy person who is having routine blood work done in the doc's office, this is not necessary. There are any number of people who are a hard stick, no matter what their level of hydration. Cholesterol is not affected by dilution as stated above. Some of the other elements of the blood are, such as red cells, sodium and blood urea nitrogen. In a person with healthy kidneys, adrenal glands and endocrine response, the water bolus you drink will be dealt with as an increased urine output to your bladder. The body has a number of monitoring and response systems to maintain a proper fluid balance and blood volume. Since it is quite common for people to have a large battery of blood tests at one time, therefore necessitating only one stick, there are other lab tests being done that could be affected by a dilutional state, such as the tests I described above. It would take a good chunk of water to do this....like a liter or two about a half hour before before drawing. Unless you are a hard stick due to being dehydrated (and it taked more than 12 hours of fasting to do this....if you are not taking in fluid, the body will conserve water and urine output will decrease....there is no reason to drink a bunch of water before a blood draw and several reasons for not doing it. There are people who are a difficult stick....ie, the elderly, those on big doses of diuretics, the chronically ill, people who been having alot of sticks for things like chemo, morbid obesity, bigtime steroid users and those who simply have difficult veins. If you have had a problem in the past with venous access, it woudl best to ask your MD about fluid loading prior to a blood draw. I would suspect in most cases, the doc would tell you not to do it.

[lawrencea]

Will donateing a pint of blood lower your cholesterol or other blood fats?