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RedDeerGuy1's avatar

Why do we need to fast before certain blood tests?

Asked by RedDeerGuy1 (20735points) 2 months ago

Just wondering.

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12 Answers

LadyMarissa's avatar

Depending on the test, certain foods or drinks can alter the outcome of the test. My doctor always had me leave off any sodas before most of my blood tests because the results could be altered to give the doctor a false result. Water & black coffee seem to be the only 2 drinks that don’t change the results.

gondwanalon's avatar

Food adds a variables into the blood stream. Fasting offers a more consistent blood chemistry state.

If you are testing the blood for cholesterol and lipids then you don’t want to eat bacon and eggs or any fatty food just before having your blood drawn. The fat will become visually present in your serum and also greatly elevate the cholesterol and lipids. Also you don’t and to eat a candy bar just before testing your blood’s sugar level.

Blackwater_Park's avatar

The worst culprit is sugar and starch. It will mess up your blood sugar and lipid/triglyceride readings. I’m sure other readings will be messed up too.

jca2's avatar

My endocrinologist told me four hours is the time limit for eating. Many blood lab places will say “nothing by mouth after midnight” but if, by chance, you’re doing blood work in the middle of the day, you can eat up to four hours before and not alter the results.

I used to go for blood work and have coffee and 2% milk and artificial sweetener, within a few hours and I’d admit it to the person doing the blood. My cholesterol and glucose are always good but I would tell them just to be on the safe side.

zenvelo's avatar

My Cardiologist wants a 10 hour minimum fast (12 hrs preferable) for lipid screenings.

When I had a PET scan a couple years ago, only wtaer was permitted for 12 hours. Part of the test was measuring how well your body metabolized glucose, then they inject you with a radiocative dye tied to sugar.

JLeslie's avatar

The normal range for the test is based on fasting. If you eat your numbers for most tests that require fasting will be much higher than if you are fasting. For instance sugar and cholesterol, those will likely be elevated if you don’t fast. There are other reasons too, depends on the test.

smudges's avatar

The normal range is based on a normal population, like male adults, female adults, children and their varying ages, etc.

As others have said, the reason for fasting is to prevent interference with certain blood tests – for example, alcohol affects triglycerides a LOT, like several hundred units. For some tests, I remember learning not to chew even sugarless gum because it causes your stomach to produce gastric fluids. As far as how strict the fasting should be, some say water and coffee are ok, others say nothing, some say 10 hours, some 12 hours; I’ve never heard of 4 hours, but haven’t been in the field for a while. When I fast, I do 12 hours, water if I really need it.

lab tech for 12 years

filmfann's avatar

If you eat within the four hours before the test, your blood sugar level will be wildly inaccurate.

smudges's avatar

Well, it’ll be a postprandial test, so it won’t be accurate for a fasting test if that’s what you mean by “wildly inaccurate”. Reactive hypoglycemia, also called postprandial hypoglycemia, is a drop in your glucose level. This usually happens within four hours after eating and isn’t related to diabetes.

SnipSnip's avatar

The blood has to be drawn when your body is not working to digest food.

smudges's avatar

^^not at all true

JLeslie's avatar

Yeah, not at all true.

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