General Question

zenele's avatar

When one is instructed to fast for 12 hours prior to blood tests, how strict is it and the relevance of what and when something is eaten during that time?"?

Asked by zenele (8260points) July 13th, 2010

In other words – can I have a beer or a glass of wine exactly 12 hours before? Can I have a glass of tea 6 hours before? What if I forgot and ate a snack 6 hours before – you get the picture.

Thanks, Docs/smarty jellies who know this from experience.

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19 Answers

dpworkin's avatar

It fucks with the results, and they become useless as a diagnostic tool. Be strict about it, so your MD will have good data.

MissA's avatar

It is my understanding that 12 hours should be adhered to. (I have a procedure tomorrow.)

I’ve been told that it has to do with the way your body reacts to different parts of the procedure and/or the medications involved. As @dpworkin said, don’t mess up your chances of securing valuable data.

JLeslie's avatar

I try to make it 14 hours to be safe. It is very important to fast as instructed, because the normal ranges your results will be evaluated against are fasting numbers. My cholesterol is about a 40 point difference fasting and non-fasting. Sugar also, it would be very important. There are many more tests where it really does matter.

Usually it is ok to take prescription medication, if you normally do, but you might want to double check, it would depend what they are evaluating.

marinelife's avatar

It depends on the test how strict you have to be.

Some tests even tea would mess up. It is water only.

Some do not want you to have even water for some hours beforehand.

Forgetting and having a snack would totally mess up the point of the test and the results.

josie's avatar

@JLeslie is right. Sugar and lipids in particular change with fasting. You need a predictable “standard” and fasting is easy to reproduce. Take it seriously.

sleepdoc's avatar

So fasting for tests… here is the thing. Some of the levels are not altered greatly by eating something, but others are. You don’t want to get a diagnosis based on the fact that you couldn’t go without food for 12 hours. After all some of the medications for high triglycerides are not the most fun to take.

CMaz's avatar

Water, bread. Anything else could throw off the test. I mean if you have to eat something you have to eat. Just keep it “bland.”

Usually those tests are in the morning. Most of that 12 hours are spent sleeping instead of binge eating.

JLeslie's avatar

Do not listen to @ChazMaz this time bread is sugar in it’s simplest form, you just screwed up your blood sugar test. It is not about bland or not bland, this is not a stomach virus.

JLeslie's avatar

What’s the big deal about fasting 12 hours anyway? Is the test late in the day or something? You can break fast and have a party afterwards :).

bunnygrl's avatar

<hugs Zen>
I had to have bloods done last month and the nurse gave me the earliest appointment (8.30am). She asked me as soon as I arrived when I had last eaten, I told her that I had a slice of toast at about 9.30pm, with my night time medication before bed, I have a terrible time swallowing pills so I usually hide them in a tiny piece of bread or toast to avoid that awful retching I get. Other than that I’d just had a bottle of water with me everywhere. She said that was fine. I even asked her if toothpaste was ok because I wasn’t going to not brush my teeth lol so I’d done that night before and that morning.

I hope you feel better soon, having bloods done is such a pain, and it always worries me too. <throws hugs for our dear Zen> xx

CMaz's avatar

@JLeslie – Is right. I was waiting for that. :-)

Jeruba's avatar

Water only. Don’t fool around with this. You may as well not have the test at all if it is going to give misleading results that could lead to wrong treatment or failure to treat something.

I usually do the blood draws at around ten in the morning to give me a little leeway early in the evening and then fast from about 8 p.m. on. That’s less likely to kill you than whatever you’re being tested for. Be tough.

If you forget and eat something, you’re going to have to postpone the test. No two ways about it.

YARNLADY's avatar

I love Kaiser Permanente. I have to get a 12 hour fast test, and my doctor gave me a printout that shows exactly what I have to avoid, and what might be allowed. Ask your doctor for a list of what you can and can’t have.

zenele's avatar

Thanks guys.

I’m going to follow @YARNLADY ‘s advice and just ask and get an exact list. It was a 19:30 – 0730 fast – so the hours were reasonable enough.


Flavio's avatar

It depends on the test. If you are having your cholesterol/blood sugar measured, you are OK drinking water or other fluids provided there is no sugar in them. If you are getting a more complicated test, especially some endocrine ones, better stick to the guidelines. If you are going in for a procedure, dont eat. You want your stomach empty so you don’t choke after the sedate you.

jazmina88's avatar

i had a part of diet mt dew to take some meds…...and ti was suggested It would mess with the results, which I get back Monday.

some water, that is it.

Flavio's avatar

What tests?

mattbrowne's avatar

I’m always strict about it.

LuckyGuy's avatar

Don’t forget to avoid sex, DRE, or ejaculation for 48 hours before having blood drawn for a PSA test. They will artificially increase your PSA reading, scaring you into visiting your Urologist unnecessarily.

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