Why can’t you eat past a certain time at night if you have to take a blood test the next morning?



I am supposed to take a blood test in the morning and my doctor told me that I have to do it on an empty stomach, and can’t eat anything after 9pm. I understand how alcohol, for one, could impact blood test results but why food? Asking cause I’m hungry 🙁

In: Biology

Certain blood tests require you to be fasting. Blood sugar, cholesterol (I think) and others. So you have to be hungry when you take the test. Eating causes sugar and other compounds to go into your blood, and will cause the test results to be wrong.

For example, if you were to eat before the blood work, the blood work could come back that you have diabetes, when you don’t.

Well for one, carbs can affect your blood sugar levels! And other nutrients can influence your blood composition

This is related to what your doctor is testing for, which in turn is related to what kind of condition or disease you might have. Many common blood tests, such as ABO/Rh typing and antibody or qPCR tests for particular pathogens, include no such restrictions for the patient.

E: I’d guess you’re being tested for some sort of metabolic disorder, but obviously you’re not required to disclose information like that to strangers on the internet.

Depends on the test. For diabetes, they want your overnight blood sugar. Obviously if you drink a Coke before the test, your sugar levels will be higher but that isn’t necessary a problem. Same for cholesterol. If you eat a steak for breakfast, that will be higher than your normal level.

Best to get what those numbers are when you are fasted so you can make a comparison against other people’s.

Just to piggyback onto OP’s question.

Does fasting *too* long also affect the test? Usually they want you to not eat or drink (except for water) for 12 hours. If you go 17 hours before the test would it also skew the results? When they say 12 hours is that a hard fast rule?

Edit: (Basic cholesterol /triglyceride tests)

Depends on the tests done but most commonly fasting is required when testing for blood sugar levels. The reason is, depending upon when you eat, what you eat and how much you eat, your blood sugar can vary drastically. So it is difficult to make an interpretation of the blood glucose levels in so many varying situations. Scientists found that a fasting period of 8 to 12 hours allows any excess sugar in the blood to be metabolised and gives a more standardised, comparable picture of the glucose levels.