I’m going in for a physical soon. I’d like to get my cholesterol checked, my thyroid levels checked, my iron checked, and checked to see if I have pre-diabetes.
Do I need to fast for these tests? If so, for how long? (So I know what time of day to schedule the appointment.)
IANAD, and of course your doctor can answer the question easily, but I think that generally they want you food and liquid free for at least 8-12 hours.
Again, I don’t know for sure. I imagine that even the receptionist at your doctor can give you the correct answer.
I’ve had all those tests numerous times except perhaps the diabetes one (I might have but I can’t remember) and I never had to fast for any of them. In fact, they didn’t even know they were going to do them until I got in the office. But it might be best to call your own doctor’s office and ask.
The Hemoglobin A1C test needs to be done fasting. It’s a test to see what your blood sugar levels have averaged over the past two weeks. The phlebotomists and the doctor will be able to tell you whether or not you should fast for the test.
Good luck.
Cholesterol and blood sugar testing require fasting for several hours. If your appointment is in the morning, don’t eat breakfast. If it is in the afternoon, call the office to see how long you should avoid eating. To my knowledge, thyroid and iron levels do not require fasting.
Neither does Hgb A1c, which measures average blood sugar levels over several months (not weeks). It is not used for diabetes screening but for long term monitoring of patients already diagnosed.
You can drink water, however.
I am diabetic and all of my blood sugar tests must be done fasting at least 12 hours before the test.
Although cholesterol screening can be done without fasting, a full cholesterol check requires a 10 to 12 hour fast. “Several hours” is inadequate.
Glucose testing for “pre-diabetes” would also need to be done fasting.
Ethilrist stated that the hemoglobin A1c needs to be done fasting, and USCDiver replied that it wasn’t necessary. In theory, fasting isn’t required for that test BUT some assays of hemoglobin A1c might (unfortunately) be affected if you don’t. Specifically, certain older techniques measure the total glycosylated hemoglobin, not just the hemoglobin A1c. Turns out that the total glycosylated levels can go up acutely after a meal (if the sugar has also gone up).
There is fair evidence that prolonged fasting, especially without the ingestion of water, or fasting with continued use of things like coffee (i.e. things that cause a lot of urine to be produced) can lead to falsely elevated levels of cholesterol. Basically what happens is that you can dry out so much that your blood, and its contents, become too concentrated. Since labs measure the concentration of cholesterol, sugar, etc., the result can be misleadingly high.
So, some advise “1) that patients fast no longer than 12 h, and 2) that, during fasting, patients avoid unnecessary physical activity, avoid hot dry environments, ensure a liberal intake of water, and avoid diuretic substances such as caffeine” (Clin Invest Med 1994 Dec;17(6):570-6.)
You likely won’t have the blood tests the same day as your doctor appt. Most clinics will give the patient a sheet with their lab’s specific fasting needs.
Labs can use different equipment for similar tests.