Not every type of blood work requires you to fast beforehand. But certain common blood tests do require fasting. These include glucose and sometimes, triglyceride tests. Tests for glucose measure your blood sugar, while those for triglycerides measure part of your cholesterol lipid panel.
Most of the time, you cannot drink coffee the morning of a fasting blood sugar test. The caffeine in your morning cup of joe may raise or lower your blood sugar levels. Therefore, to get an accurate test, it's best to wait until after your test to drink your coffee.
Coffee Before a Glucose Test
Many annual wellness checkups require an overnight fasting blood test. Your doctor wants you to fast because food and drink may alter your results, according to MedlinePlus. Sugar, fats, enzymes, minerals and cholesterol will absorb into your bloodstream and may change the test results. Alcohol should also be avoided during your fast.
Coffee before a glucose test can affect the results, especially if you add cream and sugar, according to Mercy Health, a chain of hospitals and health care clinics in Ohio and Kentucky. Added sugar and fat from the cream can make your test results inaccurate. Even without these ingredients, however, coffee and a blood sugar test don't go well together.
Mercy Health suggests asking your doctor for detailed instructions before a fasting test. Also, if you make a mistake and accidentally eat or drink something before your test, let the lab staff know, and they will determine if you need to reschedule the procedure.
Most fasting tests, however, do allow you to have water. Water hydrates your veins and makes it easier to draw the blood out of them, according to Medline Plus.
Test for Type 2 Diabetes
Health care professionals will ask you to fast for eight to 12 hours before you take a blood glucose test, says the Mayo Clinic. Many doctors recommend you get the test first thing in the morning to make it easier on yourself, having had nothing but water. Medline Plus also suggests that you don't exercise, smoke or chew gum before this procedure.
The glucose tolerance test for Type II diabetes requires several steps, says the Mayo Clinic. First, you'll give a sample of blood from a vein in your arm — that number will provide your fasting blood glucose level. Then you'll drink a glass of glucose solution containing 75 grams of sugar. After two hours, your glucose will be measured again.
If your blood glucose level is normal, it will be below 140 milligrams per deciliter of blood. If it measures between 140 and 199 milligrams per deciliter, you may have impaired glucose tolerance, also known as pre-diabetes. This means you're at risk of developing diabetes and heart disease.
If the level is above 200 milligrams per deciliter, you may have diabetes. Your doctor may want you to repeat the blood work test if that's the case.
Test for Gestational Diabetes
If you're at low risk for developing diabetes while pregnant, your doctor may request a nonfasting test. The American College of Obstetricians and Gynecologists recommends a one-hour blood glucose challenge test between 24 and 28 weeks of pregnancy.
You may get tested earlier in your pregnancy if you have any of three possible risk factors, according to the Mayo Clinic:
- If you had gestational diabetes in an earlier pregnancy.
- If you have a family history of diabetes.
- If you are obese.
- If you have a metabolic condition associated with the development of diabetes, such as metabolic syndrome or polycystic ovary syndrome.
If you are found to be at risk, or your one-hour test results have a suspicious value, then you'll be asked to take a three-hour fasting blood test similar to the blood test given for people with type II diabetes. That will mean no eating or drinking anything but water for eight to 12 hours before your test.
Caffeine and Blood Sugar
If you don't have diabetes, the Mayo Clinic says drinking coffee shouldn't noticeably affect blood sugar levels. Most healthy young adults can safely take in up to 400 milligrams of caffeine a day, which is more than the typical caffeine found in two 8-ounce cups of coffee.
If you do have diabetes, however, caffeine may affect your blood sugar levels. Caffeine may raise blood sugar for some diabetics, while it may lower blood sugar for others. Caffeine affects everyone differently, the Mayo Clinic says.
There aren't a lot of clinical studies measuring the effects of coffee on a blood glucose test. A small study of coffee drinkers in the September 2012 edition of the journal Metabolism measured the effects of caffeine from espresso coffee. It found only marginal differences in glucose measures between coffee drinkers and noncoffee drinkers.
Triglycerides and Fasting
Tests for triglycerides are usually part of your blood cholesterol level tests. The triglyceride test was once done after fasting for 10 to 12 hours, according to Harvard Health, because the concentration of fatty triglyceride particles stays in your bloodstream for hours after you eat a meal.
The reasoning was that if you didn't fast, you might skew the numbers. Doctors are looking for a result of fewer than 150 milligrams per deciliter of blood for a good result. This number is also used to calculate your level of LDL, or bad cholesterol.
But Harvard Health now says that fasting before a cholesterol test doesn't affect the numbers as previously thought. A nonfasting test during your regular diet, including coffee if you drink it, gives a better picture of lipid levels. The U.S. guidelines, including Medline Plus, still recommend fasting before a cholesterol test. But, Harvard Health adds, in Europe, fasting isn't typically required.
Other Fasting Blood Tests
Some other blood tests may require fasting too, according to Mercy Health. These can vary, however, so you'll need to check with your doctor before your test. These tests usually include:
- Anemia (iron blood test).
- Electrolyte disorder (metabolic test).
- Kidney function (renal function panel).
- Liver disease (gamma-glutamyl transferase test).
- Vitamin B12 levels.
Remember, if you're having a blood test for anything other than what's been discussed above, you probably can go ahead and have your morning coffee and breakfast before your blood is drawn. But remember to always check with your doctor for detailed instructions, and follow those instructions exactly for the best results.
- Mayo Clinic: "Glucose Tolerance Test"
- MedlinePlus: "Fasting for a Blood Test"
- Mayo Clinic: "Caffeine: Does It Affect Blood Sugar?"
- Metabolism: "A Cross-Over Study of the Acute Effects of Espresso Coffee on Glucose Tolerance and Insulin Sensitivity in People With Type 2 Diabetes Mellitus"
- Harvard Health: "Fasting Before a Cholesterol Test"
- Harvard Health: "What Blood Tests Require Fasting?"
- Mercy Health: "Why to Follow Those Pesky Rules Before a Lab Test"