One woman taking blood from another for a fasting blood glucose test
Healthy Living

Why to Follow Those Pesky Rules Before a Lab Test

Nov 23 2018
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You may be tempted to eat as usual, but here’s why to skip eating before your lab test

A fasting blood test is when you’re asked to stop eating or drinking a few hours before a doctor draws your blood. Not all blood tests require fasting. Certain tests require fasting for results that are more accurate. Most of the time, you’ll be asked to stop having food or liquid anywhere between 8 to 12 hours before your bloodwork, depending on the test. Sometimes your doctor will ask you to fast for 24 hours before a blood test.

For some tests, you’re allowed to have a few sips of water, plain coffee or black tea. Other tests don’t allow even a sip of water. Many healthcare professionals agree that people should avoid coffee and gum before a fasting lab test. Both substances affect your digestive system, which can change the results of your test.

Fasting may be necessary for a few reasons. Food and drinks affect your results because sugar, fats, enzymes, minerals and cholesterol absorb into your bloodstream. The test you’re having may need to keep all of these substances out of your bloodstream for an accurate diagnosis.

Why people need fasting blood tests

Your doctor may want to test your blood to see if you have any conditions. These are some conditions that can require a fasting lab test:

  • Anemia (iron blood test)
  • Diabetes (blood glucose test)
  • Electrolyte disorder (metabolic test)
  • High cholesterol (blood cholesterol test or lipid profile)
  • Kidney function (renal function panel)
  • Liver disease (gamma-glutamyl transferase test)
  • Vitamin B12 levels

It’s important to fast to measure the sugars, minerals, fats and enzymes in your blood to diagnose these conditions. It pays to follow instructions before the test. A wrong or missed diagnosis of your lab test can cause more health problems. However, it may be fine to take your regular medication while you’re fasting. It’s always best to ask your doctor for detailed instructions.

If you make a mistake and accidentally eat or drink something, tell the lab when you check in. That way, they can determine if you need to reschedule your test.

Symptoms that might require a fasting lab test

You may have nonspecific symptoms that are hard to diagnose. Or, you may have the following symptoms that could prompt your doctor to order a fasting lab test:

  • Excessive urination and thirst could indicate diabetes.
  • Swollen feet and ankles could indicate kidney disease.
  • A high blood pressure reading could indicate high cholesterol.
  • A yellowish tint to your skin and eyes may indicate a liver condition.
  • Fatigue and an irregular heartbeat could indicate an electrolyte imbalance.
  • Pale skin and leg cramps could indicate anemia or a vitamin B12 deficiency.

Many other symptoms could make you need a fasting lab test. Your doctor might order a fasting lab test to rule out some of these conditions.

Lab tests for diagnosis

Most fasting lab test results are available for your healthcare provider about 24 hours after they take your blood sample. Certain tests may take longer to complete. The report that the lab provides for your doctor checks levels of sugars, minerals and vitamins in your blood. The report is a good tool to use to diagnose your condition.

The fasting lab test results can confirm or rule out a condition. Or, your doctor may order further tests if there’s no clear diagnosis for treatment. If you do get a diagnosis, your doctor will recommend appropriate treatment. For example, your doctor may recommend cholesterol-lowering statin medication to reduce the levels in your blood. In that case, you may need to have another fasting lab test a month or two after you start your medication to see if your numbers are lower.

It can be worrying when you don’t feel quite right. A blood test can help your doctor learn what you’re going through and get you on the path to treatment. Visit Mercy.com to make an appointment with a primary care doctor and schedule your bloodwork.


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