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Tests That Require Fasting
Fasting before a blood test is typically required for fasting glucose, fasting lipid panel, fasting metabolic panel, fasting cholesterol, HDL or triglyceride tests. These tests look for cholesterol and glucose levels in your blood, and any food consumed before the test will provide an inaccurate reading.
How Long Should You Fast
Your doctor may recommend a certain time period for fasting; however, the standard amount of time is about eight to 12 hours of no food before having blood drawn. Schedule your blood test for early in the morning so you’ll only need to fast overnight to meet requirements.
Fast for eight hours before a glucose test. The test is typically conducted in the morning while your body is still in a resting place for a more accurate reading.
You must fast at least 12 hours before taking
a cholesterol blood test to get an accurate reading on triglycerides. The American Heart Association warns against quick cholesterol tests you find in malls and health fairs because fasting is imperative to obtaining an accurate result.
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Fasting Dos and Don’ts
Stay hydrated and drink plenty of water. Take any medication that your doctor prescribed to you except for corticosteroids, estrogen or androgens, oral contraceptives, some diuretics, anti-psychotic medications including haloperidol, some antibiotics and niacin.
Do not smoke, drink any other liquid than water or exercise during your fast. Even chewing gum is off-limits. Any of these elements can adversely affect your test results.
Because some tests are ordered at the same time--some requiring fasting of eight hours and others of 12, still others no fasting--in effort to be efficient, follow your doctor's orders regarding fasting and "npo" status (nothing by mouth--including water) before blood tests.Source: ehow.com