When fasting for a routine or diagnostic blood test, the only item allowed on your menu is water. Most beverages, including tea and milk, contain water, but their additional components can affect your blood makeup and alter your test results. Any caffeine, sugar and plant or animal compounds that you consume change your levels of digestive acids, glucose and enzymes, so testing laboratories place a temporary moratorium on all foods and most drinks. Follow your doctor's advice closely to prepare for a fasting blood test.
Type of Test
If substances such as tea or coffee might jeopardize the objectivity of a blood test, fasting is routinely ordered for all patients. You will be instructed to fast for eight to 12 hours when your blood sample will undergo some blood glucose and blood cholesterol tests. Fasting is required for broader assessments such as basic metabolic panels, which evaluate blood glucose and blood mineral levels, and lipid panels, which measure cholesterol and triglycerides.
Time of Test
Morning blood draws allow you to sleep through most of your fast but do require you to forgo an initial cup of tea, coffee and other caffeinated beverages, along with all other foods and drinks except for water that day. You can't drink tea until after your morning appointment. If you are an early riser and schedule your blood draw for later in the day, you can drink tea and add milk, lemon or sugar -- as long as you stop drinking at the hour designated by your doctor to begin the eight- to 12-hour fasting period.
How Tea and Coffee Threaten Test Results
Both the water and the caffeine in brewed beverages can change the elements of your blood sample, potentially subjecting you to misdiagnoses or delays in treatment if you need to be retested. When you consume tea, the water volume decreases your body levels of electrolytes, which your kidneys would normally adjust. The diuretic aspect of caffeine, however, makes them excrete more liquid than usual at the same time that your stomach produces more acid. This can affect both your mineral and your pH levels to skew your blood work.
Successfully Completing Your Fast
Rather than trying to drink more tea to "stock up" before your fast begins, stick to your usual daily schedule of tea drinking. A morning cup eight to 12 hours before an afternoon blood test is appropriate. Drinking tea in the evening before a morning appointment, however, might affect your sleep habits. If you usually enjoy tea in the morning and must skip your dose to complete your fast, simply put it off until after your test and then indulge along with your breakfast meal.
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