The Effects of Drinking Coffee Before Blood Tests

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People often wonder about how to prepare for medical tests. Each test is different and only your doctor can definitively tell you how to prepare for your blood tests. In what way or manner does drinking coffee before blood tests affect the results. For tests which require pre-test fasting, coffee will affect your blood, your metabolism and your blood test results. Only water is allowed for certain tests, otherwise an inaccurate reading may result.

Food and Beverages

Caffeine is present in nuts, soft drinks, chocolate, energy drinks, tea and coffee. Even “decaffeinated” beverages contain trace amounts of caffeine. A cup of decaffeinated coffee contains 5 mg. of caffeine compared to 140 mg. in a regular cup of coffee. You can also find synthetic caffeine, man-made caffeine, rather than that extracted from fruits, plants and seeds. Synthetic caffeine is available in pill form and added to food and beverages.

Caffeine Affects Blood Sugar Levels

Caffeine can affect your blood in several ways. Blood tests can determine blood cell numbers, variety, percentage, concentrations, quality of blood cells, blood pressure and flow. Caffeine is readily absorbed in 30 to 45 minutes. In about three hours its effects go away. During that time when it is present in your body, it raises blood sugar levels and affects results for some blood tests.

Tests and Guidelines

Following are some common blood tests, and general guidelines for avoiding food and beverages including caffeine for 8-12 hours prior to your blood test.

Amylase - Pancreatitus disorder and pancreatic disease. NO fasting.

ANA - (Antinuclear Antibody test) lupus, polymyosistis, autoimmune diseases. NO fasting.

aPTT - (Activated Partial Thromboplastin Time) evaluates risk of excessive bleeding prior to a surgical procedure. NO fasting.

AIC - (Hemoglobin A1C or Glycohemoglobin) diabetes and prediabetes. NO fasting.

BMP - (Basic Metabolic Panel) diabetes and kidney disease. You may be asked to fast for 10 to 12 hours prior to test.

CBC - (Complete Blood Count/Hemogram) general health, anemia or infections, toxic substance exposure. NO fasting.

CMP - (Comprehensive Metabolic Panel) kidneys, liver, electrolyte and acid/base balance, blood sugar and blood proteins. You may be asked to fast for 10 to 12 hours prior to test.

Electrolytes - (Electrolyte Panel) fluids and electrolyte balance. NO fasting.

Glucose - (Fasting Blood Sugar) blood glucose level, diabetes, pre-diabetes, and hypoglycemia. Fast 8-12 hours pre-test including avoidance of caffeine.

HIV Antibody - (Human Immunodeficiency Virus) HIV infection. NO fasting.

Lipid Profile - Coronary heart disease, stroke, blood vessel blockage, hardening of arteries. Avoid food and beverages, except water before your test.

Lyme Disease - (Lyme Antibodies Detection) exposure to Lyme disease. NO fasting.

Mono - (Mononucleosis) NO fasting.

Talk to your doctor to find to what degree does drinking coffee before blood tests affect the results. If advised to fast before your test, avoid beverages with caffeine including decaffeinated coffee.


labtestsonline, Test Index

National Institutes of Health, Common Blood Tests

American Heart Association, Caffeine