How to prepare for a blood test

It’s best to ask the lab or your doctor before going for testing to make sure that there are no special requirements for preparation for the tests you have scheduled, and if there are specific requirements, to adhere to these carefully to avoid invalid test results and waste your time and money. Most tests do not require special preparation, but check it out before arriving at the test location.


For cholesterol and glucose tests, you must fast for at least 8 hours prior to having your blood drawn, unless your doctor tells you otherwise. Fasting means no eating or drinking for at least 8 hours before the test, except water. After your blood is drawn, you may resume your regular diet. Most blood lab tests that don’t mention “fasting” in their names do not require any special preparation. However, a few tests, like the Fasting Blood Sugar and the Glucose Tolerance (which does not mentioned “fasting” in the title) do require special preparation and the lab will provide that information at your request prior to your arrival for testing. Some panels, such as Cardiac Risk include a test (e.g., Apo A and B, triglycerides) that should be done on a fasting specimen, often this is mistakenly or overlooked.


Drink plenty of water before your blood test. This helps keep your blood pressure from dropping. The leading cause of fainting during a blood test is a drop in blood pressure. Avoid coffee or other caffeinated drinks before your test because they actually cause your body to expel water.


Unless fasting is required for your testing, eat breakfast to help keep your blood sugar up. This will help you feel better after your blood draw and prevent lightheadedness and dizziness. If you think you might be nauseous during the blood draw don’t eat immediately before your appointment.


If you take a blood-thinning medication, such as heparin or Coumadin (warfarin), tell the phlebotomist about these medications before your blood is drawn. After your blood is drawn, the phlebotomist will closely observe the puncture site to see that bleeding has stopped before you leave the collection location.


If you are anxious about what is going to take place, ask the person taking your blood to explain everything he or she is doing. Or think of something entirely different, like your vacation or what you are going to do after your blood test.


Eat a snack after you have your blood drawn. Take a snack with you if you will not be going directly back home or to work. That way you can eat it directly after the blood draw.

Here are a few tests with special preparation rules:

Stool examination for Occult Blood does require quite a rigorous adherence to diet for several days before and during collection of the specimen. False-positives may be the result of failure to observe these instructions, and the consequences involve extensive follow-up testing which can be as expensive as they might be unpleasant.

Sputum Cytology (sputum=mucus) specimens are best collected after deep coughing early in the morning because the richest accumulation of cells of diagnostic quality is to be found in this type of sputum sample.

Semen analysis must be performed on a clean, fresh, warm specimen, best collected at the laboratory facility.

Vitamin B12 blood specimens require testing after refraining from taking multivitamins for at least a week and may only be valid when a specimen is collected several weeks after the last B12 injection.

Remember, it is always wise to contact the doctor, lab rep or receptionist at the lab of your choice beforehand to be certain that there are no special preparations you need to undertake before going to the lab for any testing.

understand your blood test results

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