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MARCH 7, 2019

Is fasting necessary before a blood test? Precautions and preparations

When doctors ask you to take blood tests, they can ask you to be fasting for some time. But what does that mean? Why do you need to fast before getting your blood sampled?

To make things clear for you, we answer the most frequently asked questions. Can I drink my coffee, take my medicine or smoke a cigarette before my blood test?

Some advice will be given to prepare you for the appointment and facilitate the experience.

Why do you need to fast before a blood sample?

After eating food or drinking a liquid that is caloric, the blood circulating throughout your body transports nutrients from the digestive system to the various organs, tissues, muscles, etc. The ingested food contains lipids and glucids (fats and sugars) which, circulating in the sampled blood, falsify the results. The presence of fat in the samples affects the blood quality and stops it from mixing well for some tests.

Test examples requiring fasting:

  • Glucose
  • Hyperglycemia
  • Cholesterol, HDL, LDL and triglyceride
  • B.12 vitamin
  • Folic acid

What to do before blood tests?


The day before the appointment, eat a light meal with water. It is important to not change your eating habits too much before the appointments so you can get the results linked to your lifestyle.

Drink water in the evening so you are not too hungry. In the morning, drink a large glass of water, this will help you do your sample during the appointment, will keep you hydrated and will make the sampling easier, especially if you need to give a urine sample. Do not hesitate to drink, because dehydration reduces the water presence in blood and the width of veins, which makes sampling more difficult for you and the nurse.


No, be it black coffee or with sugar and creme, coffee contains nutrients, which could falsify the results.


It is not recommended to smoke a cigarette before the appointment. It could have an effect on some results. Smoking can increase the white blood cells level in blood, so it is better to avoid smoking during the 10 to 12 hours before the sampling.


It is important to respect the 48 hours fasting frequently requested with blood tests. A single drink can alter results, causing a wrong interpretation by the doctor. The quality of the serum (blood) could be affected, which can abnormally increase triglycerides. Triglycerides allow the transportation of fatty acids, and are also a primary source of energy for the body.


Like cigarettes, cannabis consumption through inhaling (smoke) increases the number of white blood cells in the blood. Large cannabis intake could influence the testosterone level. Cannabis can also have effects on blood pressure, it is therefore important to avoid its consumption before a cardiac test like an ECG or a Holster.

Consumption can bring about a decrease in body temperature, which affects the venous system and makes blood sampling more difficult. It is recommended to avoid cannabis consumption, of all forms, 24 hours before the appointment.


It is recommended to take your medicine before the appointment like always, but some medicine can influence the results. We mention the exceptions during the appointment scheduling. Medical marijuana consumption is part of that category of medicine.


Sadly, some pregnancy routine tests require you to be fasting. However, the recommended fasting is only 8 hours in this precise situation.


It is recommended to avoid intense physical activities 24 hours before the appointment to avoid irregularities in the sampled blood. Creatine kinase can be falsely high and the PSA (prostate test) can also be high after a long period on a bicycle. However, there is no problem with light training.


For some, blood samples can be a source of stress. For both fear of needles or pain, it is advised to practice breathing for the blood sampling. By taking deep breaths through the nose and exhaling through the mouth, it is possible to decrease blood pressure.

It is possible to get the ‘’Emla’’ cream in pharmacies. It has to be applicated on the skin of the creases of the elbow about 30 minutes before your appointment to desensitize the skin and therefore reduce pain.

If you have receding veins, this simple advice can make the experience much easier: simply warm up the elbow area with a magic bag to dilate the veins. This will make the veins ‘’expand’’ and will facilitate the sampling.

It is possible that after all those steps the stress will still be there, so we recommend our at-home services with no additional fees to make the experience easier.

During the appointment

No matter if it is at home, in a company or in a clinic, we offer a calm and comfortable environment to make the experience better. Calm and tranquility make blood sampling easier, so it is recommended to inhale well through the nose and exhale slowly through the mouth to relax. Do not be afraid to communicate with the nurse to reassure yourself about any possible questions. Our nurses are qualified and near the patients.


At JMA Diagnostics, we have a policy of two stings per appointment. When conditions are not optimal for blood sampling, we move the appointment to the next earliest date or following your preferences. It prevents appointments from being too painful and prevents negative blood sampling experiences.


Some tests do not ask for fasting, but must be done in the morning or afternoon. Some results can change depending on the time of the day. For example, a serum cortisol test is often required in the start and the end of the day, because the result varies depending on the time of the day.


It is not necessary to fast for urine tests. To be fasting or not has no influence on the result. However, some tests need to respect some restrictions, like quantity, delay between two urines or the first stream or the half stream.


There are no food restrictions for stool tests. You can keep your eating habits with no worry. However, it is recommended to avoid laxatives during the collecting period.

After the blood test


Whether the sampling asked for a fasting or not, it is recommended to have a small bite after the blood test. No matter if it is a muffin or a tender bar, it is better to accompany it with water.


It is possible that a bruise forms after the blood sampling, it is a completely normal reaction. You do not have to worry. Applying pressure on the sting after the sampling reduces the risk of bruises.


If you are prone to reflex syncopes (pressure drops), we recommend to sit or lie down after the samples are taken. This makes you avoid drops and it accelerates recuperation.


When you choose JMA Diagnostics, the results will be transmitted to you and your doctor in the following 24 to 48 hours for the vast majority of the tests. Some specific tests require 6 open days, but they are rare.


To bring up the important parts one more time, it is mandatory to follow the fasting requested for the blood tests. You need to avoid eating or drinking something other than water from up to 12 hours before the appointment. This means no coffee or cigarettes. You also have to avoid alcohol 48 hours before the blood sampling for cholesterol. Medicine intake is done as usual, except if a health professional indicates the contrary.

24 hours before the appointment, you need to avoid physical activity and stress. Controlling your breathing can help manage stress before and during the appointment. Share your expectations and fear to the nurses, they will know how to reassure you. After the appointment, eat a little and drink a glass of water.

Our by appointment services improves the blood sampling experience because you know the exact hour when you need to start fasting. Be it in the comfort of your home, in our clinics or in the office, the JMA Diagnostics team makes sure to offer a professional service of quality, respectful towards the person.

For an appointment or more information, contact us!