Home » Why do you have to go fasting for a blood test? Can you drink water?

Why do you have to go fasting for a blood test? Can you drink water?

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Why do you have to go fasting for a blood test?  Can you drink water?

05/21/2023 at 20:00


Pregnant women and patients with poorly controlled diseases should have a blood test every three months

Specialists recommend performing a blood test at least once a year to be able to know the state of health of our body and, above all, if everything works correctly.

And it is that in an analysis it can be verified:

  • The levels of red and white blood cells and platelets, cholesterol, blood glucose, iron, thyroid enzymes…

However, there are patients who must have a general check-up every six months. It is the case of patients diabetics, those over 65 years of age, or those with comorbidities, that is, they have two or more diseases at the same time.

This frequency can be reduced to three months in pregnant women or people with certain pathologies that are not fully controlled.

  • Therefore, as explained by the Spanish Society of Primary Care Physicians (SEMERGEN), “the frequency with which analytical controls must be carried out is very variable and largely depends on the profile of the patient before whom we are”.

Do medications alter the results of a blood test?

It is one of the most worrying issues among patients who take medication on a daily basis.

And the reality is that it is important to notify both the doctor and the laboratory of the treatment being followed, to confirm if it will interfere with the results of the blood test.

  • The taking painkillers and anti-inflammatories can alter liver enzymes (if the liver is working properly) or creatine (measure kidney function), as indicated by SEMERGEN.
  • Oral corticosteroidson the other hand, they can alter upwards blood glucose numberswhich could indicate that you have diabetes.
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Fasting before an analysis: myth or truth?

Fasting before a blood test is essential.

  • Experts indicate that for a normal blood test, fasting time should be around 8-12 hours.
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Otherwise, parameters such as glucose, cholesterol or triglycerides may appear altered in the results.

  • So it is recommended that you have an early dinner the night before, between 8:00 and 9:00 p.m., and perform the blood extraction first thing in the morning.

Another of the most repeated questions among patients who are going to undergo a blood test is whether they can drink water before the test.

As already mentioned, respecting fasting is essential so that the results obtained are reliable and are not altered by food intake during the previous hours.

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But, nevertheless, it only refers to the consumption of solid and liquid foods, such as coffee (with or without sugar), tea… and not water. During fasting to perform a blood test you can drink water, in small amounts and in moderation.

Otherwise, parameters such as:

  • Red blood cell count (if it is high it may be a sign of polycythemia vera, a disease of the bone marrow).
  • Hemoglobin (which if the level is abnormally low can be a symptom of anemia, which can result in shortness of breath, fatigue, weakness, pale skin or cold hands and feet).

And what about the urinalysis? Do you have to fast?

The collection of the urine sample is carried out by the patient himself at home, so it does not require any prior preparation.

You should also fast for about eight hours, between dinner (if possible light) and first thing in the morning.

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You have to collect the first urine in the morning, avoiding the first stream, with a minimum retention of four hours.

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Beforehand, genital hygiene should be carried out and the sample kept refrigerated and taken to the health center as soon as possible.

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