Fasting for blood tests is very important and must be respected when necessary, as the intake of food or water can interfere with the results of some tests, especially when it is necessary to assess the amount of some substance that can be altered by food, such as cholesterol or sugar, for example.
The fasting time in hours depends on the blood test that will be performed, but some examples are:
- Glucose: It is recommended that 8 hours of fasting be done for adults and 3 hours for children; Cholesterol: Although it is no longer mandatory, it is recommended to fast for up to 12 hours in order to obtain results more faithful to the person's condition; TSH levels: It is recommended to fast for at least 4 hours; PSA levels: It is recommended to fast for at least 4 hours; CBC: Fasting is not necessary, as this test only evaluates components that are not altered by food, such as the number of red blood cells, leukocytes or platelets. Know what the blood count is for.
In the case of people with diabetes, who need to take blood glucose measurements several times a day, the times and time after eating should be guided by the doctor during the consultation.
In addition, the fasting time may vary according to the laboratory where the test will be performed, as well as which tests will be performed on the same day, and therefore it is important to seek medical or laboratory guidance about the fasting time. required.
Is it allowed to drink water while fasting?
During the fasting period it is allowed to drink water, however, only the amount sufficient to quench thirst should be ingested, as the excess can alter the test result.
However, other types of drinks, such as sodas, teas or alcoholic beverages, should be avoided, as they can cause changes in blood components.
Other precautions before taking the exam
When preparing for a blood test for glycemia or cholesterol, in addition to fasting, it is also important not to perform rigorous physical activities 24 hours before the test. In the case of a blood test for PSA measurement, sexual activity should be avoided in the 3 days prior to the test, in addition to situations that may increase PSA levels, such as riding a bicycle and taking some medications, for example. Learn more about the PSA exam.
In all cases, on the day before the blood test, smoking and drinking alcoholic beverages should be avoided, as they influence the results of the analysis, especially in the measurement of blood glucose and triglycerides. In addition, some remedies, such as antibiotics, anti-inflammatories or aspirin influence the results of the blood test, and it is important to tell the doctor which remedies are used for guidance on suspension, if necessary, and for them to be taken into account. consideration at the time of analysis.
See also how to understand the results of the blood test.
