Table of contents:
The fasting time may vary according to the exam to be performed, and fasting may not be necessary, or a fasting period of 4, 8 or 12 hours may be indicated. Fasting for blood tests is very important and must be respected when necessary, as the intake of some foods can interfere with the results of some tests. In general, fasting can be classified into:
- Mandatory fasting,in which the exam is only performed with the person fasting;
- Desirable fasting,in which fasting is not mandatory, but if it is performed more accurate results can be obtained. However, performing these tests without fasting also provides adequate results;
- Fasting not mandatory,in which the person can perform the exam with or without fasting, without interfering in the results.
The fasting time may vary according to the laboratory in which the exam will be performed, as well as which exams will be performed on the same day, and therefore it is important to seek medical or laboratory advice about the time of fasting required.
Fasting time for blood test
The following table shows the fasting times for the main blood tests requested.
| Exam | Fasting time |
|---|---|
| Hemogram | Fasting not mandatory, but desirable for 4 hours |
| Glycemia | Mandatory fasting of 8 to 12 hours |
| Glycated hemoglobin | Fasting not mandatory |
| Total cholesterol and fractions/ lipidogram | Fasting not mandatory, but fasting for 8 to 12 hours may be indicated by the doctor |
| TSH | Fasting not mandatory |
| T4 | Fasting not mandatory |
| Serum iron | 8-hour mandatory or non-mandatory fasting, depending on laboratory analysis method |
| Ferritin | Fasting not mandatory, but desirable and 3 hours, and it is also recommended to suspend the use of biotin and food supplements containing biotin 3 days before the exam |
| Vitamin B12 | Fasting not mandatory |
| Creatinine | Fasting not mandatory |
| Vitamin D | Fasting not mandatory |
| PSA (free and full) | Fasting not mandatory |
| Total bilirubin and fractions | Fasting not mandatory but desirable for 4 hours |
| C-reactive protein (CRP) | Fasting not mandatory |
| TGO/AST | Fasting not mandatory |
| TGP/ALT | Fasting not mandatory |
It is important to take into account that the fasting time of some exams may be different according to the laboratory analysis method and the doctor's orientation, who may indicate that the exam be performed with more or less time of fasting according to the purpose of the analysis and the person's he alth condition.
Common questions
The following are the main questions related to fasting to take a blood test:
1. Is it allowed to drink water during fasting?
During the fasting period it is allowed to drink water, however only enough water should be ingested to quench thirst, as excess can change the test result.
However, other types of beverages, such as soft drinks, teas or alcoholic beverages, should be avoided, as they can cause changes in blood components.
2. Is it necessary to stop taking medication?
It is not necessary to stop taking medication to have a blood test, unless it is recommended by your doctor. However, it is important to inform the laboratory about the use of any medication, so that it is possible to take this factor into account at the time of analysis.
3. What to do when exams have different fasting times?
If the doctor recommends performing tests with different fasting times, follow the instructions for the test with the longest fasting time. For example, if a blood count (non-mandatory fasting) together with glucose (mandatory fasting from 8 to 12 hours) has been indicated, it is recommended to adopt a fasting period of 8 to 12 hours, because this way it is possible that the glucose measurement happens correctly and fasting does not interfere with the result of the blood count.
However, in some cases, if fasting is very prolonged, the laboratory may indicate that some tests are not performed, and the person should return another day for their performance.
4. Can I consume alcohol the day before the blood test?
No, because alcohol can directly interfere with glucose and triglyceride levels, influencing the test result. Thus, it is recommended that alcoholic beverages be avoided at least 72 hours before the exam.
5. Can I exercise before the blood test?
It is recommended to avoid rigorous physical activity the day before the exam, as exercise can promote a decrease in glucose and triglyceride levels, as well as favoring the formation of metabolites such as lactic acid and creatine phosphokinase (CPK)), which circulate in greater concentration, directly interfering with the test result.
In addition, in the case of PSA measurement, for example, sexual activity should be avoided in the 3 days prior to the exam, in addition to situations that may increase PSA levels, such as riding a bicycle and taking some medicines, for example.
6. Does the cigarette interfere with the test result?
Yes, especially in terms of glucose levels and, therefore, it is recommended to avoid its use before the blood test.
