How should you prepare for a blood test? How long should a patient fast before the scheduled time of sample collection? Does exercising, drinking coffee or smoking affect the results?

Blood tests—how to prepare? Do physical exertion, menstruation or medications affect the results?

Appropriate preparation for a complete blood count (more information about which can be found here: https://sklep.alablaboratoria.pl/badanie/7935/morfologia_krwi_c55_) is an important diagnostic factor that helps ensure reliable results. Most people know the general rules for having blood collected, such as arriving fasting or refraining from drinking alcohol. However, few patients are still aware of the effect that, for example, physical exertion, menstruation or medications can have on test results. It is worth remembering that:

  • before a complete blood count (and other tests performed using the collected sample), one should refrain from intense physical exertion. On the day before the test, it is advisable to skip any planned workout, and in the morning—before the blood is collected—even avoid a brisk walk to the laboratory. Physical exertion affects parameters such as cortisol, glucose and insulin levels, and even leukocyte and platelet counts;
  • menstruation itself does not significantly affect blood test results; however, if possible, it is advisable to have a complete blood count at another point in the cycle. Importantly, blood tests for sex hormone levels must be performed on a specific day of the cycle—therefore, in the case of such a test, one should always seek medical advice;
  • when it comes to medications taken regularly or as needed, blood is usually advised to be collected before the morning dose (if this is, of course, possible). The situation is different when blood tests are intended to determine changes in the concentration of the active substance contained in medications or the body’s response to the medications being taken. In that case, the doctor referring the patient for the blood test should provide instructions regarding whether the medications should be taken, their doses and the time they should be taken.

Blood tests—how many hours should you fast? How long should fasting last before blood is collected?

Why are fasting blood tests so important? Eating food the day before or on the day of the test affects many basic parameters analyzed in a laboratory using a blood sample. These include glucose and insulin levels, as well as triglyceride concentrations, liver parameters and even the urea concentration measured in blood.

The optimal interval between an adult patient’s last meal and blood sample collection is 12 hours. For younger people or patients with health problems, this period may be shortened to 8 hours (although any such changes should always be discussed with a doctor). It is advisable for dinner on the evening before the test to be light and easy to digest. The prohibition on consuming food also applies to products such as chewing gum and breath-freshening lozenges.

Preparing for a blood test: alcohol, smoking and caffeine consumption

As already mentioned, before blood is collected, patients should refrain from consuming alcohol (preferably for several days before the test). This substance affects, among other things, the complete blood count parameter known as MCV (mean corpuscular volume), the activity of liver and pancreatic enzymes, and blood glucose levels in general. This prohibition also applies to cigarettes—it is advisable to stop smoking the day before the test and, most definitely, from the morning of the blood collection. Nicotine can distort results covering lipid and diabetes profiles, stress hormone levels and other indicators. Caffeine also affects cortisol, adrenaline, cholesterol and glucose levels; therefore, on the day before the test it is advisable to limit, and on the day of the test to completely avoid, coffee, tea and energy drinks.

In the morning before blood is collected, you may drink a glass of still water without any additives.

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