When we go for a blood donation many times, we hear that we should be "on an empty stomach". But what does that mean exactly? Is it enough just not to eat anything right before the test? Does eating a meal affect my laboratory test result? Find out before which test you should be "fasted" and why.

Usually saying that we should be"fasting"we mean that we should eat our last meal about 12 hours before blood sampling. This is to exclude improper preparation of the patient for the examination. Consumption of a meal before the test will have a direct impact on most laboratory parameters.

Being "fasted" - why can't you eat before the test?

Eating even a small amount of a meal before laboratory tests may affect the results. Why? An example is glucose, which should be within a certain range of values ​​when fasting.

Eating a meal causes a rise in glucose and testing immediately afterwards would be too high, indicating impaired glucose tolerance.

In addition, eating a meal (especially when it is high in fat) causes a significant increase in blood triglycerides. What, apart from distorting the determination of this parameter, may cause interactions with some reagents used in the laboratory.

Fasting and drinks

Being “fasted” also includes limiting your intake of fluids before testing specific fluids. In the morningbefore the test, you can drink a glass of still water . It is even advisable, as it will facilitate blood collection, especially when we need to perform several tests at the same time.

However, more is not recommended as it may lead to overhydration and false results of some tests, e.g. hematocrit. In fact, it is best to drink enough water the day before your test.

It is forbidden to drink on the day of the test:

  • juice (even freshly squeezed),
  • teas,
  • coffee
  • and water with lemon or honey.

Coffee and tea contain caffeine, which can affect, for example, glucose or cortisol levels. In addition, on the day of the test, do not even chew gum, as it may also affect the test results.

Fasting and stimulants

Being "fasting"it also includes abstaining from alcohol consumption, both on the day of the test and the day before. On the day of the test, smoking is not recommended, because the nicotine contained in cigarette smoke may increase the concentration of glucose and cortisol.

Many patients wonder if they can take the medications and / or supplements they usually take before the test.

There is no clear answer here, so it is best to consult your attending physician before the test and ask if you can take medications and / or dietary supplements before the test.

Testing to be performed "on an empty stomach"

Remember, however, that not all tests must be performed on an empty stomach. Laboratory tests thatshould absolutely be performed on an empty stomachare:

  • glucose,
  • lipidogram (triglycerides, total cholesterol, LDL and HDL cholesterol),
  • iron,
  • total iron binding capacity (TIBC),
  • total protein,
  • liver tests (ALT and AST),
  • INR clotting time,
  • electrolytes (sodium, potassium, magnesium, chlorides),
  • vitamin B12,
  • urea,
  • creatinine,
  • uric acid,
  • insulin,
  • prolactin,
  • cortisol.

Tests that do not need to be performed on an empty stomach

However, tests thatdo not have to be performed on an empty stomachare:

  • blood group test,
  • glycated hemoglobin (HbA1c),
  • CRP protein,
  • testing the concentration of specific antibodies, e.g. IgM and IgG against Lyme disease, any-TPO.
  • Blood count: how to read the result
  • Blood morphology - what this blood test is about
  • Blood test - how to prepare for it

Category:

Research