Laboratory tests, which require fasting

Fasting samples

With some exceptional cases a laboratory test requires an 8-12 hour fast before taking the sample. A fasting sample can be identified by the little f letter at the beginning of the abbreviation of the laboratory test (for example fP-Gluk). However, there’s no reason for fasting, unless it has been expressively instructed and there’s the ‘f’ prefix in the abbreviation of the test.

During the fast before the laboratory test, one cannot drink a lot of water or any drinks with energy, such as juices or alcoholic beverages. Coffee, tea and cigarettes can also affect the fasting sample, so it is good to avoid them 8 to 12 hours before taking the sample. A glass of water can be had without a worry, but bigger amounts of liquids can affect the results, for example by diluting blood and urine.

Why is fasting needed before a laboratory test?

Food affects a lot of the concentrations of substances’ in the blood, for example blood sugar, meaning the concentration of glucose increases after a meal. This is, because the sugar from the food and the glucose from the starch are absorbed from the intestines to the bloodstream. The blood sugar reference, meaning the normal value, is different during fasting than it is straight after a meal.

Glucose value, which is measured after a meal is challenging in laboratory tests, since it is affected by the size of the meal, the quality of the meal, as well as the time passed since the meal. For example, when trying to determine if diabetes is in question or not, the blood sugar level, meaning the glucose in the plasma is always examined from a fasting sample. Other tests, which require fasting are for example cholesterol, triglyceride and bile acid tests.

Tests in which fasting is necessary:

Read more:

Instructions to prepare for a laboratory test