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:

Test Abbreaviation of the test Test number

A-vitamin

fS-A-Vit 1113

Adrenocorticotropin

P-ACTH 1020

Angiotensin1-1convertase

fS-ACE 1092

Ammonium-ion

fP-NH4-ion 1071

Aldosterone

fP-Aldos 6336

B6-vitamin

fP-B6-Vit 6080

B1-vitamin

fP-B1-Vit 4948

E-vitamin

fS-E-Vit 3169

Gastrin

fS-Gastr 1448

Gastropanel

fP-GastPan 9225

Gastropanel, concise

fP-GastPSu 9293

Glucose, fasting

Pt-Gluk 1468

Glucose test, oral, short

Pt-Gluk-R1 1483

Glucose test, oral, short (0,2h)

Pt-Gluk-R 14831

Glucose test, oral, long

Pt-Gluk-R2 1484

Folate, erythrocytes

fE-Folaat 1414

Folate, serum

fS-Folaat 1416

Calcitonin

fS-CT 2008

Growth hormone

fS-GH 2035

Lactose test

Pt-LaktR 21971

Parathyroid hormone, intact

fP-PTH 4560

Pepsinogen I

fS-Pepstin1 2464

Iron

fP-Fe 4529

Renin, laying down

fP-Renin-M 3627

Renin, standing up

fP-Renin-P 3628

Renin, concentr.

fP-Reninmr. 6338

Fatty acids, free

fS-FFA 2561

Transferrin

fS-Transf 2756

Read more:

Instructions to prepare for a laboratory test