Preparing for laboratory tests
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Laboratory tests requiring fasting
Eating affects the levels of components in the blood, which is why it is important to fast before certain tests. In laboratory tests that require fasting, you should not eat or drink for 8–12 hours before the sample collection.
General information on laboratory tests requiring fasting
Usually, you do not need to avoid eating or drinking before taking the sample. In exceptional cases, laboratory tests are taken as a fasting sample in the morning after fasting overnight. Then, you should not eat or drink for about 8–12 hours before the test.
- You can see that your test is a fasting sample if there is a small f in front of the test name, for example, fP-Gluk, which comes from the English word 'fasting'. However, there is no reason to follow fasting unless expressly ordered to do so.
Do not drink large amounts of water or any energy-containing beverages, such as juices or alcoholic beverages, during the fasting period before giving the fasting sample. Coffee, tea and tobacco can affect the fasting sample, so it is advisable to avoid them 8–12 hours before taking the samples. You can drink a glass of water, but larger amounts of fluid may affect the results, for example, by diluting the blood and urine.
Why do you need to fast before the test?
Food affects the levels of many components in the blood. For example, the blood sugar, or glucose level, increases after a meal. This is because the glucose in sugar and starch in food is absorbed from the gut into the bloodstream. The reference, i.e. normal, blood glucose value is different after fasting than after a meal.
The glucose value measured after a meal is challenging as a test, as it is influenced by the size and quality of the previous meal as well as the time elapsed since the meal. For example, when you wish to find out whether you have diabetes, your blood sugar, or plasma glucose, is always tested from a fasting sample. Other tests requiring fasting include, for example, folate and bile acid tests.
Test for which fasting is necessary
| Test | Test abbreviation | Test number |
|---|---|---|
| Vitamin A | fS-A-Vit | 1113 |
| Adrenocorticotropin | P-ACTH | 1020 |
| Angiotensin 1–1 convertase | fS-ACE | 1092 |
| Ammonium ion | fP-NH4-ion | 1071 |
| Aldosterone | fP-Aldos | 6336 |
| Vitamin B6 | fP-B6-Vit | 6080 |
| Vitamin B1 | fP-B1-Vit | 4948 |
| Vitamin E | fS-E-Vit | 3169 |
| Gastrin | fS-Gastr | 1448 |
| Gastro panel | fP-GastPan | 9225 |
| Gastro panel, concise | fP-GastPSu | 9293 |
| Glucose, fasting | P-Gluk | 1468 |
| Glucose test, oral, short | Pt-Gluk-R1 | 1483 |
| Glucose test, oral, short (0.2 h) | Pt-Gluk-R | 14831 |
| Glucose test, oral, long | Pt-Gluk-R2 | 1484 |
| Folate, serum | fE-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, lying down | fP-Renin-M | 3627 |
| Renin, upright | fP-Renin-P | 3628 |
| Renin, concentrated | fP-Reninmr. | 6338 |
| Fatty acids, free | fS-FFA | 2561 |
| Transferrin | fS-Transf | 2756 |
