Can I eat and drink before having a blood test ?

If you are having a "fasting blood test", you cannot eat or drink anything (other than water) for 8 or 12 hours prior to the blood collection. It is also advised not to smoke during this period.

If you are having a "fasting blood test", you cannot eat or drink anything (other than water) for 8 or 12 hours prior to the blood collection. It is also advised not to smoke during this period.

Most test results will not be altered after a meal. However, ingesting food may completely skew some results, particularly those related to carbohydrates and fats. Other parameters may fluctuate after a meal, but usually less significantly.

Fasting is ESSENTIAL if your prescription includes one of the following tests:

  • Calcium, phosphorus (or a calcium and phosphorus test)
  • Blood glucose, insulin, C-Peptide
  • OGTT (oral glucose tolerance test)
  • Triglycerides, total cholesterol, LDL, HDL, A or B apolipoprotein
  • Fatty acids
  • Homocysteine
  • Gastrin

Fasting is PREFERABLE if your prescription includes one of the following tests:

  • Blood electrolytes: Na, K, Cl
  • Iron, transferrin saturation
  • Serum folates, erythrocyte folates
  • Sedimentation rate
  • White blood cells + count
  • Albumin, proteins, electrophoresis
  • BUN, creatinine, uric acid
  • CA 125, Ca19.9, CA 15.3, PSA
  • Alkaline phosphatases, transaminases (for other enzyme tests, it is not necessary to be fasting, but it is important to avoid postprandial periods (3 hours after a meal) because lactescent serum may falsify results)

In specific instances, your doctor will ask you to take a blood test after a meal. You must follow his recommendation.

Some elements are not modified by meals, but their levels can vary widely based on the time of the sample collection (circadian rhythm or day/night cycle). This is the case for many hormones.

If one of these elements is being tested, it is essential for the sample to be systematically collected at the same time of day.

List of elements that must be sampled in the morning (unless otherwise instructed by the prescribing doctor):

  • Crosslaps (CTX)
  • Aldosterone, renin
  • Cortisol, ACTH

List of elements that should preferably be sampled in the morning (unless otherwise instructed by the prescribing doctor):

  • Testosterone
  • Prolactin
  • Thyroid function tests Adrenaline, noradrenaline
  • Iron
  • FSH, LH, Oestradiol

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