What you need to know about fasting test

"This examination requires you to fast. You cannot eat after 8 o'clock tonight. If you take oral antihypertensive drugs in the morning, you can only drink small sips of boiled water." I believe everyone has heard similar instructions from medical staff.

Why do you need to fast before some tests?

As far as blood tests are concerned, the normal reference value ranges for clinical test items are based on the test results of fasting blood samples from healthy people. 12 to 14 hours after a meal, the digestion and absorption activities of the gastrointestinal tract have basically been completed, and food factors have basically no effect on blood components. Therefore, the various biochemical components in the blood are relatively constant and stable. The various values ​​measured at this time can more truly reflect the biochemical changes of the body [1], which is helpful for the diagnosis of diseases.

In digestive tract ultrasound examination, fasting examination is also an effective measure to eliminate interference factors and ensure the reliability of examination results. First, eating and drinking will cause increased abdominal distension, which will block the examination site. Second, eating will cause gallbladder contraction. Some gallbladder polyps and even small stones are easily hidden in the folds formed by contraction [2], and cannot be identified by ultrasound.

What is considered “appropriate fasting”?

The “Guidelines for Venous Blood Sample Collection” issued by the National Health Commission in 2020 clearly stated that fasting blood draws should be scheduled between 7:00 and 9:00 in the morning, with a minimum fasting time of 8 hours, preferably 12 to 14 hours, but not recommended to exceed 16 hours [3].

So, how can we do the fasting check without any hassle?

Please read the precautions carefully before the examination. If you have any questions, be sure to consult relevant medical staff in advance, and follow the doctor's instructions for "appropriate fasting" and "effective examination."

【References】

Zhao Lin, Xu Hao. Do you need to fast for laboratory tests?[J]. Chinese Aging, 2021(19):49-50.

Yu Jing. Talk about fasting and holding urine during ultrasound examination[J]. Jiangsu Health Care, 2021(04):29.

Wang Beili, Guo Wei, Pan Baishen. Interpretation of the health industry standard "WS/T 661-2020 Guidelines for Venous Blood Specimen Collection"[J]. Chinese Medical Journal, 2021, 101(21): 1610-1613.

Author: Song Shiyao, head nurse, Bachelor of Medicine from Sichuan University.