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This article comes from the official account of Wechat: back to Park (ID:fanpu2019), by Wang Chenguang and Li Changqing

As far as the current situation is concerned, fasting before a medical examination is necessary in a considerable number of cases, but in either case, the ban on drinking water does not make any scientific sense.

Written by Wang Chenguang (PhD in biology, former professor of Union Medical College), Li Changqing (MD, medical practitioner in the United States)

A friend recently had a physical examination and found that his blood pressure was much lower than usual. I asked the author about the reason, and the author replied that it may be because he did not drink water before the physical examination (low blood pressure in healthy people in the morning is usually related to dehydration). My friend said, where can I drink water for physical examination? It is required that you should not eat or drink on an empty stomach, and send the physical examination notes of the medical examination institution:

Dear customers: please eat a light diet the day before the physical examination, do not drink, do not be tired, pay attention to rest; in the morning of the physical examination, please fasting, fasting, water prohibition, do not take medicine (except hypoglycemic drugs, antihypertensive drugs).

Most people in China have the experience of being asked to have an fasting physical examination, so many people go hungry while waiting in line. A similar situation is also common among inpatients, because the time for inpatients in most hospitals in the country to draw blood is the same in the morning, and patients will also be required not to eat and drink water before drawing blood. Occasionally, a patient will be canceled after eating and drinking water before drawing blood.

So is it true that you can't eat or drink for blood tests? This question can be broken down into several parts.

Not only should we not ban water before physical examination and blood sampling, but should we drink more water before blood test? The answer is: yes, and drink as much as you want, as long as you drink only white water and avoid alcoholic drinks, fruit juices, sodas and other drinks with sweeteners.

Drinking tea and coffee does not affect most blood tests, as long as you drink enough water, because tea and coffee are diuretics. Some studies have found that caffeine may affect thyroid function, so blood tests involving thyroid indicators are involved, and caffeinated drinks are not recommended before blood sampling. Caffeine also affects blood pressure and heart rate-so the best option is to drink only white water.

Alcohol affects liver and kidney function, and drinking alcohol does not quench thirst and replenish water, but can lead to dehydration of the body. Not only can not drink alcohol before the physical examination, it is generally recommended not to drink alcohol within 24 hours. Alcoholic drinks often contain sugar or certain electrolytes, which can affect the accuracy of blood tests.

Drinking water does not affect the test results, but not drinking water may affect many test results, including blood routine, renal function, blood pressure and so on. The rule of not allowing drinking water not only does not help to improve the accuracy of the test results, but reduces the accuracy. This is because blood concentration during dehydration can lead to a relative increase in blood cell count, and some people may even have abnormal renal function indicators as a result of dehydration.

The domestic ban on water before physical examination may be due to the fear of affecting blood volume (refers to the effective circulating blood volume of the whole body). In fact, the general drinking water is mainly to supplement the loss of water, will not lead to an unlimited increase in blood volume, to a certain extent, can not increase, drink stomach support is also useless. However, dehydration can lead to a decrease in blood volume and blood pressure, in which case peripheral blood vessels contract compensatively to maintain the blood supply of important organs such as the heart and brain. If it is not regulated in time, it may lead to syncope due to transient cerebral ischemia.

Studies have shown that dehydration affects a number of blood indicators. Hematocrit is the first to be affected. This refers to the volume of red blood cells (RBC) in the blood, which is a part of blood routine. Dehydration is the most common cause of high hematocrit because the volume of fluid in the blood decreases, resulting in an increase in the number of red blood cells per unit volume of blood. After a night's sleep, the human body is usually in a certain degree of dehydration in the morning, when the blood routine is easy to appear that this index is higher than the normal range. But this does not mean that there is a health problem. As long as you drink enough water, the hematocrit will return to normal.

If a person is severely dehydrated, hemoglobin increases accordingly. This is because hemoglobin and hematocrit are calculated based on whole blood volume, and a decrease in liquid composition will inevitably lead to a corresponding increase in other components.

Dehydration affects renal function in many ways. The loss of water in the body will lead to an increase in blood osmotic pressure and induce the release of vasopressin, resulting in urine concentration, thus affecting the test results. The two most common indicators used by blood tests to diagnose kidney disease and evaluate renal function are creatinine and blood urea nitrogen (BUN). These two indicators are too high to indicate that there is something wrong with kidney function, because both substances are wastes produced by the body's metabolism in the blood and eventually need to be excreted from the kidneys to form urine. Dehydration reduces blood flow to the kidneys and increases blood urea nitrogen and creatinine levels.

If you do not drink enough water before the blood test, it will also bring difficulties to the examination. When the body is dehydrated, it will be more difficult for health care workers to find veins. Many physical examinations also require the retention of urine samples, dehydration, urine volume is reduced, urine will be concentrated. Some people leave urine for physical examination, worried that they will not be able to urinate, so they specially do not go to the toilet in the morning and leave it until the time of physical examination, so they have to endure the double suffering of holding back urine and being thirsty.

Fasting before physical examination: established and untenable conditions some routine physical examination items, such as blood routine, liver and kidney function, infectious disease indicators (mainly AIDS and hepatitis virus antibodies), thyroid function indicators are not affected by eating. The main factors that will be affected are the detection of fasting blood glucose and blood lipids, which increase after eating. The main purpose of testing blood glucose is to screen and follow up diabetes, while the detection of blood lipids is to screen and follow up hyperlipidemia.

Why do routine physical examinations generally check blood sugar and blood lipids? This problem must be clear to many people: once diabetes and / or hyperlipidemia are diagnosed, it means that there is an increased risk of cardiovascular, kidney and nervous system diseases in the future, requiring timely intervention, including diet, exercise and drugs.

Such being the case, is it necessary to have an empty stomach before drawing blood for the purpose of diagnosis and follow-up of diabetes and hyperlipidemia?

Let's start with diabetes. The diagnosis of diabetes is mainly divided into symptomatic and asymptomatic. The typical symptoms of diabetes are "three more and one less", that is, drinking, eating, urinating and weight loss. Most type 1 diabetes and a small number of type 2 diabetes have obvious symptoms. For patients with symptoms, only one index is needed, that is, the value of random blood glucose is not lower than 200mg / dl (translated into a more commonly used unit in China, that is, about 11mmol / L) to diagnose diabetes, and fasting blood glucose is not needed.

For asymptomatic diabetes, two of the following three indicators are needed to diagnose: random blood glucose is not lower than 200mg / dl (that is, 11mmol / L), postprandial 2-hour blood glucose is not lower than 126mg / dl (equivalent to 7mmol / L), and glycosylated hemoglobin (HbA1c) is not less than 6.5%. If only one indicator is met, the test needs to be repeated the next day.

At present, the common international norm is to use glycosylated hemoglobin to screen and monitor diabetes. The advantage of this index over blood sugar value is that it is relatively stable and can reflect blood sugar in the past few months, while blood sugar value can fluctuate instantly. If the random blood sugar of the examiner is above 200mg / dl and the glycosylated hemoglobin is higher than 6.5%, then diabetes can be diagnosed. Thus it can be seen that fasting blood glucose is not necessarily required to diagnose diabetes.

Let's talk about the diagnosis of hyperlipidemia. Blood lipids in physical examination are not a single index, but include high-density cholesterol, low-density cholesterol, triglyceride and other indicators. Among them, high-density cholesterol is a kind of blood lipid which is beneficial to the human body, and this index is little affected by eating. Eating is mainly affected by low-density cholesterol and triglycerides, especially foods that contain more oil. High levels of low density cholesterol and triglyceride in the blood are considered to be adverse factors for health, but the adverse direction is different. the increase in the risk of cardiovascular disease mainly comes from low density cholesterol, and the risk of triglyceride is mainly hyperlipidemic pancreatitis.

The detection of blood lipids after eating will bring errors to the measurement of low density cholesterol. In fact, it is not so much a measurement as a calculation error. Many of the blood lipids are in the form of chylous particles, the ratio of low-density cholesterol to triglycerides in chylous particles is about 1:5 on an empty stomach, and after eating, especially after a high-fat diet, this ratio can be reduced to 1:20. If the proportion is calculated at 1:5 after eating, especially if the triglyceride is high, then the calculated low density cholesterol value will be very different from the actual value. Some laboratories can directly measure low-density cholesterol, in which case the effect of triglycerides is negligible and can be examined after meals.

Therefore, whether the detection of blood lipids requires fasting depends on what the test target is. If it is a patient with a history of high triglyceride lipids and a family history, fasting examination is recommended. If low-density cholesterol is taken as the test target, it depends on whether the laboratory carrying out the test is measured or calculated. If it is measured, then there is no need for fasting.

For hospitalized patients, some blood tests are targeted and are not used to detect blood sugar and blood lipid levels. Most of these targeted indicators are not affected by diet, so there is no need to fast. For healthy people's physical examination, blood sugar and blood lipids and other indicators that are greatly affected by diet are usually compulsory examination items, in terms of the current technical status, it is still necessary to fast. And physical examination items usually include abdominal ultrasound, digestive endoscopy and so on, all of which need to be carried out on an empty stomach. Diet other than drinking water is prohibited, which is also reflected in the precautions of mainstream medical institutions in other countries and regions before physical examination and blood sampling.

However, in any case of physical examination, the ban on drinking water is unreasonable. In China, almost all physical examination institutions and hospital laboratory departments have listed drinking water and diet as prohibited items before physical examination and blood sampling. From a scientific point of view, refusing to drink water before blood examination is not only useless, but also has a lot of harm, which should be deleted from the requirements of blood examination as soon as possible.

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