Can’t drink water before a physical examination? This “physical examination notice” is not only useless, but also harmful!

As far as the current situation is concerned, fasting before a physical examination is still necessary in quite a few cases; but no matter what the physical examination is, there is no scientific reason to prohibit drinking water.

Written by | Wang Chenguang (PhD in Biology, former professor of Peking Union Medical College) , Li Changqing (PhD in Medicine, practicing physician in the United States)

A friend recently had a physical examination and found that his blood pressure was much lower than usual. He asked me why. I replied that it might be because he did not drink water before the physical examination (low blood pressure in the morning for healthy people is usually related to dehydration). My friend asked how could he drink water during the physical examination? He was required to be on an empty stomach, not to eat or drink, and he sent me the physical examination instructions from the physical examination institution:

Dear customers: Please eat a light diet, do not drink alcohol, do not overwork, and take enough rest the day before the physical examination. On the morning of the physical examination, please fast, do not eat or drink water, and do not take any medicine (except hypoglycemic and antihypertensive drugs).

Most people in China have been asked to fast for a physical examination, and many people have to go hungry while waiting in line. Similar situations are also common among hospitalized patients, because most hospitals in China have their blood drawn in the morning, and patients are also required not to eat or drink before the blood draw. Occasionally, patients who have eaten or drunk before the blood draw will have their blood drawn.

So is it true that you can’t eat or drink during a blood test? This question can be broken down into several parts.

Before a physical examination and blood drawing, you should not only not abstain from water, but drink more water

Can I drink water before having my blood drawn? The answer is: Yes, provided that I drink only water in moderation and avoid alcoholic beverages, juices, sodas, and other beverages with added sweeteners.

Drinking tea and coffee does not affect most blood test indicators, provided you drink enough water, because tea and coffee are both diuretics. Studies have found that caffeine may affect thyroid function, so for blood tests involving thyroid indicators, it is not recommended to drink caffeinated beverages before blood draw. In addition, caffeine also affects blood pressure and heart rate - so the best choice is to just drink plain water.

Alcohol affects liver and kidney function, and drinking alcohol does not quench thirst or replenish water, but instead causes dehydration. Not only should you not drink alcohol before a physical examination, it is generally recommended not to drink alcohol within 24 hours. Alcoholic beverages 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, kidney function, blood pressure, etc. The rule of not drinking water not only does not help improve the accuracy of the test results, but reduces the accuracy. This is because the blood is concentrated in a dehydrated state, which will increase the blood cell count. Some people may even have abnormal kidney function indicators due to dehydration.

The reason for not drinking water before a physical examination in China may be due to the concern about affecting blood volume (referring to the effective blood volume circulating throughout the body). In fact, drinking water in general is mainly to replenish water loss, and will not lead to an unlimited increase in blood volume. It will not increase beyond a certain level, and drinking until the stomach is full is useless. However, dehydration will lead to a decrease in blood volume and blood pressure. In this case, peripheral blood vessels will compensatorily contract to maintain blood supply to important organs such as the heart and brain. If the adjustment is not timely, it may cause fainting due to transient cerebral ischemia.

Studies have shown that dehydration can affect many blood parameters. The first thing to be affected is the hematocrit. This refers to the volume of red blood cells (RBC) in the blood and is an item in a blood test. 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 state of dehydration to a certain extent in the morning, and this indicator is likely to be higher than the normal range when doing a blood test at this time. But this does not mean that there is a problem with your health. As long as you drink enough water, the hematocrit will return to normal.

If a person is severely dehydrated, hemoglobin will also increase accordingly. This is because hemoglobin and hematocrit are calculated based on the total blood volume, and a decrease in fluid components will inevitably lead to a corresponding increase in other components.

Dehydration affects kidney function in many ways. The loss of water in the body can increase blood osmotic pressure and induce the release of vasopressin, which can lead to concentrated urine and affect test results. The two most common indicators of blood tests used to diagnose kidney disease and evaluate kidney function are creatinine and blood urea nitrogen (BUN). Excessive elevation of these two indicators indicates that there is a problem with kidney function, because these two substances are waste products produced in the blood by the body's metabolism and ultimately need to be excreted from the body in the form of urine from the kidneys. Dehydration reduces blood flow to the kidneys and increases the levels of urea nitrogen and creatinine in the blood.

If you don't drink enough water before the blood test, it will also cause difficulties in the examination. When the body is in a state of dehydration, it will be more difficult for medical staff to find a vein. Many physical examinations also require urine samples. In a dehydrated state, the urine volume decreases and the urine will also be concentrated. Some people save urine for physical examinations, worrying that they will not be able to urinate at that time, so they deliberately get up in the morning and don't go to the toilet, and then urinate when the physical examination is performed. This means that they have to endure the double torture of holding urine and thirst.

Fasting before a medical examination: conditions for validity and non-validation

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, etc. are not affected by eating. What will be affected are mainly fasting blood sugar and blood lipid tests, which will increase after eating. The purpose of blood sugar testing is mainly to screen and follow up diabetes, while the purpose of blood lipid testing is to screen and follow up hyperlipidemia.

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

In this case, for the purpose of diagnosing and following up diabetes and hyperlipidemia, is it necessary to fast before drawing blood?

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

For asymptomatic diabetes, two of the following three indicators are required for diagnosis: random blood sugar not less than 200mg/dl (11mmol/L); blood sugar 2 hours after a meal not less than 126mg/dl (equivalent to 7mmol/L), and glycosylated hemoglobin (HbA1c) not less than 6.5%. If only one indicator is met, the test needs to be repeated the next day.

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

Let’s talk about the diagnosis of hyperlipidemia. Blood lipids in physical examinations are not a single indicator, but include multiple indicators such as high-density cholesterol, low-density cholesterol, triglycerides, etc. Among them, high-density cholesterol is a kind of blood lipid that is beneficial to the human body, and this indicator is little affected by eating. Low-density cholesterol and triglycerides are mainly affected by eating, especially foods that contain more oil. High levels of low-density cholesterol and triglycerides in the blood are considered to be unfavorable factors for health, but the direction of the unfavorable situation varies. The increased risk of cardiovascular disease mainly comes from low-density cholesterol, and the risk of triglycerides is mainly hyperlipidemia pancreatitis.

Testing blood lipids after eating will cause errors in the measurement of low-density cholesterol. In fact, it is more of a calculation error than a measurement. Many blood lipids in the blood exist in the form of chylomicrons. When fasting, the ratio of low-density cholesterol to triglycerides in chylomicrons is about 1:5. After eating, especially after a high-fat diet, this ratio can be reduced to 1:20. If you still calculate according to the 1:5 ratio after eating, especially when triglycerides are high, the calculated low-density cholesterol value will be very different from the actual value. Some laboratories can directly measure low-density cholesterol. In this case, the impact of triglycerides can be ignored and the test can be done after a meal.

Therefore , whether fasting is required for blood lipid testing depends on the test target . If the patient has a history of high triglyceride blood lipids and a family history, fasting is recommended. If the test target is low-density cholesterol, it depends on whether the laboratory conducting the test is conducting actual measurement or calculation. If it is an actual measurement, then fasting is not necessary.

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 physical examinations of healthy people, indicators such as blood sugar and blood lipids that are more affected by diet are usually required. As far as the current technical status is concerned, fasting is still required. In addition, physical examination items usually also include abdominal ultrasound, digestive endoscopy, etc., which also need to be performed on an empty stomach. It is prohibited to eat anything other than drinking water. This is also reflected in the precautions before physical examinations and blood draws in mainstream medical institutions in other countries and regions.

But no matter what kind of physical examination, the rule of prohibiting drinking water is unreasonable. In China, almost all physical examination institutions and hospital laboratories list drinking water and eating as prohibited items before blood drawing. From a scientific point of view, prohibiting drinking water before blood drawing is not only unhelpful, but also has many harmful effects, and it should be removed from the blood drawing requirements as soon as possible.

This article is supported by the Science Popularization China Starry Sky Project

Produced by: China Association for Science and Technology Department of Science Popularization

Producer: China Science and Technology Press Co., Ltd., Beijing Zhongke Xinghe Culture Media Co., Ltd.

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