How deep should blood be drawn for disease screening

There is no uniform fixed value for the needle depth of blood collection for disease screening. The needle depth of commonly used clinical blood collection (fingertips, newborn heel) is usually controlled at 1~2mm, and the needle depth of conventional venous blood collection is mostly in the range of 2~5mm. The specific value will vary depending on the blood collection object, blood collection site, and operating habits.

Everyone usually performs blood tests, rapid infectious disease screening, fingertip blood sugar and other projects, and most of them come across fingertip blood collection. In fact, the industry has always had different tendencies in the choice of this depth. When doing screening for childhood diseases or encountering subjects who expressly express fear of pain, many nurses will choose shallow puncture. The needle will be inserted about 1mm and just breaks the dermis layer. The amount of bleeding will be enough for rapid detection. The pain is basically the same as being bitten by a mosquito in summer. Especially for newborn heel blood screening, the operating regulations of the National Health and Medical Commission clearly require that the depth of the needle should not exceed 2mm, because it is afraid that the delicate periosteum will be damaged if the needle is penetrated deeply, and the child will suffer in vain. However, there are also many old nurses who work in physical examination centers and community outpatient clinics all year round who prefer to insert needles of 1.5~2mm. When I helped record the physical examination data of the elderly in the community, I heard them say that shallow punctures look gentle, but when it comes to middle-aged and elderly people or strong young men with thick skin stratum corneum, if enough blood cannot come out for a long time, they have to pinch their fingertips repeatedly. Squeezing, the squeezed blood is mixed with a lot of tissue fluid, which will lower the accuracy of indicators such as blood sugar and red blood cell count. There was a young man who came for diabetes screening before. After the first shallow prick, he squeezed three times to get enough blood. The result was that the blood sugar was 0.8mmol/L lower than the actual value. In the end, he pricked again, but it hurt a lot more.

After talking about peripheral blood collection, let’s talk about the venous blood collection that everyone is more afraid of - the kind where a tourniquet is tied on the arm and a three- to four-centimeter-long needle is used to draw blood. Many people get dizzy when they look at the length of the needle, thinking that it has to be inserted all the way in. They are really overthinking it. The depth of venous blood collection basically depends on the thickness of the subcutaneous fat. For thin girls or people who exercise regularly, blood vessels are clearly floating on the surface of the arm. Sometimes blood will return as soon as the needle is inserted 2mm. If the needle is inserted deeper, it will easily puncture the blood vessel, and the arm will be blue for several days. If you encounter a subject with a large body weight and thick subcutaneous fat, the blood vessels are buried deeply, and sometimes the blood vessels can only be touched after piercing 4 or 5 mm. This situation is also particularly common. There are different voices in the industry regarding the control of this depth. School textbooks generally clearly recommend that the depth of the needle should not exceed 5mm to avoid damaging the deep nerves and blood vessels. However, many veteran nurses with more than ten years of practical experience will say that if they really encounter people who are particularly fat and have blood vessels buried particularly deep, they can occasionally penetrate as low as 7mm. As long as the needle is inserted slowly and the blood returns are stopped immediately, there will be no problem. After all, the standards are set for most people, and special circumstances must be adjusted flexibly.

Many people think that the deeper the piercing is, the more painful it will be. In fact, this is not the case. The greater pain is related to the speed of needle insertion and whether it accidentally touches the nerve endings of the epidermis. Just like when you are packing things and accidentally scratch the edge of an A4 paper, you will wince in pain even though it only hurts the epidermis. If the nurse inserts the needle quickly and accurately, even if it is a little deeper, you may just feel a little itching and all the blood will be collected. I have met a legendary "flying needle" nurse before. Her hand speed is so fast that many people were stunned for two seconds after collecting venous blood and asked, "Is this the end?"

To be honest, there is really no need to worry about how deep the nurse will insert the blood before taking blood. Nurses in regular medical institutions have been systematically trained and will adjust the depth at any time according to your age, weight, and blood collection items. If you are particularly afraid of pain, or know that it is difficult to find blood vessels, just tell the nurse in advance. If you talk to each other more, you may be able to avoid suffering once and the results will be more accurate.