How deep should blood be drawn for disease screening?

The needle depth for routine venous blood collection for disease screening is usually in the range of 0.5 to 1.5 cm, while the needle depth for peripheral blood collection is only 1 to 2 mm. The specific value will vary depending on the blood collection object, blood collection site, and operating habits. There is never an absolutely unified standard answer.

Damn, I have been working as a blood collection nurse at a grassroots screening site for almost three years. I have seen too many people who ask "How deep do you want to pierce me" as the first thing I ask when I sit down. Last time, there was a little girl holding her sleeves to hide, saying that the prick was painful for a week during the previous physical examination. The nurse must have pricked it too deeply. I pinched her slender arm, and the blood vessels were floating on the surface of the skin. I smiled and said to her, "I'll give you a 0.5cm prick, which is too much. I'll make sure it only hurts." Finally, I injected 0.3cm and saw blood coming back. After taking two tubes of blood, she didn't even frown.

Don't tell me, this level of depth really depends on who chooses to choose. If you encounter a subject who weighs 200 pounds and has subcutaneous fat on his arm that is two centimeters thick when pinched, the blood vessels are buried deep and it takes a long time to touch. It is common for blood to return after the needle is inserted 1.5cm. Even when encountering particularly difficult-to-find blood vessels, it is not uncommon to have to penetrate to 2cm under ultrasound guidance. For children or elderly people with particularly thin skin, the needle has to be inserted shallower. For children, 0.3~0.8cm is usually enough. Blood vessels in the elderly are fragile. If the needle is inserted deeply, it will easily pierce the back wall of the blood vessel, causing bulges and bruises. People who have been exercising all year round have another situation. The subcutaneous fat is less and the blood vessels are slippery, which looks obvious. Instead, the angle and depth should be controlled when inserting the needle. If it is too shallow, it will easily slip away, and if it is too deep, it will stick into the muscle.

Oh, by the way, don’t cover all blood collection items with the depth of venous blood collection. If your screening only requires fingertip blood collection, the depth of the prick is less than one-tenth of that of venous blood collection. Generally, 1~2 mm is enough to break the skin and squeeze out the blood. However, if the blood prick is deeper, it will easily damage the periosteum of the phalanx and cause pain for several days. You must be careful when collecting heel blood for newborn screening for genetic metabolic diseases. The depth of the needle must not exceed 2mm, otherwise it will easily damage the cartilage of the heel and affect its development.

Clinically, there are actually two different operating habits regarding the depth of needle insertion. After so many years, there is no absolute right or wrong. One school of thought is "stop as soon as blood is seen." Stop inserting the needle as soon as you see blood returning to the end of the needle. This method causes the least damage to the blood vessels and will hardly puncture the back wall of the blood vessels. It is also not easy to bruise after drawing blood. It is especially suitable for the elderly and children with poor blood vessel conditions. However, if you need to collect several tubes of blood, occasionally the bevel of the needle will come out of the blood vessel and it will have to be re-tied. The other group is used to "insert the needle 2~3mm after the blood has returned" and send the entire bevel of the needle into the blood vessel. The blood collection process is very stable and there will be no problem if four or five tubes are collected. It is suitable for adults with thick blood vessels and tight subcutaneous tissue. As long as the operation is done properly, it will not increase too much pain. To be honest, when we practice the operation in private, we always say that the feeling of inserting the needle is very much like poking a soft-boiled egg. When the eggshell breaks open and encounters the resistance of the egg white, and then goes a little further inside and encounters a soft flowing feeling, it means that the blood vessel has been inserted. Stop when you need to, and you really won't get stuck with the ruler to measure the depth.

There is also a misunderstanding that many people have. They think that the deeper it is inserted, the more painful it will be. This is not true. The pain is mainly related to the speed of needle insertion and whether it touches the nerve endings. Sometimes it seems that the needle is shallow, but it just hits the ending of the cutaneous nerve, but it will hurt more than if the needle was inserted deeper. Last time, there was a young man with very good blood vessels. He didn't feel anything when I inserted the needle almost 1cm in. He even asked me, "Did you take the injection?", which really amused him.

To put it bluntly, you really don’t need to worry about how deep to pierce your skin when you go for screening. Nurses in formal institutions have learned this through hundreds or thousands of practical exercises. Relaxing when you expose your arms and don’t tense your muscles is much more effective than repeatedly asking “how deep to pierce.” If you are really afraid of pain, tell the nurse in advance and they will try to choose the shallowest suitable depth for you. There is no need to worry in advance.