Why are some people's veins 'hard to find'?

Most people will take a blood test at some point in their life, and commonly, this requires blood to be drawn from the vein on the inside surface of the elbow. However, this simple procedure can sometimes become a bruise-inducing ordeal when health care providers struggle to locate patients' veins.

But why are some people's veins so "hard to find?"

There are several reasons why patients may have been told their veins are difficult to locate, including the depth and size of their veins, whether they were dehydrated and their stress levels, Dr. Mark Whiteley, a consultant venous surgeon and phlebologist at The Whiteley Clinic, a vein treatment center chain in England, told Live Science in an email.

However, people's veins can also be hard to find due to external factors, such as the technique used to draw blood or the temperature of the room, Whiteley said.

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"The standard technique for doing this [drawing blood] is to place a tourniquet on the upper arm around the biceps, with the aim to dilate the veins in the lower arm," he said. Once those veins are dilated, or widened, blood can be drawn using a "hypodermic" needle that's inserted beneath the skin, and a syringe. "However, if a tourniquet is placed too tight or too loose, veins may remain hidden," he said.

Arterial blood is pumped down the arms through the arteries at high pressure, while venous blood flows back out of the arms at a lower pressure. "If the tourniquet is too tight, no blood can get into the lower arm and so the veins do not fill," Whiteley said. "If the tourniquet is too loose, venous blood can leak out under the tourniquet … resulting in deflated veins."

Temperature is important because surface veins are part of the body's temperature control system. "When we are very hot, all of our veins dilate, helping us lose heat. When we are cold, our bodies secrete hormones to constrict the veins, reducing their diameter and making them virtually impossible to dilate," Whiteley said.

Setting these external factors aside, it is possible for a person's veins to be too small or too deep to be easily detected. "Patients who have veins that are deeper under the skin, particularly those with a high body mass, can make it very difficult for veins to be palpable through the skin," Whiteley said. In addition, some people have "rolling veins," meaning their veins move easily when touched and can change position during needle insertion, according to Cleveland Clinic.

People's stress and anxiety levels may make their veins difficult to spot because the blood vessels in the skin constrict as part of the "fight or flight" response, while the veins that supply muscles dilate. "Patients who are very nervous go pale, as all of the blood vessels in the skin constrict. As such, it is very difficult to dilate the veins to take blood," Whiteley said.

Veins may also be hard to find due to dehydration and other conditions that affect the volume of blood in the body, like surgical shock, he added. Past injuries can also make people's veins difficult to stick.

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"Patients who have had a great many blood tests, or who have had drugs injected into veins, or who have had surgery affecting the veins in the arm can have scarred or even absent veins that are too damaged to take blood from," Whiteley said.

This article is for informational purposes only and is not meant to offer medical advice.