Can you hit an artery when drawing blood?

Accidentally puncturing an artery during a routine blood draw is possible but genuinely rare, occurring in roughly 1 out of every 34,000 donations in one large study of over 400,000 blood collections.¹ The reason it happens at all has everything to do with how close certain arteries run to the veins that phlebotomists target, and why it stays uncommon has to do with training, technique, and the fact that most people’s anatomy is forgiving enough to allow a safe stick. But the consequences of hitting an artery, while usually manageable, can occasionally be serious enough that the topic deserves a closer look.

How Often Arterial Punctures Actually Occur

The best available data on accidental arterial puncture during standard phlebotomy comes from a study that tracked 410,000 whole-blood donations and found exactly twelve cases where an artery was hit instead of a vein. That works out to about 0.003 percent of all draws.¹ To put that in perspective, you’d need to observe tens of thousands of routine blood draws to expect even a single arterial puncture. The rarity partly reflects the skill of trained phlebotomists, but it also reflects a reality of arm anatomy: veins and arteries do not run in a tangled jumble. The veins targeted for blood draws sit, on average, in predictable locations that are usually far enough from the nearest artery to provide a margin of safety.

That said, “rare” does not mean “impossible,” and the risk isn’t distributed equally across all people. Factors like body composition, anatomical variation, hydration status, and even the specific vein chosen for the draw all shift the odds. The arm’s inner elbow, known as the cubital fossa, is the most common site for venipuncture, and it’s also where the relevant arteries and veins come closest together.

Why the Anatomy Makes Arterial Puncture Possible

The veins most commonly used for blood draws at the inner elbow are the median cubital vein, the cephalic vein, and the basilic vein. The artery that poses the greatest risk is the brachial artery, which runs through the same region on its way to splitting into the radial and ulnar arteries further down the forearm. An ultrasound study that measured the distances between these structures found that the median cubital vein, the single most popular target for blood draws, sits a median distance of only about 3.6 millimeters from the brachial artery. In some individuals that gap was as narrow as 1 millimeter.²

The basilic vein, another common option, sat a median distance of about 12 millimeters from the brachial artery, while the cephalic vein was the farthest away at roughly 21 millimeters.² These numbers explain why the cephalic vein is generally considered the safest site for venipuncture from an arterial-puncture standpoint, even though it can be harder to access in some people. They also explain why a needle that overshoots the median cubital vein by even a few millimeters could plausibly enter an artery.

Keep in mind that these are median distances. In some arms, the gap between the median cubital vein and the brachial artery can be as wide as 14 or 15 millimeters. In other arms, the artery essentially sits right underneath the vein, separated by barely a millimeter of tissue. You can’t see these structures from the surface, and a phlebotomist working by feel alone has no way to know which scenario they’re dealing with.

Anatomical Variations That Raise the Risk

Not everyone’s arteries follow the textbook pattern. The brachial artery branches and courses differently in a meaningful fraction of the population. A systematic review and meta-analysis found that the most common variant, the brachioradial artery, appears in about 6 percent of limbs. The superficial brachial artery, which runs closer to the skin surface than normal, also shows up in around 6 percent of limbs.³ An accessory brachial artery, essentially an extra branch, is found in about 4 percent of limbs.

The superficial brachial artery is particularly relevant to blood draws because it courses closer to the skin than the standard brachial artery does. A person with a superficial brachial artery might have an artery sitting in a position where the phlebotomist expects only veins. Another systematic review noted that these superficial variants also carry clinical implications for nerve compression, since the artery and the median nerve run in close proximity.⁴ None of these variants are something you’d know about ahead of time unless imaging had been done for another reason. They’re silent anatomical quirks that only become relevant when a needle enters the wrong structure.

How You Can Tell an Artery Was Hit

In the study of 410,000 blood donations, the most common giveaway was speed. Eleven of the twelve arterial punctures produced an unusually fast blood flow, filling the collection bag in under four minutes instead of the usual seven to ten. Three-quarters of the arterial units were noticeably bright red, and in a third of cases the needle was visibly pulsating.¹ These are the classic trio of signs: rapid flow, bright red color, and pulsation.

However, these signs aren’t perfectly reliable in every setting. A case report describing unintentional arterial cannulation during peripheral IV placement noted that while pulsatile flow of bright red blood and high pressure in the line are key indicators, blood color alone can be misleading in patients with low oxygen levels, since their arterial blood may look darker than expected.⁵ In a healthy person sitting in a blood-draw chair, the bright red color is a strong clue. In a hospitalized patient who is already hypoxic, you might miss it.

For a person having their blood drawn, the experience of an arterial puncture can feel different from a normal venipuncture. Some people report a sharper or more intense pain, and the sensation of pulsation near the needle is sometimes perceptible to the patient. But plenty of normal blood draws are uncomfortable, so pain alone isn’t diagnostic. The key sign the phlebotomist watches for is the speed and character of blood flow. If the tube fills almost instantly, something unusual is happening.

What Can Go Wrong After an Arterial Puncture

Most accidental arterial punctures during routine blood draws are recognized quickly and managed without lasting harm. But arteries are under higher pressure than veins, which means bleeding is harder to control and complications can escalate if the puncture isn’t handled properly.

• Hematoma: This is the most common complication. In the study of arterial punctures during blood donation, a third of the affected donors developed hematomas at the puncture site.¹ Arterial hematomas tend to be larger and more painful than venous ones because of the higher blood pressure feeding them.
• Pseudoaneurysm: If the hole in the arterial wall doesn’t seal properly, blood can leak out and form a contained, pulsating mass next to the artery. One blood donor developed a brachial artery pseudoaneurysm three days after donation that turned out to be a clot-walled mass measuring over 3 centimeters, compressing the median nerve and requiring surgery to remove.⁶ Another case involved a 17-year-old who developed a brachial artery pseudoaneurysm after donating blood at a high school blood drive.⁷
• Nerve damage: The brachial artery runs alongside the median nerve, so bleeding or swelling from an arterial puncture can compress the nerve. One reported case documented severe median nerve compression in the forearm caused by a large hematoma from an accidental radial artery pseudoaneurysm.⁸
• Arteriovenous fistula: In rare cases, an abnormal connection can form between the punctured artery and a nearby vein. One case report described an elderly woman who developed swelling, bruising, and dilated vessels in her arm after routine venipuncture, ultimately found to have an acquired arteriovenous fistula between her cephalic vein and a small branch artery.⁹
• Compartment syndrome: This is the most dangerous potential outcome, where continued bleeding into a closed muscle compartment raises pressure enough to compromise blood flow and nerve function. Although compartment syndrome is more commonly associated with intentional arterial puncture for blood gas sampling, it has been documented after arterial line procedures, particularly in patients who are already critically ill.¹⁰

The thread connecting these complications is that arterial pressure doesn’t stop just because you pull the needle out. Veins are low-pressure, and a venous puncture site usually seals quickly with light pressure. Arteries push blood outward much harder, and that force can continue driving blood into surrounding tissue through a small hole that would be trivial in a vein.

What to Do If an Artery Gets Hit

The immediate response matters quite a bit. For a routine blood draw or donation where the phlebotomist suspects an arterial puncture, the standard approach is to withdraw the needle and immediately apply firm, direct pressure. One nursing case report recommended applying an elastic tourniquet over the puncture site immediately, followed by an ice compress to help constrict the vessel and reduce local bleeding and swelling.¹¹ The pressure needs to be maintained longer than for a normal venipuncture, often for ten minutes or more, because the arterial pressure working against the clot is substantially higher.

In more complex scenarios, such as when a large catheter accidentally enters an artery during central line placement in a hospital, the situation gets trickier. A systematic review of inadvertent arterial injury during central venous catheterization found that simply pulling out the catheter and pressing on the site was actually a poor management strategy with a high rate of complications.¹² In those cases, surgical or interventional repair is often needed. That’s a very different situation from a standard blood-draw needle, which is much smaller and causes a much smaller hole. But it illustrates the principle that arterial injuries demand more respect than venous ones.

If you’ve had blood drawn and you notice unusual swelling, a growing lump, persistent pulsating near the site, or increasing pain in your arm over the following hours or days, those are reasons to seek medical attention promptly. The donor who developed a pseudoaneurysm in the large study didn’t have an obvious problem at the time of donation; the pulsatile mass only became apparent three days later.¹ Delayed symptoms are uncommon but real.

How Ultrasound Guidance Reduces the Risk

For routine blood draws in healthy people with visible veins, ultrasound guidance isn’t standard practice and would be overkill. But in settings where the anatomy is harder to navigate or the stakes of an accidental arterial puncture are higher, ultrasound has proven valuable. A randomized trial of ultrasound-guided versus conventional femoral venipuncture during cardiac catheter ablation found that the ultrasound group had fewer inadvertent arterial punctures, fewer extra puncture attempts, and higher rates of first-pass success.¹³

A review examining ultrasound-guided puncture for interventional pulmonary embolism therapy reached similar conclusions, noting that widespread adoption could reduce access-related complications and cut down on accidental arterial punctures, particularly in patients with obesity, unfavorable anatomy, or known anatomical variations.¹⁴ These studies focused on femoral venipuncture in procedure suites rather than arm draws in a clinic, but the underlying principle applies broadly: when you can see the artery and the vein before you insert the needle, you don’t accidentally confuse them.

The case report of the elderly woman who developed an arteriovenous fistula after routine venipuncture specifically raised the idea that ultrasound guidance could benefit high-risk populations, including elderly patients with fragile veins and neonates with tiny vessels.⁹ Whether routine outpatient labs will ever adopt ultrasound for standard blood draws is debatable, but the technology exists and works.

Children and Other Higher-Risk Groups

Venipuncture in children is harder across the board. A review of pediatric venipuncture practice noted that blood collection in children carries an inherently higher risk of procedural errors compared to adults because of anatomical constraints, limited cooperation, and smaller blood volumes. Sampling failures, including multiple puncture attempts, are more likely in children with dehydration, obesity, chronic illness, or difficult vein anatomy.¹⁵ While the review focused primarily on venous sampling failures rather than arterial puncture specifically, the same factors that make veins harder to find in children also make it easier to miss and hit an adjacent artery. A child’s arm is simply smaller, and the distances between veins and arteries are proportionally tighter.

Elderly patients with fragile, thin-walled veins present a different kind of challenge. Their veins may collapse easily, leading to repositioning attempts that push the needle deeper than intended. Patients on blood thinners face compounded risk, not because they’re more likely to have an artery punctured, but because any arterial bleeding that does occur is harder to stop. The same goes for patients with coagulation disorders.

Hospital inpatients are another population where the risk profile shifts. Patients who are critically ill, dehydrated, or on vasopressors may have veins that are difficult to access, leading to more attempts and deeper needle angles. In the intensive care unit, arterial blood gas sampling is a separate and intentional form of arterial puncture, but even that routine procedure can occasionally lead to serious complications in patients who are already compromised.

Why Vein Choice Matters More Than Most People Realize

If you’ve ever had a phlebotomist spend time feeling around your arm before choosing a site, they were doing more than looking for the biggest vein. They were also assessing location, depth, and proximity to pulsation. The ultrasound data showing that the cephalic vein sits more than 20 millimeters from the brachial artery while the median cubital vein sits less than 4 millimeters away makes a strong case for preferring the cephalic vein when it’s accessible.² In practice, the median cubital vein remains the most popular site because it’s usually easy to find, stays relatively still during the draw, and produces reliable samples. But that popularity comes with a tradeoff in arterial proximity that most patients are never told about.

The basilic vein, which runs along the inner aspect of the arm, offers a middle ground: closer to the artery than the cephalic vein but still considerably farther away than the median cubital. Some phlebotomy training programs already emphasize checking for pulsation near the intended insertion site before proceeding. If you feel a pulse where you’re about to insert a needle, that’s the artery telling you to move. The problem is that arterial pulsation isn’t always palpable through overlying tissue, particularly in overweight patients or those with deeper-lying variants of the brachial artery.

For patients who get blood drawn frequently, such as those undergoing chemotherapy, dialysis, or chronic disease monitoring, asking which vein the phlebotomist plans to use and requesting the cephalic vein when feasible is a reasonable step. It won’t always be possible, since veins eventually wear out from repeated use, and the cephalic vein isn’t always large or accessible enough. But when given a choice, the anatomy favors the vein that’s farthest from the artery.

When Arterial Draws Are Done on Purpose

It’s worth noting that arteries are sometimes punctured intentionally. Arterial blood gas sampling deliberately targets the radial artery at the wrist (or occasionally the femoral or brachial artery) to measure oxygen, carbon dioxide, and acid-base levels. This is a common procedure in emergency departments and intensive care units. Even when done on purpose by experienced practitioners, the procedure carries a small but real risk of complications, including bleeding, hematoma, and in rare cases compartment syndrome.¹⁰

The difference between an intentional arterial puncture and an accidental one during a blood draw is preparation and expectation. When a clinician targets an artery on purpose, they use a smaller-gauge needle, choose a site with collateral circulation (the radial artery at the wrist, where the ulnar artery can maintain hand blood flow), and apply extended pressure afterward. When an artery is hit accidentally during a routine venipuncture, the needle may be larger, the site may not have ideal collateral flow, and the phlebotomist may not realize what happened right away. That delay in recognition is where most of the risk lives.