Butterfly Needle in Phlebotomy: When and How to Use Winged Infusion Sets

What Is a Butterfly Needle?

A butterfly needle — also called a winged infusion set — is a short-bevel needle attached to a length of flexible tubing, which then connects to a hub or luer adapter. The name comes from the two plastic wings on either side of the needle shaft. You pinch those wings together to hold the needle during insertion, then fold them flat against the skin to secure it in place with tape.

That tubing between the needle and the collection hub is the feature that sets butterflies apart from straight needles. It gives you slack. You can move the tube, reposition the collection device, or swap tubes without disturbing the needle in the vein. With a straight needle inserted into a holder, the whole assembly is rigid — any movement of the holder moves the needle tip.

The tubing also lets you hold the needle at a much lower insertion angle than a standard straight needle. That matters a lot when you are working with shallow veins that would blow at a steeper angle.

When to Use a Butterfly Needle

Butterfly needles are not the default device for most adult draws. Straight needles with an evacuated tube holder are faster to set up, cheaper, and perfectly adequate for a good median cubital or cephalic vein. You reach for a butterfly when the anatomy or the situation calls for it.

Small or Fragile Veins

Elderly patients often have veins that are thin-walled, easily ruptured, and prone to rolling. The flexible tubing on a butterfly absorbs small movements during tube changes, which reduces the torque on the needle tip. Less torque means a lower chance of blowing the vein or causing a hematoma.

Hand and Wrist Veins

Hand veins are the most common reason a phlebotomist picks up a butterfly. The dorsal hand veins are frequently visible and palpable when antecubital veins are not accessible, but they are shallow, they move easily, and the anatomy does not accommodate the rigid angle of a straight needle well. The butterfly's low-profile wings lie flat against the hand and the flexible tubing handles tube changes without pulling the needle.

Wrist veins are similar. Use caution near the wrist — nerves and tendons run close to the surface — but when you do draw from this area, a butterfly is the appropriate device.

Pediatric Patients

Children have small veins and limited cooperation. A butterfly gives you better control during insertion and lets you secure the needle flat against the skin while you collect. Many pediatric draws use 23-gauge butterflies.

Short or Partial Draws

When you only need a very small volume — a few tubes or a partial fill — the butterfly can be easier to manage than a straight needle assembly. Some point-of-care tests require only a milliliter or two, and the butterfly gives you the control to collect exactly what you need.

Rolling Veins

A vein that slides away from the needle during insertion can sometimes be caught more reliably with a butterfly because your thumb and forefinger on the wings give you a low, stable grip close to the insertion point. Anchor the vein below the site with your non-dominant thumb and insert at a shallow angle with the wings flat.

Gauge Selection

The two most common butterfly gauges in phlebotomy are 21 gauge and 23 gauge.

• 21 gauge — The standard choice for adult hand vein draws. Flow rate is adequate for filling multiple tubes without excessive hemolysis. This is the gauge you will use for most butterfly draws on adult patients.
• 23 gauge — Used for pediatric patients, very small or fragile veins, or when you need extra precision on a difficult site. The smaller lumen means slower flow. With a 23-gauge butterfly, draw blood slowly and steadily. Pulling back too hard on a syringe or allowing a high vacuum to pull quickly through a narrow lumen can shear red blood cells and cause hemolysis, which ruins the sample.

You may occasionally see 25-gauge butterflies for neonatal or extremely difficult draws, but 21 and 23 are the standard exam-relevant gauges. When the NHA CPT asks about butterfly needle gauge selection, 23 gauge for pediatric and small vein draws is the expected answer.

Technique Differences from a Straight Needle

The insertion mechanics are similar to a straight needle draw, but there are a few differences worth knowing cold.

Insertion Angle

Insert a butterfly at a 10 to 15 degree angle, shallower than the 15 to 30 degrees typical for a straight needle in the antecubital fossa. Hand and wrist veins sit close to the surface. A steep angle will go through the vein rather than into it.

Holding the Needle

Pinch the wings together between your thumb and forefinger. Once you are in the vein and see the flashback of blood in the tubing, release the wings and fold them flat against the skin. Secure with tape before you begin filling tubes.

Flashback

Butterflies have a short length of tubing between the needle and the collection end. When you enter the vein, blood flashes back into that tubing — this confirms venous access. Do not advance the needle further once you see flashback.

Tube Changes

The flexible tubing makes tube changes easier on fragile veins. Still, hold the needle wings flat against the skin with one finger while you swap tubes to prevent any movement at the tip.

Needle Disposal

Most modern butterfly needles have a safety mechanism — either a retractable needle or a sheath that covers the tip when you remove the needle from the vein. Activate the safety feature immediately after withdrawal, before doing anything else. Butterfly needles historically had a higher needlestick injury rate because the exposed needle could spring back during cleanup. The safety mechanism is there for a reason.

The Discard Tube Rule

Here is the problem. The flexible tubing in a butterfly needle holds a small volume of air — the dead space. When you push the light blue citrate tube onto the hub, that air gets drawn into the tube before blood follows. A sodium citrate tube must be filled to exactly the line on the tube. The ratio of blood to anticoagulant is 9 parts blood to 1 part citrate, and that ratio has to be precise. If air is drawn in first, the tube ends up underfilled, the ratio is off, and coagulation test results — PT, PTT, INR — will be falsely prolonged.

The solution is to draw a discard tube before the citrate tube. The discard tube purges the air from the tubing so that the citrate tube fills with blood only.

What to Use as the Discard Tube

• Plain red top (no additive) — the most common choice. It does not interfere with coagulation testing and gets discarded after the draw.
• Second light blue top — acceptable if a red top is not available. The second tube gets discarded. The first tube in line is the one you keep and send to the lab.

The discard tube does not need to be completely filled. You only need to purge the air from the tubing — a partial fill is fine. Some facilities require a full fill as a matter of policy, so follow your institution's protocol, but the functional requirement is clearing the dead space.

When the Discard Tube Rule Applies

This rule applies when drawing a light blue citrate tube with a butterfly needle. If you are using a straight needle and evacuated tube holder, there is no dead space tubing, so no discard tube is needed before the blue top. The rule is specific to butterflies (and to syringes, which have their own transfer considerations).

In the order of draw, the discard tube is drawn first, then the light blue tube follows in its normal sequence position.

Cost Considerations

Butterfly needles cost more than straight needles. A standard straight needle and holder combination typically runs a fraction of the cost per unit compared to a winged infusion set. In a high-volume draw station, that difference adds up quickly across thousands of draws per month.

This is why butterflies are not the default device. When a patient has accessible antecubital veins, a straight needle is the right choice — clinically equivalent and significantly cheaper. Butterflies are appropriate when the clinical situation requires them, not as a convenience or preference for every draw.

Some facilities track butterfly usage rates as a quality metric. Consistently high rates in a given phlebotomist's draw data may prompt a review of technique or patient assessment skills, since overuse of butterflies on patients with good antecubital access suggests skipping the primary assessment step.

Practice Questions