The gauge of needles ranges from 14 to 30, but not all these gauges are used for routine blood collection. Lab test results must be accurate for a proper diagnosis for any patient. So, to ensure that blood drawn is suitable for testing, the hole of needles must be wide enough to accommodate for a fast flow of blood and for the safe passage of blood components.
So, what are these 3 most common gauge needles used? The 21, 22, and 23 gauges respectively. (Note that in some countries this may change. It may be instead the 20, 21, and 22 gauges).
Of all 3, the 21 gauge is the most common used when drawing blood. It is the go-to gauge for most adults with plump veins. The hole through which blood flows is wide enough to allow for a fast flow of blood while maintaining the integrity of the sample.
When we talk about the gauge of a needle we are referring to the size of the hole in the needle. The higher the gauge, the smaller the hole. The lower the gauge the wider the hole.
The 21 Gauge Vs The 23 Gauge Needle
The 21 gauge needle is the standard gauge needle for drawing blood. Most adults who are healthy have plump and bouncy veins. Those veins are usually big enough for the use of a 21 gauge needle.
The hole of the 21 gauge is wide and it allows the cells in the blood to pass through the needle quickly without damage. I have been surprised many times how fast blood flows through the 21 gauge needle. When filling tubes pay close attention because if you don’t you may draw more blood than you need. Depending on the size of the tube and the flow of blood, a tube will fill in 2 to 10 seconds.
When used with syringes, pulling the plunger is easy as blood flows quickly and tube vials fill up fast with the Evacuated Tube System. Four 8 ml tubes can fill in under 60 seconds. Blood will flow really fast depending on the rate of flow.
Note that collection tubes have a vacuum inside which controls how fast blood enters the tube and the amount of blood that is collected. Also, when filling tubes with syringes allow the vacuum inside the tube to pull on the plunger without pushing the plunger itself.
The 23 gauge on the other hand is not the same. The rate of flow of blood is slow. Every time I catch myself waiting for a tube to fill when I am using a 23 gauge. And pulling that plunger on the syringe is slow going.
However, it has its purposes. Use a 23 gauge for small veins. The size of the needle must correspond to the size of the vein. If you use a 21 gauge on a small vein it could tear the vein and cause bleeding.
23 gauges are used for children and adults with small or thin veins.
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What About The 22 Gauge Needle?
The 22 gauge needle falls somewhere between the 21 and 23 gauge. Use it for adults who have medium-sized veins and for older children.
The idea here is that the vein is smaller than a vein on which you would use a 21 gauge needle but bigger than a vein on which you would use a 23 gauge needle.
The flow of blood is a fraction slower than the 21 gauge but collection tubes fill quickly enough.
The Butterfly Needle System
The butterfly needle comes as a unit. It can range from 18 to 27 gauge. The needle is attached to a soft tube that ranges between 8 to 15 inches long. The end of the tube is joined to an adapter that can be screwed into a tube holder if using the evacuated tube system or unto a syringe.
The length of the needle can be from 1/2 to 3/4 an inch long. Use the 21, 22, and 23 gauges. Use the 21 gauge on adults with plump veins. Use the 22 gauge on smaller veins and use the 23 gauge on small and thin veins.
The 24 or 25 gauge needle may be used on patients with difficult veins including neonatal/pediatric, geriatric, oncology and burn patients.
What Gauge Needles Are Used For Donating Blood?
Donating blood is different from having your blood drawn for a blood test. A large amount of blood is drawn during donation. It can vary from 200 to 550 ml depending on the country but 450 to 500 ml is typical. A 16, 17, or 18 gauge needle is used. This will vary from country to country. A 16 gauge may be the go-to in one country while an 18 gauge may be more common in another.
The large hole of the needle facilitates the flow of blood and within a few minutes a pint of blood is collected.
What About Higher Gauge Needles?
The other gauge needles like 25 to 30 are too small for drawing blood, although the 25 gauge may be used to draw blood from very small and fragile veins. The holes in those needles are too small for blood cells to pass without getting damaged. When red blood cells get damaged, hemolysis occurs, which can alter lab test results.
The flow of blood is also very slow. When the flow is so slow the blood can clot. Those gauge needles are more commonly used for shots or injections.
| Needle Gauge | Color | Length (In inches) | Flow of Blood | Patient |
| 21 | Green | 1 1-1/4 1-1/2 | Fast | Most Adults |
| 22 | Black | 1 1-1/4 1-1/2 | Medium-fast | Older Children Some Adults |
| 23 | Light Blue | 1 1-1/4 1-1/2 | Slow | Children Adults with small or thin veins |
I created a phlebotomy needle gauge chart. Click here to go check it out.
Related Questions
What is the difference between gauge needle numbers? The numbers refer to the size of the hole in the needle. Lower numbers mean that the hole in the needle is wide. Higher numbers mean that the hole in the needle is small. So, as the numbers get lower the hole gets wider and as the numbers get higher the hole gets smaller.
Which is bigger 23 gauge or 25 gauge needle? The 23 gauge needle is bigger. The hole in the 23 gauge needle is wider than the hole in the 25 gauge needle. The 23 gauge is used for small or thin veins. The 25 gauge may be used for pediatric patients but it may cause hemolysis.
How do I know what gauge needle to use? The size of the needle must match the size of the vein. A 21 gauge is used on a big vein. A 23 gauge is used on a small vein. If the vein can’t be seen use the tip of the index or middle finger to search and feel for the vein. Feeling the vein will give you an idea of its size. Choose a gauge needle accordingly.