Gauge refers to the interior measurement or diameter of the needle. Needles are routinely available in various gauge sizes, including 18, 21, 22, and 25 gauge, as shown in the image. The higher the gauge number, the smaller the diameter of the needle.
The needle gauge becomes a consideration when the patient's vein is narrow, fragile, or superficial. In such cases, a gauge size with a larger number (e.g., 25 G) may be preferred over a routine needle gauge (e.g., 21 G) to minimize damage to the blood vessel as well as minimize any associated pain with the collection.
However, with a larger gauge size comes a smaller bore and a smaller internal diameter of the collection needle. When blood cells are forced by the vacuum pressure of large-volume evacuated tubes to quickly enter into the tight space of a small gauge needle, hemolysis may occur. Hemolysis can result in inaccurate results (slight to significant) when testing certain analytes. Potassium, for example, can be falsely increased in a hemolyzed sample.
If smaller gauge needles (large diameter) are used inappropriately, there can be damage to the vein as well.
Phlebotomists must exercise judgment between maintaining patient comfort and maximizing sample integrity when selecting an appropriate needle gauge for each patient.
