The page below is a sample from the LabCE course Minimizing Pre-Analytical Variability During Venipuncture and Urine Sample Collection Processes. Access the complete course and earn ASCLS P.A.C.E.-approved continuing education credits by subscribing online.

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Needle Gauge

The gauge refers to the inner measurement or opening of the needle. Needles are routinely available in a variety of gauge sizes, including 18, 21, 23, and 25 gauge, as shown in the image.
The needle gauge becomes a consideration when the vein of the patient is narrow, fragile, or superficial. In such cases, a gauge size with a LARGER number (eg, 25 G) may be preferred over a routine needle gauge (eg, 21 G) to minimize damage to the blood vessel, as well as minimize the associated pain with 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 tiny needle gauge, hemolysis may occur. Hemolysis can cause inaccurate results (slight to significant) when testing several analytes. Potassium, for example, can be falsely increased in a hemolyzed sample.
Phlebotomists must exercise judgment between maintaining patient comfort and maximizing sample integrity when selecting an appropriate needle gauge for each patient.