Needle Gauge Sizing Explained

Hmm...It's easier for a camel to pass through
the eye of a needle than avoid Miss Bruiser's
painful ministrations!

Sizing of most medical implements made sense, gloves were measured by running a tape measure across the mid section of the hand, the measurement in inches was the glove size. Catheters were measured in Fr. and one French was equal to 1/3 mm. a 32 Fr. catheter was most definitely larger than a 26 Fr.  Endotracheal tube diameters are measured in simple millimeters. Nice and straightforward for foolish folks like myself.

Needle caliber sizing is a horse of a different color. Miss Bruiser, my favorite nursing instructor insisted that physical discomfort (pain) was required for a  student nurse's proper education, so when she gave us a choice of needle size for injection practice sessions it was obviously a trick question. Her beady little eyes shifted like a pinball in play when she asked, "Now which size needle would you like me to use on your tender deltoid when I demonstrate an intramuscular injection technique, A 16ga or a 22ga?" Well a 16ga sounded like the best choice because the logical thought process would deduce that 16 is less than 22 so the needle would be of a smaller caliber  and hurt less when harpooned by a towering Miss Bruiser. Well we were dead wrong and had the bloody aching arms to prove it.

 Every nurse knows that the smaller the gauge number, the bigger the needle, but how in blue blazes did this come about? Up until the dawn of the 20th century there was no standard for needle size other than puny, medium, and jumbo. Thanks to the British wire industry things were about to change.

Needle sizing is a direct descendant of ye ol'  Birmingham wire gauge which was widely used as an industrial standard in merry old England. According to this standard a #1 gauge wire had a diameter of 6.26mm  which was used as a starting point for the measurement of wire diameters. The number of wires that could fit in this defined area was the gauge. It's easy to see that for an area of 6 mm diameter (or there abouts) to accommodate 25 strands of wire they would have to be much, much less of a diameter than say #12 gauge to occupy the exact same area. The greater the number of wires ram rodded into the same sized area, the lesser the diameter. So that's why the higher the needle gauge, the smaller the diameter of the needle. I'll take getting harpooned with 27 gauge needle over a 16 gauge any day!

As time went on, the Birmingham wire gauge was further improved and made more exacting for the sizing of medical needles by meticulously defining the steps between the different gauges, for 7-14 gauge needles the size increased by .025 mm for each larger size needle, to 19 gauge the size increased by .013mm, and lastly with the small needles size increased by .0064 mm. When you think about tiny needles, just a small decrease in gauge can make a really big difference so that's why the steps between gauges are so small.

The year 1955 was an occasion for old nurses to celebrate. Roeher Products introduced the disposable Monoject syringe which allowed for a choice of needle size which was independent of the syringe. The color coding of needle sizes was born and every nurse knew a bright  red needle guard signified a 25 ga needle and a blue guard meant a 21 ga. I never could figure out why a 20 ga needle and a 25 ga had the same reddish  color, although the 20 guage was of a very light shade. 

The needle that gave every novice nurse the tremblors was not a monster 16ga, but rather the mighty 2 inch long.Imferon needle with the purple guard. I shudder to think of the misery Miss Bruiser could inflict with one of these monsters as it bounced off periosteum with a loud clunk followed by an ear splitting shriek!