By convention, single lumen catheters, like needles, are identified by gauge and multi-lumen catheters are labeled by French size. When used to describe the catheter size, both terms refer to the outside diameter of the catheter. Each lumen of a multi-lumen catheter is also conventionally referred to by its gauge. Unlike catheter gauge descriptions, luminal gauges do not have direct correlation to needle gauge. To establish luminal gauge descriptions, catheter manufacturers test the flow through a catheter lumen and compare it to the flow through a needle of a known gauge. The flow potential of the lumen is then described as the lumen’s gauge.