Needleless connectors (NCs) for vascular access have limited needlestick injuries, but complications including occlusion, thrombosis, and infections have increased despite reduced needlestick injuries. These complications relate to the ability of an NC design to limit volume fluctuations that can lead to fluid reflux with potential for microbial contamination. Different NC designs requiring specific usage protocols and training, a lack of clarity in NC function relative to manufacturer-designated categories, and confounding results from a limited number of studies comparing different NCs have resulted in confusion, ultimately leading to complications from undesirable fluid movement within the vascular access. The authors therefore quantified the magnitude of reflux with current commercially available NCs using a venous stimulator. Thirteen blinded NC designs spanning the categories of negative and positive displacement, neutral, and antireflux were tested to quantify fluid movement upon disconnection and reconnection from a representative intravenous pressure (3 NCs per design; 10 trials per NC). Trials for each NC tested followed consistent displacement trends leading to tight error bars. Blinded NCs were then characterized according to their function and compared with their category designation after unblinding. All positive and negative NCs functioned in a manner consistent with their respective category designations. Conversely, all NCs categorized as neutral actually functioned with negative displacement (ie, reflux upon disconnection; 4/5 NCs) or positive displacement (1/5 NCs). Only NCs classified as antireflux functioned as neutral, which was confirmed in a blinded bidirectional flow test. These results suggest that the neutral NC-marketed category may be confusing to users unless the particular NC design has an integrated antireflux component.