Apparatus and methods for concentrating platelet-rich plasma is described herein. One variation may generally comprise a tube having a length and defining a channel within and one or more ports located at a proximal end of the tube and in fluid communication with the channel. A plunger may slidably translatable within the channel while forming a seal against an inner surface of the channel and a float may have a pre-selected density and defining a concave interface surface, wherein the float is slidably contained within the channel such that the concave interface surface is in apposition to the one or more ports.