The OPB350 can detect a number of different fluids in a tube as long as the tube is transmissive to the wavelength of the LED (typically 850nm-900nm, although other wavelengths can be used for custom applications).  The device can identify several different kinds of fluid.  Some examples are saline, pure water, dialysate fluid, and blood.  It can also determine if the tube is empty or contains air, if there are bubbles in the solution, or even if the tube is present or not.