Air-in-line sensor systems and methods of using same are provided. The system includes an adapter (60) including first and second cylindrical portions defining a fluid flow channel, the first cylindrical portion comprising two adjacent wedge-shaped protrusions (68). Each wedge-shaped protrusion is infrared transmittive and defines an outer surface and an inner surface. The system further includes a tube and an infrared reflective sensor having an infrared light emitter (42,52) and an infrared light detector (44,54). The infrared light emitter and the infrared light detector are positioned at or near an adapter so that an infrared light can be transmitted to the adapter and at least a portion of the infrared light reflected off the adapter can be detected by the infrared light detector.