A reference T-sensor system is provided for detecting the presence and/or measuring the concentration of analyte particles in a sample stream. The system includes: a) a laminar flow channel; b) three or more inlets in fluid connection with the laminar flow channel for respectively conducting into the laminar flow channel (1) an indicator stream which may include an indicator substance which indicates the presence of analyte particles by a detectable change in property when contacted with the analyte particles, (2) the sample stream, and (3) a reference stream, which can be a control stream and/or an internal standard stream; c) wherein the laminar flow channel has a depth and/or width sufficiently small to allow laminar flow of the streams and a length sufficient to allow particles of the analyte to diffuse into the indicator stream to form a detection area; and (d) an outlet for conducting the streams out of the laminar flow channel preferably to form a single mixed stream.