A system, a sensor and a control unit for determining validity of a result of a lateral flow test, whereby a lateral flow stick that has a sample pad, a conjugate pad having antibody treated gold particles, and a porous membrane for receiving a capillary flow of a milk sample. The porous membrane has a test line treated with a progesterone reference which binds antibody treated gold particles of the milk sample and thereby brings the test line to change colour tint when exposed for milk comprising a progesterone level lower than a threshold limit, and a control line treated with an antibody reference which binds antibody, and thereby brings the control line to change colour tint when exposed for milk comprising antibody treated gold particles.