The invention relates to a microfluidic system comprising a microfluidic device having a first and a second opposite surfaces and an optical detector, the microfluidic device comprises a flow channel with a detection channel section having a length of at least about 1 mm, the microfluidic device comprises at least one aperture section comprising at least a part of said detection channel section and a transparent window into said detection channel section, the optical detector is arranged to determine at least one optical property of said aperture section as a function of time. The flow channel may have capillary dimensions and/or it may wholly or fully be arranged to drive a fluid flow by applying external forces. The microfluid device may be used to determine various properties of a sample fluid, for example it may be used to determine a samples coagulation properties and/or reactions of one or more components in a fluid sample as a function of time.