Additive channels

A method of adding reagent to a fluid sample in a microfluidic device, comprising:

• a) providing
• i) a fluid sample comprising a first anticoagulant, and
• ii) a micro fluidic device comprising a microchannel comprising first and second sides and first and second ends, an input port in fluidic communication with said first end of said microchannel, and first and second additive channels in fluidic communication with said a first end of said microchannel, said first additive channels positioned on said first side of said microchannel near said input port, said second additive channel positioned on said second side of said microchannel near said input port, said first and second additive channels comprising
• iii) a first reagent solution comprising one or more reagents capable of re-activating a coagulation cascade,
• said microfluidic device further comprising one or more additional additive channels in fluidic communication with said second end of said microchannel,
• said one or more additional additive channels comprising
• iv) a second reagent solution comprising a second anticoagulant;
• b) introducing said fluid sample into said microchannel of said microfluidic device under conditions such that a portion of said fluid sample contacts said first reagent solution as said fluid sample moves through said microchannel so as to create a treated portion;
• and
• c) contacting said treated portion with said second reagent solution.