A microfluidic cartridge for performing an antigen:antibody agglutination reaction, the microfluidic cartridge comprising: a) a body member (20, 101, 901) comprised of a substrate or substrates, said body member comprising a reaction channel, said reaction channel having a first end and a second end and having a narrowest dimension configured for enabling laminar flow conditions; b) a first and a second intake channel (1,2) fluidly joined to said first end of said reaction channel at a staging union; said first intake channel for conveying a first fluid and said second intake channel for conveying a second fluid; characterized in that said staging union is configured with a dual capillary stop, said dual capillary stop comprising a first capillary stop interposed between said first end of said reaction channel and said first intake channel and a second capillary stop interposed between said first end of said reaction channel and said second intake channel, the dual capillary stop for simultaneously releasing said first and second fluids into said reaction channel in response to a start signal, thus enabling said fluids to contactedly flow in parallel within said reaction channel as lamellae, one next to the other, and enabling formation of a horizontally stratified laminar diffusion interface; c) a downstream flow control channel fluidly joined to the second end of said reaction channel, wherein the downstream flow control channel comprises an elongate runoff channel segment configured with microfluidic dimensions for enabling capillary flow therein, and said runoff-channel segment having reverse tapered walls that broaden from upstream to downstream for sustaining and modulating a passive laminar flow regime in said reaction channel at Reynolds Numbers ranging from 0.1 to 10 for a duration up to about ten minutes; and d) a vent or check valve downstream of or terminating said downstream flow control channel. a) a body member (20, 101, 901) comprised of a substrate or substrates, said body member comprising a reaction channel, said reaction channel having a first end and a second end and having a narrowest dimension configured for enabling laminar flow conditions; b) a first and a second intake channel (1,2) fluidly joined to said first end of said reaction channel at a staging union; said first intake channel for conveying a first fluid and said second intake channel for conveying a second fluid; characterized in that said staging union is configured with a dual capillary stop, said dual capillary stop comprising a first capillary stop interposed between said first end of said reaction channel and said first intake channel and a second capillary stop interposed between said first end of said reaction channel and said second intake channel, the dual capillary stop for simultaneously releasing said first and second fluids into said reaction channel in response to a start signal, thus enabling said fluids to contactedly flow in parallel within said reaction channel as lamellae, one next to the other, and enabling formation of a horizontally stratified laminar diffusion interface; c) a downstream flow control channel fluidly joined to the second end of said reaction channel, wherein the downstream flow control channel comprises an elongate runoff channel segment configured with microfluidic dimensions for enabling capillary flow therein, and said runoff-channel segment having reverse tapered walls that broaden from upstream to downstream for sustaining and modulating a passive laminar flow regime in said reaction channel at Reynolds Numbers ranging from 0.1 to 10 for a duration up to about ten minutes; and d) a vent or check valve downstream of or terminating said downstream flow control channel.