An assay result reading device for reading the result of an assay performed using a porous liquid transport carrier, the device comprising: a light source; a photodetector generating signals representing the presence or absence of a liquid sample; and a central processing unit or microcontroller responsive to the signals so as to calculate a flow rate therefrom and compare the calculated flow rate to predetermined upper and lower limits and reject the assay result if the calculated flow rate is outside the upper and lower limits; characterised in that: the device comprises at least two light sources capable of emitting light upon at least one of two or more spatially separated zones of the carrier, and at least two photodetectors so positioned as to be capable of detecting light emanating from each of the two said zones, wherein the first light source is capable of emitting light incident on the first, upstream zone, and the first photodetector detects light emanating from said zone; and the second light source is capable of emitting light incident on the second, downstream zone, and the second photodetector detects light emanating from said zone; and further wherein the flow rate is calculated for a liquid flowing between the two said zones on the liquid transport carrier from the detection of the presence of the liquid sample at, or passage thereof through, the first, upstream zone and the detection of the presence of the liquid sample at, or passage thereof through, the second, downstream zone. a light source; a photodetector generating signals representing the presence or absence of a liquid sample; and a central processing unit or microcontroller responsive to the signals so as to calculate a flow rate therefrom and compare the calculated flow rate to predetermined upper and lower limits and reject the assay result if the calculated flow rate is outside the upper and lower limits; characterised in that: the device comprises at least two light sources capable of emitting light upon at least one of two or more spatially separated zones of the carrier, and at least two photodetectors so positioned as to be capable of detecting light emanating from each of the two said zones, wherein the first light source is capable of emitting light incident on the first, upstream zone, and the first photodetector detects light emanating from said zone; and the second light source is capable of emitting light incident on the second, downstream zone, and the second photodetector detects light emanating from said zone; and further wherein the flow rate is calculated for a liquid flowing between the two said zones on the liquid transport carrier from the detection of the presence of the liquid sample at, or passage thereof through, the first, upstream zone and the detection of the presence of the liquid sample at, or passage thereof through, the second, downstream zone. the device comprises at least two light sources capable of emitting light upon at least one of two or more spatially separated zones of the carrier, and at least two photodetectors so positioned as to be capable of detecting light emanating from each of the two said zones, wherein the first light source is capable of emitting light incident on the first, upstream zone, and the first photodetector detects light emanating from said zone; and the second light source is capable of emitting light incident on the second, downstream zone, and the second photodetector detects light emanating from said zone; and further wherein the flow rate is calculated for a liquid flowing between the two said zones on the liquid transport carrier from the detection of the presence of the liquid sample at, or passage thereof through, the first, upstream zone and the detection of the presence of the liquid sample at, or passage thereof through, the second, downstream zone.