A lateral flow diagnostic device comprising: a substrate of porous material; channel boundaries defining a first fluid flow channel within the substrate; and a first test site disposed at a predetermined location along the first fluid flow channel, the first test site comprising an analyte detection substance for detecting presence of a target analyte in fluid flowing along the first fluid flow channel; in which: the first fluid flow channel comprises a constricted portion (Lc) with a smaller cross-sectional area than another portion of the fluid flow channel; and the first test site is disposed within the constricted portion of the first fluid flow channel; the lateral flow diagnostic device comprising a plurality of fluid flow channels, at least two of the fluid flow channels comprising a respective test site comprising the same analyte detection substance, wherein the at least two fluid flow channels include at least: the first fluid flow channel comprising said constricted portion; and one of: a second fluid flow channel comprising a constricted portion with a second cross-sectional area different from a first cross-sectional area of the constricted portion of the first fluid flow channel, or a second fluid flow channel without said constricted portion and having a larger cross-sectional area than the constricted portion of the first fluid flow channel. a substrate of porous material; channel boundaries defining a first fluid flow channel within the substrate; and a first test site disposed at a predetermined location along the first fluid flow channel, the first test site comprising an analyte detection substance for detecting presence of a target analyte in fluid flowing along the first fluid flow channel; in which: the first fluid flow channel comprises a constricted portion (Lc) with a smaller cross-sectional area than another portion of the fluid flow channel; and the first test site is disposed within the constricted portion of the first fluid flow channel; the lateral flow diagnostic device comprising a plurality of fluid flow channels, at least two of the fluid flow channels comprising a respective test site comprising the same analyte detection substance, wherein the at least two fluid flow channels include at least: the first fluid flow channel comprising said constricted portion; and one of: a second fluid flow channel comprising a constricted portion with a second cross-sectional area different from a first cross-sectional area of the constricted portion of the first fluid flow channel, or a second fluid flow channel without said constricted portion and having a larger cross-sectional area than the constricted portion of the first fluid flow channel. the first fluid flow channel comprises a constricted portion (Lc) with a smaller cross-sectional area than another portion of the fluid flow channel; and the first test site is disposed within the constricted portion of the first fluid flow channel; the lateral flow diagnostic device comprising a plurality of fluid flow channels, at least two of the fluid flow channels comprising a respective test site comprising the same analyte detection substance, wherein the at least two fluid flow channels include at least: the first fluid flow channel comprising said constricted portion; and one of: a second fluid flow channel comprising a constricted portion with a second cross-sectional area different from a first cross-sectional area of the constricted portion of the first fluid flow channel, or a second fluid flow channel without said constricted portion and having a larger cross-sectional area than the constricted portion of the first fluid flow channel. the first fluid flow channel comprising said constricted portion; and one of: a second fluid flow channel comprising a constricted portion with a second cross-sectional area different from a first cross-sectional area of the constricted portion of the first fluid flow channel, or a second fluid flow channel without said constricted portion and having a larger cross-sectional area than the constricted portion of the first fluid flow channel. a second fluid flow channel comprising a constricted portion with a second cross-sectional area different from a first cross-sectional area of the constricted portion of the first fluid flow channel, or a second fluid flow channel without said constricted portion and having a larger cross-sectional area than the constricted portion of the first fluid flow channel.