A lateral flow assay device of the present disclosure has numerous features to ensure correct operation of the device under gravity, such as vent pockets for enabling the flow of sample fluid from one chamber to the next when the vent pocket is unsealed. The vent pockets have protrusions to help prevent accidental resealing. The device also can have a gasket to ensure free air movement between open vent pockets. Flow control features that protrude horizontally into a chamber proximal to an outlet act to deflect the flow to the side of the chamber opposite from the outlet, prior to the flow entering the outlet. Such features can reduce the flow velocity of the sample fluid and increase the effective fluid flow path length, enhance mixing of reagents with a sample, and enabling more accurate control of fluid flow in the device. A flexible circuit with patterned metallic electrical components disposed on a heat stable material can be in direct contact with fluid in the chambers and has resistive heating elements aligned with the vent pockets and the chambers.