A test kit for determining qualitatively or quantitatively the presence of one or more analytes in a fluid sample, comprising an assay device together with a reading device which engages with the assay device and wherein precisely located engagement of the assay device with the reading device is essential for accurate reading of the assay result, wherein precisely located engagement of the assay device with the reading device causes a 'lock-and-key' interaction, ie. a unique 3-dimensional interaction, between the assay device and reading initiation means of the reading device. Preferably the reading initiation means includes switch actuating means comprising at least one fixed projecting portion and at least one displaceable projecting portion, and a contact portion of the assay device casing comprises a recess shaped to accommodate the fixed projecting portion of the switch actuating means but not the displaceable projecting portion thereof, the contact portion also comprising an interface portion that contacts and displaces the displaceable portion of the switch actuating means when the fixed projecting portion is accommodated within the recess.