A test kit for determining the presence of one or more analytes in a fluid sample, including 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, where precisely located engagement of the assay device with the reading device causes a `lock-and-key` interaction between the assay device and reading initiation means of the reading device. Preferably the reading initiation means includes switch actuating means including at least one fixed projecting portion and at least one displaceable projecting portion, and a contact portion of the assay device casing includes a recess shaped to accommodate the fixed projecting portion of the switch actuating means but not the displaceable projecting portion.