There is provided a lateral flow immunoassay test device. The test device comprises a test strip. The test strip comprises a test membrane having a translucent section including a control zone and a test zone. The device is adapted to house the test strip such that at least the translucent section is exposable to ambient light. The device comprises a backing structure for backing the test strip which comprises a first optical detector for detecting ambient light which has passed through the test zone, and a second optical detector for detecting ambient light which has passed through the translucent section in a further zone outside of the test and control zones. Using the detector outputs an amount of analyte in a test liquid may be calculated.