A test device is configured for diagnostic testing and includes an optical readable medium, in turn including a pattern of spots of material arranged on a surface of the device. Several patterns may be provided. The patterns accordingly formed may be human and/or machine readable. They may notably encode security information, e.g., indicating whether the device has already been used. The spots may notably be inkjet spotted. In addition, a method is provided for decoding information encoded in a pattern of such a test device. In embodiments, liquid is introduced in the device, which comprises additional spots having a substantially different solubility than spots forming the actual pattern. Thus, the additional spots get solubilized in and flushed by the liquid as the latter wets them, and an initially hidden pattern may be read, which is formed of the remaining spots (not solubilized). Encoding methods are also provided.