Apparatus and method are provided for performing solid phase tests for mobile components in fluids, especially in vitro diagnostic tests. A solid phase device is used having a fixed component immobilized on its surface which is in contact with a fluid sample when inserted therein. The improvement involves utilizing a receptacle, into which the device is inserted, having the same fixed component immobilized on the inner surface of the receptacle. The test is performed by placing a fluid sample, having a mobile component reactive with the fixed component, into the receptacle and in contact with the insert for a period of time and measuring a change which is a function of the concentration of the mobile component.