Figure 5 shows a test strip that can detect the presence of an anabolic steroid in an athlete's urine. The end of the test strip is dipped in urine. The urine moves up through the test strip. The test area and the control area contain a dye. The dye turns blue when monoclonal antibodies bind to it. Suggest the purpose of the control area in the test strip.

The control area serves as a check to confirm that the test strip is functioning correctly. It contains immobilised antibodies that bind to any free monoclonal antibodies, regardless of whether they are bound to the steroid or not. If a blue line appears in the control area, it proves that: 1. The urine has flowed correctly up the strip. 2. The monoclonal antibodies have been successfully picked up by the urine and have travelled up the strip. If the control line does not appear, the test is invalid and the result cannot be trusted, even if the test line is also blank.