On a home pregnancy test, the letter “C” typically indicates the control line. This line’s appearance confirms the test is functioning correctly. For example, if the control line does not appear, the test is invalid and should be repeated with a new device, regardless of any other result indicated.
The control line serves as an internal check, verifying that the test strip has absorbed sufficient urine and that the reagents are working as intended. Its presence is vital because it assures the user that the test mechanism itself is operational, providing a reliable basis for interpreting the result indicated by any other lines or symbols.