It means to reduce the concentration of a particular substance.
Diluted means that a certain protein that was tested for drugs not present enough in your urine to render a conclusive test. The urine is "diluted". There are certain levels of drug tests, and some...
when you dilute a solution, you are usually adding some additional solvent, so you are decreasing the concentration of the solute.
Dilute means to make it less strong. i.e. You can dilute syrup to with water to thin it and make it less strong or sweet.
Yes they can mean the same thing.