wrote in message
ups.com...

It would. However, this is why the "reference material" used is another
endogenous steroid, not some testosterone taken off the shelf. (Those
who perform studies using stable isotopically labeled tracers are
exceedingly familiar with the need to obtain a baseline or background
sample prior to tracer administration to account for the natural
abundance of, e.g., 13C in whatever is being traced. The principle is
the same here.)

As I was saying Andy, using another steroid might not be a good idea. And
the sample may have been poluted with other carbon sources that are more
immediate than steroid production which is a great deal more detailed. So it
is possible that there can be radically different ratios of C12/C13 in one
component of urine and another component.