The half life of hgh is very short, less then an hour. Your natural growth hormone comes in pulses so it would have no affect on gh level unless you are getting the test done right after injection. The only thing that using hgh consistently has an influence on is igf.
Gh levels will be different from hour to hour so gh level testing is fairly pointless unless you did injection righ before. That is why they do stimulation tests when properly measuring natural gh levels. Without that it is a useless test