Is 500 ng/ml on urine test possible?

I have spent hours trying to find out how high a human being's THC level can be in a urine test. The bar charts and graphs I have found only go up to 300. But does that mean anything over 300 is rare, or indicative of very, very heavy daily use? I have no idea and I cannot find an answer on the Internet anywhere.

My tests result showed 500 ng/ml, smoking 1/2 oz or less a week. I have difficulty believing that the samples did not get mixed up. I used STAT as per instructions, have sailed through tests before that way. How can I suddenly go from below the minimum threshold to such an incredibly high level?

Much is riding on these results; verifying urine is/is not mine will cost $2,000. I will pay it if a mix-up seems likely. But if it's probable that my heavy use simply caught up with me, I can't afford to waste the money.