I have a high bun/ creatinine level (28) and a low creatinine level (0.5) . should i be concerned or do something?

No: Low creatinine never means anything bad. Reference ranges are set so that a few percent of healthies fall outside arbitrarily. The ratio could be up if you ate a couple of burgers. You're physician's not worried. Stay physically fit and you'll probably have a long and happy life. Best wishes.

BUN/Creat Ratio: I personally would not worry about it since the high ratio is due to a slightly elevated BUN. This might be due to a diet high in non-essential amino acids (nothing to truly be concerned about) or a mild lack of water intake. If you are still concerned, please see your primary doctor and go over all of that with her/him.