http://www.truthaboutpetfood.com/articles/science-says-antiquated-protein-testing-led-to-2007-pet-food-recall.html

There is also a link that goes into great detail on testing methods in this article.

Total Protein Methods and Their Potential Utility to Reduce the Risk of Food Protein Adulteration

http://onlinelibrary.wiley.com/doi/10.1111/j.1541-4337.2010.00114.x/full