I asked the manufacturer about the biuret content in Miracid, as I use it and it had come up here before. Here's the response I got:

"...The Scott's Company does have a specification around the maximum amount of biuret allowed in the urea we buy for all of our products. In this case, the maximum amount of biuret possible in this formula is 0.9%. Due to manufacturing variability in urea production, we can not provide the actual concentration. What we can say is that it will never be higher than this number and typical concentrations are likely to be around 0.6% or less..."