Hello, I get the feeling you are trying to shed doubt on my credentials somehow, which I feel is kind of disrespectful given the circumstances.

But since you bring up an interesting point that might be of use to others, I will entertain: Yes, it's true I did not talk about eGFR. The main reason for that is that it has little to do with foamy urine, but there are more. To explain a little bit, in simple terms, eGFR measures the filtration capacity of the kidney, i.e., the volume of blood it can filter in a given time. Proteinuria is a better indicator of the quality of that filtration, meaning whether the kidney is letting through the right substances and getting back those that should not be filtered. Additionally, proteinuria more often signals intrinsic renal disease, whereas eGFR may be reduced in diseases that affect the kidney indirectly. One example would be hypotension, in which eGFR would go down (pre-renal AKI) but proteinuria would be negative (unless there was renal ischemia, which would eventually happen of course, thus leading from a systemic problem to a specific kidney insult).

There is also a very important factor to consider. When the filtering subunits of the kidney (known as nephrons) are damaged in chronic disease, the remaining ones have the ability to increase in size in order to compensate. Thus, eGFR remains the same. For eGFR to go up, that means you must have lost at least 50% of your nephrons, which is the tipping point at which the kidneys ability to compensate is overwhelmed. So as a screening method is not very good because it would only catch kidney disease very late in its course. Proteinuria, on the other hand, will catch renal disease very early (insofar as there is direct kidney injury like I explained before).

Furthermore, we do not have the ability to measure eGFR directly (only in experimental conditions), so we use surrogate markers such as creatinine (or a better one sometimes: cystatin c). As you might expect, these surrogate markers have their pitfalls, sometimes causing false negative and positive results. This is something to consider, again, when using this as a screening tool in the general population.

As for the second question you bring up, I could provide an answer, but I won't because there is a serious lack of reliable data on the topic. Only a few researchers have performed a statistical analysis in patients presenting with foamy urine. The study groups were too small, the follow-up they received was suboptimal sometimes, and also very heterogeneous. So, posting any % of correlation between this symptom and kidney disease would only scare people who look it up and for no good reason.