I want to share with everyone my experience over the past few weeks regarding proteinuria, as I know some questions have come up about this on /r/keto before.
First, a brief history, as I have been lurking for 2+ years but rarely post:
I was an overweight kid for almost all of my childhood. I knew this fact, but the reality of the situation didn't really hit me until age 13 or so when my pediatrician showed me the BMI chart and how there was not even a space on it for a kid my size (too short, too fat). I was "morbidly obese". I decided something had to change around Christmas of 2012 (Age 16) when I went shopping for a Christmas sweater and had to, for the first time in my life, buy a men's size XL – I was 5' 2" and ~240 lbs. I don't want to bore anyone with the details, but fortunately I found /r/keto, stuck with it for a few months, grew a little bit, got back on keto for a while, grew a bit more, and now (age 18) I am 5' 9" 160 lbs (and much more muscular).
In September I went for my annual physical and my urine showed high levels of protein. I had heard of this happening on Keto, but I had not been very strict the past few weeks and wasn't even sure that I was in ketosis, though I had definitely been eating very low carb. In either case, when I told my doctor (GP) about my high fat, medium protein, low carb diet she did not think that the diet was the cause. She ordered a repeat urinalysis, as well as a 24 hour urine sample, and over the course of 3 weeks my urine continued to show high levels of protein. She ordered some blood tests to check for leukemia, diabetes, myeloma, and some other scary diseases but fortunately I came up clean. Stumped, she sent me to a nephrologist (kidney doctor) for further examination, and told me to be sure to mention my diet as she was now starting to believe that might be the cause.
The nephrologist was also pretty confused, as he had never had a patient with proteinuria as young as me. He did say that sometimes adolescents just get proteinuria for a while after taking certain medicines and that it was likely nothing too serious. I told him about the keto diet, how I had lost 1/3 of my bodyweight over the past 18 months, and how sometimes people report proteinuria on keto and he said he would look into it, but he did not believe there would be a correlation. Regardless, he ordered a slew of blood tests and a kidney/bladder ultrasound to check for viruses or some other kidney issues and after all the tests came back fortunately the protein was gone.
So, while the doctors still do not know exactly why I showed protein in my urine for those 5 weeks, they were also never concerned about my diet, weight loss, or keto. Again, this was two general physicians as well as a nephrologist who both told me that I could continue my keto diet even while undergoing kidney function testing.
In conclusion, to anyone who might be worried about their ketostrips or urinalysis showing protein, I recommend getting checked out by your doctor. Some small amounts of protein in the urine might happen on keto every once in a while and can sometimes be caused by under or over consumption of water, but if you are showing a significant amount of protein or are showing it for an extended period of time (according to my doctor) it is probably not because of keto. It can be nothing, as in my case, or it can be something far more serious, but it is definitely not something to ignore.
I may have left some things out so if you have any more specific questions about discussions I had with my doctors or testing results just ask!