How is kidney function determined

I was diagnosed with polycystic kidney disease about 25 years ago.  I have had regular bloodwork done since then.  I have watched my creatinine level which has alway been between .9 and 1.1 until this last fall.  I had gone to my eye doctor with glaucoma symptoms, and found out that I had psydotumor cerebri.  This is excess spinal colum pressure in my brain pushing my optic nerves into my eye.  I was starting to lose vision.  He had me do a very complete blood test that showed slightly elevated sugars, 111, and my createnine was 1.3.  A month later, it was back to 1.14.  When I saw my doctor, he told me that I had 50% kidney function.  It was not my normal doctor, I didn't know what to ask.  I later found that my filtration rate was low, around 40.  

My question is, what is used to determine kidney function.  I thought I was doing fine as my creatinine was low.

My older sister had a transplant several years ago, and my sister 2 years younger had her kidneys removed in November and is awaiting a transplant.  She told me that her function was 50% a year before.  I will see my doctor and review bloodwork I will have taken this week next week.  I want to make sure I ask the correct questions.

I still have high pressure in my spinal column.  When I have a spinal tap, my blood pressure is easier to control for a week or so after.  Is there any relation to high spinal column pressure (35) normal is less than 20, and kidney function.

Any help will be appreciated.