How to know how much my kidneys are working?

Glomerular-filtration Rate: It is a rate at which blood is filtered through glomeruli in a minute. It tells us that how much the kidneys are functioning. It can be measured or estimated. There are various formulas for measuring GFR:
1. Cockcroft Gault formula (CGF)
2. Modified diet MDRD
3. Creatinine clearance (Urine)
4. Others
How to estimate GFR:
1. Inulin
2. DTPA
Most commonly used are CGF and DTPA
Using this GFR we can differentiate chronic kidney disease(CKD) into different
stages
Stage 1
Stage 2
Stage 3
Stage 4
Stage 5
Stage 5 ND (not on dialysis)
Stage 5 D (on dialysis)
Stage 5 T (post transplant)
The patient can himself calculate GFR easily by computed formulas which are easily available on mobile applications and internet.