The Cystatin C test measures the amount of cystatin C in your blood. Cystatin C (CysC) is a cysteine protease inhibitor that is present in all tissue and produced at a constant rate, filtered freely and neither secreted into the renal tubules nor absorbed into the bloodstream from the renal tubules. Your body produces Cystatin C constantly and it is found in various body fluids, including blood, spinal fluid, etc. When your kidneys are in a healthy state, they filter cystatin C out of the blood through urine. Cystatin C test is used to assess the functioning of your kidneys or renal system. The levels of protein measure glomerular filtration rate (GFR). Levels of GFR shows how well the kidneys are working. Doctors may also use it to evaluate the progression of a particular kidney disease that a patient might have. When your doctor suspects a kidney-related disease, he might prescribe a Cystatin C test. This is because, in most cases, the levels of creatinine serum are not reliable. Creatinine serum determines the amount of creatinine in your blood. The concentrations of cystatin C in the blood are normal when the renal system functioning is normal. However, as kidney function deteriorates, the level begins to rise. This increase in cystatin C occurs as the glomerular filtration rate falls. Cystatin C levels change with a change in glomerular filtration rate, which is why it is a reliable indicator for the evaluation of renal disorders. Some symptoms might prompt your doctor to recommend a Cystatin C test, like a change in the amount of urine passed in a day, water retention in different body parts, dyspnoea or shortness of breath, painful sensation in the chest or seizures. The prevalence of kidney disease in India is around 800 per million of the population.