This blood test measures cystatin C, a small protein produced by cells throughout the body and filtered out by the kidneys. The result is used to estimate the glomerular filtration rate (eGFR), which reflects how well the kidneys are filtering blood. Cystatin C levels are less affected by muscle mass, age, and diet than creatinine, so this test can be a useful alternative or complement to creatinine-based kidney assessments. The test is used to evaluate kidney function and estimate the filtration rate of the kidneys. It may be ordered when creatinine-based estimates may be less reliable, such as in people with unusual muscle mass, or to provide additional information about kidney health.