Cystatin C

Cystatin C is a protein produced by all nucleated cells in the human body. It belongs to the family of cysteine protease inhibitors—enzymes that can break down protein substances. Cystatin C inhibits the activity of these enzymes and prevents them from degrading the extracellular protein matrix. As a result, the level of cystatin C reflects the intensity of synthesis or breakdown processes of extracellular structures, including vessel walls (for example, in atherosclerosis) or myocardial regeneration (heart failure, ischemia due to heart muscle damage). These properties make cystatin C a highly sensitive laboratory marker for assessing the severity and prognosis of cardiovascular diseases.

Currently, the primary diagnostic application of measuring cystatin C levels is in evaluating kidney function. Numerous studies have shown that the synthesis rate of cystatin C in the body is constant and virtually independent of anthropometric indicators like sex, age, body weight, and muscle mass. Typically, the kidneys’ filtration capacity is the sole factor determining cystatin C concentration in the blood serum. Cystatin C, being a low-molecular-weight protein, is freely filtered in the kidney glomeruli. It is then reabsorbed and completely metabolized in the renal tubules, meaning it is broken down in the kidneys and does not return to the bloodstream but is instead excreted through the urinary bladder. The rate at which fluid is filtered through the kidney glomeruli is known as the Glomerular Filtration Rate (GFR) and is a crucial parameter for assessing kidney function. Dysfunction of the kidney’s glomerular apparatus leads to a decrease in GFR and, consequently, to the accumulation of certain substances (including cystatin C) in the blood. Thus, there is a correlation between reduced GFR and increased cystatin C levels in the blood. Taking this into account, specific formulas have been developed to accurately calculate GFR based on the serum concentration of cystatin C.