The Superfamily of Cysteine Protease Inhibitors
In 1981, the amino acid sequence of human Cystatin C was determined, but it showed no homology with any known protein superfamily at the time. It was subsequently proven to belong to a new protein superfamily. The amino acid sequence of Cystatin C was the first from the cystatin family to be examined. However, it was not until two years later, when researchers examined the structure of chicken cystatin and found 44% amino acid sequence identity between the two proteins, that the function of Cystatin C as an inhibitor of cysteine proteases was confirmed. After another two decades of research, ten additional inhibitors of human cysteine proteases were discovered, and all were found to have strong homology with Cystatin C and chicken cystatin. Therefore, they all belong to the human cystatin superfamily.
The human cystatin family currently consists of 11 identified proteins. Type 1 cystatins, comprising cystatins A and B, are primarily intracellular proteins. Type 2 cystatins, comprising cystatins C, D, E, F, S, SA, and SN, are mainly extracellular and/or intercellular transport proteins. Type 3 cystatins comprise high and low molecular weight kininogens. They are primarily intravascular proteins and, besides being inhibitors of cysteine proteases, they are also involved in the coagulation process and the generation of vasoactive peptides.
Biological Functions of Cystatin C
Besides being an inhibitor of papain-like cysteine proteases, Cystatin C has recently been confirmed to inhibit other families of cysteine proteases, such as human legumain, a classical enzyme of the family C13 peptidases. It has also been discovered that these two inhibition sites are non-overlapping. Therefore, a single Cystatin C molecule can simultaneously inhibit different types of cysteine proteases. Recent studies have also found that Cystatin C plays a role in atherosclerosis and antigen presentation and acts as a growth factor for mesenchymal stem cells. Mice lacking the Cystatin C gene ("Cystatin C knockout mice") appear more susceptible to cancer metastasis.
Distribution of Cystatins in Body Fluids
The distribution of different cystatins in body fluids varies significantly. For example, Cystatin D is mainly found in saliva and tears, whereas Cystatin C is present in various body fluids. In some compartments, such as cerebrospinal fluid, Cystatin C accounts for over 90% of the total molar concentration of cystatins. In other compartments, like plasma, its proportion is much smaller. Furthermore, the total concentration of cystatins also differs across body compartments. For instance, the total inhibitory capacity against papain in blood is approximately 12 μmol/L, whereas in cerebrospinal fluid, its concentration is less than 1 μmol/L. Although the ranges of individual cystatins partially overlap, each body fluid represents a unique set of cystatins, and different fluids exhibit specific cystatin profiles.
Serum/Plasma Cystatin C as a Marker of Glomerular Filtration Rate (GFR)
Production of Cystatin C
The structure of the human Cystatin C gene and its promoter region has been demonstrated to be that of a housekeeping gene, suggesting that the rate of Cystatin C production by most nucleated cells is relatively constant. The presence of a hydrophobic signal sequence in the pre-Cystatin C further indicates that this protein is normally secreted. Indeed, immunochemical and Northern blot studies of human tissues and cell lines have detected Cystatin C and/or its mRNA in all cell types examined. Similarly, studies on Cystatin C production by human stem cell lines in culture media found that almost all cell lines studied secreted Cystatin C. Measurements of Cystatin C levels in large patient populations show that levels are primarily influenced by GFR and are independent of other pathological conditions, consistent with Cystatin C being a constantly secreted protein. Some reports suggest that stimulating macrophages in vitro can downregulate Cystatin C secretion, but inflammatory conditions do not typically lower Cystatin C levels in vivo. A recent report indicates that Cystatin C produced by vascular smooth muscle cells may reduce the occurrence of aortic aneurysms.
Metabolism of Cystatin C
Generally, plasma proteins with a molecular weight below 15-25 kDa can almost freely pass through the glomerular filtration barrier and are then completely reabsorbed and degraded by proximal tubular cells. Theoretically, this should also apply to Cystatin C, which has a molecular weight of only 13 kDa and a slightly elliptical structure with a radius of approximately 30-35 Å. Experimentally, studies in rats administered Cystatin C showed that its clearance rate was 94% of the clearance rate of the gold standard Cr-EDTA, indicating that Cystatin C is almost freely filtered by the glomerulus. Over 99% of the filtered Cystatin C was found to be degraded by tubular cells. By partially constricting the aorta above the renal artery in a group of rats to varying degrees, thereby reducing GFR, a study found an excellent correlation (r=0.99) between Cystatin C and Cr-EDTA clearance, with the y-intercept nearly at zero. This study suggested that tubular reabsorption of Cystatin C is negligible. Immunohistochemical and Northern blot studies on human kidneys have also confirmed that Cystatin C, after passing through the glomerular filter, is normally degraded by proximal tubular cells.
Clinical Applications of Serum/Plasma Cystatin C as a GFR Marker
Since most tissues produce Cystatin C, and as a low-molecular-weight protein, it is freely filtered by the glomerulus, its serum or plasma level is a potential marker for GFR. Early studies in 1984-1985 already found that Cystatin C was at least as good a marker for GFR as serum creatinine. These studies also found that serum Cystatin C levels reflected GFR better than levels of other low-molecular-weight proteins like beta-2-microglobulin and retinol-binding protein. However, in these early studies, Cystatin C concentration was measured using radioimmunodiffusion, a very time-consuming method requiring 10-20 hours, with a relatively high coefficient of variation (around 10%). This reduced the clinical practicality of serum Cystatin C as a GFR marker. A decade later, fully automated particle-enhanced turbidimetric immunoassays (PETIA) and sandwich enzyme immunoassays were developed. These were not only rapid but also precise, significantly enhancing the potential for routine clinical use of serum Cystatin C as a GFR marker. Since 1994, following the introduction of a fully automated PETIA for serum Cystatin C, most literature on serum Cystatin C as a GFR marker has utilized commercial PETIA methods, along with a commercial particle-enhanced turbidimetric immunoassay introduced in 1998.
Although serum creatinine is still widely used as an indicator of GFR, it often remains within the normal range even when GFR has significantly decreased, sometimes by up to 50%. Serum creatinine levels as a GFR marker are influenced by factors like exercise, tubular secretion, creatinine reabsorption, and diet. Due to these significant drawbacks, researchers have sought better indicators of GFR. Some recent studies have compared serum Cystatin C and creatinine as GFR markers against "gold standard" methods involving plasma clearance of injected low-molecular-weight substances like Cr-EDTA, Tc-DTPA, and iohexol. Some studies show that Cystatin C reflects GFR better than serum creatinine, especially in patients with mild to moderate GFR reduction, while others find both parameters perform similarly as GFR markers.
Almost all researchers emphasize that serum Cystatin C, unlike serum creatinine, is not influenced by gender or muscle mass. Some studies indicate that as GFR significantly declines with age, serum Cystatin C can use a single reference range for males and females aged 1-50 years. In predicting total fasting serum homocysteine levels, serum Cystatin C performs better than serum creatinine, likely due to its close relationship with GFR.
Cystatin C and Chemotherapy
Studies on cancer patients before and during chemotherapy have found that serum Cystatin C reflects decreased creatinine clearance better than serum creatinine, especially in early renal failure. Researchers suggest using serum Cystatin C instead of creatinine clearance as a screening test before chemotherapy and for dose adjustment in patients with reduced GFR.
Cystatin C and Diabetes Mellitus
Plasma/serum Cystatin C has been reported as an effective tool for monitoring early nephropathy in non-insulin-dependent diabetes mellitus, performing better than serum creatinine and beta-2-microglobulin. Another report indicates that measuring Cystatin C concentration alone is more reliable than plasma creatinine or creatinine clearance for assessing normal renal function in diabetic patients.
Cystatin C in Neonates
Cataldi et al. studied serum Cystatin C levels in healthy pregnant women and their healthy newborns. The results showed no significant correlation between maternal and infant Cystatin C levels. Unlike creatinine, infant Cystatin C levels were not influenced by maternal serum levels. Therefore, Cystatin C can be used to monitor GFR in perinatal women. Finney et al. studied serum Cystatin C in premature infants, infants under 1 month, and older children. They found that serum Cystatin C reflected GFR better than serum creatinine in children, as it correlated closely with the maturation of renal function. Plasma Cystatin C levels in preterm infants were significantly elevated across all gestational ages and reached adult levels by 1 year of age. In children under 1 year, plasma Cystatin C levels reflect renal immaturity, eliminating the need for serum creatinine measurement, which is influenced by increasing muscle mass during growth.
Cystatin C and Preeclampsia
Pregnancy-related disorders like gestational hypertension, renal structural changes, decreased GFR, and others increase risks for both mother and fetus. Over 50,000 women die annually from pregnancy-related disorders. Therefore, sensitive and specific diagnostic tests are needed to closely monitor renal function, ensuring timely delivery before the development of toxemia and severe renal impairment. Recent studies show that serum Cystatin C has better diagnostic accuracy for preeclampsia compared to serum uric acid and creatinine. Cys C has been proven to be a valuable marker for pregnancy complicated by preeclampsia.
Cystatin C and Kidney Transplantation
Bricon et al. studied patients in the first four days after kidney transplantation and found that plasma Cystatin C levels decreased more sharply than creatinine. Plasma Cystatin C was also better than plasma creatinine for monitoring GFR decline. In studies covering four acute rejection episodes and one acute nephrotoxicity episode, serum Cystatin C concentrations correlated broadly with plasma creatinine concentrations. Interestingly, plasma Cystatin C increased more markedly than creatinine. Using serum Cystatin C as an indicator would have allowed earlier diagnosis in one acute rejection episode and the acute nephrotoxicity episode. During a 3-month follow-up of these patients, researchers compared Cr-EDTA clearance, plasma Cystatin C, endogenous creatinine clearance, and plasma creatinine as GFR markers. Results showed creatinine overestimated GFR by 30-40%, had approximately 25% false negatives, and poor diagnostic accuracy. Serum Cystatin C reflected GFR well, correlated strongly with Cr-EDTA clearance, and had fewer false negatives.
Acute rejection occurs in 20% to 40% of patients after kidney transplantation. If diagnosed and treated early, most rejection episodes can be reversed with restored renal function. Cys C is significantly superior to Scr in diagnosing acute rejection post-transplant, meaning Cys C levels rise earlier than Scr (by 2.7 ± 1.8 days). Compared to pre-rejection levels, Cys C increases by 148.9%, much higher than Scr's 43.9%. Studies show that Scr declines slowly after transplantation, becoming negative (below the cutoff of 122 μmol/L) around 1 week. In contrast, Cys C declines rapidly within the first 3 days post-surgery, especially on the first day (up to 69.2%). By the second day, 91% of patients have negative Cys C levels (below the cutoff of 1.79 mg/L). Some literature also reports an immediate post-transplant drop in Cys C of 29.3 ± 1.7%. This advantage of Cys C is particularly useful for diagnosing accelerated rejection (often occurring 2-5 days post-surgery), where Scr elevation might be masked by its post-operative decline, while Cys C can quickly turn from negative to positive, showing a clear change.
Infection is the most common complication after kidney transplantation. Within the first year post-transplant, about 70% of patients experience at least one infection, with bacterial infections accounting for over 50%. Research indicates that although the magnitude of increase during infection might not differ significantly between Cys C and Scr, Cys C levels change earlier than Scr (by 4.4 ± 1.5 days). This facilitates early detection, which has significant clinical implications for transplant patients. Infections in these patients may present with atypical early symptoms/signs, or leukocyte and neutrophil counts may not rise due to immunosuppressant use, making laboratory detection difficult. Moreover, infections can be severe and life-threatening or lead to graft loss if not treated promptly.
Cys C can rapidly indicate acute rejection and response to drug therapy
For monitoring GFR, Cystatin C is more sensitive than creatinine. In organ transplant patients, testing for Cys C can help rapidly diagnose acute rejection or potential renal damage caused by drug therapy.
Stable Cys C levels at baseline by day 6 post-transplant indicate a relatively stable post-operative course.
Cys C declines faster than creatinine on the first day post-transplant, providing a clearer indication of renal function status.
In uncomplicated cases, post-operative Cys C variation is less than 20%.
Cystatin C is a sensitive marker for diagnosing early Acute Kidney Injury (AKI)
AKI occurs in up to 5% of all hospitalized patients and up to 50% of ICU patients. Mortality rates for AKI have not improved over the past 15 years. Early AKI is often reversible, but diagnosis is frequently delayed. As a routine marker for detecting AKI, serum creatinine has significant limitations. Compared to serum creatinine, blood Cys C can detect AKI approximately 1.5 days earlier.
Serum Cystatin C and Serum Creatinine
Despite their differences, both serum Cystatin C and serum creatinine can correctly identify certain types of GFR disorders. Some renal diseases may differentially affect the filtration of Cystatin C (positively charged, 13343 Da) and creatinine (uncharged, 113 Da). Indeed, recent research shows that in type 1 diabetic patients with proteinuria, GFR estimated by Cystatin C decreases due to reduced glomerular filtration pore size, whereas iothalamate clearance (using a 613 Da marker) indicates normal GFR in these patients. Thus, Cys C can confirm reduced GFR for small molecules in the ~10-35 kDa range.
Another example is the physiological difference between serum Cys C and creatinine before the first circulation and urine excretion after kidney transplantation. During this period, Cys C might be filtered and degraded by tubular cells, potentially lowering its serum level. Conversely, serum creatinine does not decrease during this period because, although filtered, it is neither degraded nor secreted. This is supported by studies in the post-transplant phase. Other physiological examples involving differential handling of low molecular weight substances and molecules in Cys C's range during glomerular filtration occur in pregnant women, where GFR for low molecular weight substances increases, but levels of molecules in Cys C's range may decrease.
Ideal Properties of a Glomerular Filtration Rate (GFR) Marker
GFR is a crucial indicator of renal function, generally assessed by measuring "renal function markers." An ideal marker for GFR should possess the following properties:
Not bound to plasma proteins, allowing free filtration by the glomerulus.
Not reabsorbed or secreted by renal tubules.
The kidney is its sole route of elimination.
For endogenous markers, the rate of release into the bloodstream from tissues should be constant. For exogenous markers, they should be non-toxic substances not metabolized in the body.
Cystatin C: An Endogenous Marker Approaching the Ideal for Reflecting Glomerular Filtration Function
As a low-molecular-weight protein carrying a positive charge at physiological pH in body fluids, Cys C can be freely filtered by the glomerulus. It is not secreted by renal tubular epithelial cells. Although reabsorbed in the proximal convoluted tubule, it is completely catabolized and does not return to the bloodstream. The kidney is the sole organ for clearing circulating Cys C. Because the Cys C gene is a "housekeeping gene" expressed in almost all nucleated cells without tissue specificity, the body's production rate of Cys C is relatively constant.
Stability of Cystatin C
Numerous studies have shown that Cystatin C is stable in serum or plasma for at least 7 days at room temperature, several weeks at 4°C, and several months at -20°C or -80°C. Levels remain unchanged even after more than seven freeze-thaw cycles, and storage in unseparated blood for 24 hours does not affect Cys C levels.
Reference Values for Serum Cystatin C
Establishing clinically usable reference values requires a calibrator, which also helps validate the performance of relevant quantitative assay procedures. Recombinant human Cystatin C is easily produced and purified and can be used to create reliable calibrators. A first step towards international standardization involved producing a high-purity solution of recombinant Cys C, determining its concentration by quantitative amino acid analysis and spectrophotometry, and then diluting it to physiological concentrations using human serum free of endogenous Cys C. Using such calibrators and commercial particle-enhanced turbidimetric immunoassays, reference value studies have been conducted in adult and pediatric populations. Adult results typically show no gender difference, with Cys C levels increasing with age, consistent with the age-related decline in GFR. However, the GFR decline is less pronounced below age 50. Therefore, some propose using partitioned reference ranges (e.g., 20-50 years and over 50 years).
Pediatric Cys C levels differ somewhat from creatinine levels. In children over 1 year old, Cys C levels show little variation and no gender difference. Hence, it is suggested that the reference range for children over 1 year could be the same as for the 20-50 year age group: <1.02 mg/L.
Since almost all plasma proteins with molecular weights below 15-25 kDa are freely filtered by the normal glomerulus, their concentrations in the body are influenced not only by their production rate but also, to some extent, by GFR. Therefore, the production rate of low-molecular-weight proteins can be significant in disease processes. Measuring the ratio of such low-molecular-weight proteins to Cys C might offer better specificity than measuring their levels alone. For example, age affects GFR and the reference value for beta-2-microglobulin, but the ratio of beta-2-microglobulin to Cys C remains unaffected. This ratio could be a more specific marker of cellular proliferation than measuring plasma beta-2-microglobulin alone.
Recommendation for Using Serum Cystatin C as a GFR Marker
Numerous studies indicate that Cystatin C is a better GFR marker than creatinine, particularly for detecting small, early decreases in GFR—an area sometimes referred to as the "creatinine-blind GFR range." Effective clinical use of Cys C requires precise quantitative methods and the use of non-turbid samples. Current particle-enhanced turbidimetric immunoassays generally meet precision requirements, but fasting, non-turbid samples should be used unless methods are continuously improved to guarantee freedom from interference by turbidity. Combining serum creatinine and Cys C measurements can provide more accurate GFR information, especially when invasive and expensive clearance measurements are not feasible due to biomedical or economic reasons. If both Cys C and creatinine fall within their respective reference ranges, the chance of missing a decreased GFR is very small. When GFR is measured by invasive clearance methods, serum Cys C can serve as a substitute for creatinine to follow GFR changes over time.
Important Considerations for Using Serum Cystatin C as a GFR Marker in Practice
When the fully automated particle-enhanced turbidimetric immunoassay was first introduced, it was claimed to be unaffected by high triglycerides. However, with wider use, it became apparent that results could be influenced by turbidity from endogenous chylomicronemia in specimens, potentially affecting Cys C concentration measurements. This analytical interference from chylomicronemia might explain some reports of considerable biological variation for Cys C, leading some researchers to conclude that Cys C offered no significant advantage over creatinine as a GFR marker. Studies on the biological variation of Cys C, using non-turbid specimens, confirm that Cys C can fully replace creatinine as a GFR marker.
Research has also found that some rheumatoid factors can interfere with particle-enhanced turbidimetric immunoassays, causing falsely elevated results.
It is important to emphasize that although fully automated particle-enhanced turbidimetric immunoassays are more precise than the earlier immuno-diffusion methods for quantifying Cys C, their precision is still generally lower than most methods for measuring creatinine. Furthermore, the significant inter-individual variation in Cys C levels demands high-precision methods to meet the Cotlove criteria and significantly enhance the clinical utility of serum Cys C.
Introduction to the Traceability of Cystatin C Calibrators
Serum Cystatin C is recognized as an ideal marker for reflecting GFR. However, differences in calibrators and assay methods often lead to significant result variability. Therefore, in 2007, the IFCC Working Group addressed the issue of Cystatin C standardization. Laboratories including DAKO, Roche, Dade Behring, and the Institute for Reference Materials and Measurements (IRMM) were tasked with the standardization work. In May 2010, a Cystatin C reference material traceable to ERM®-DA471/IFCC was introduced.
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