Reduction in mean glomerular pore size coincides with the development of large shunt pores in patients with diabetic nephropathy

Selective glomerular hypofiltration syndrome

The estimated glomerular filtration rate (eGFR) provides insight into cardiovascular disease (CVD) risk stratification and proactive management. Accumulating evidence suggests that combining eGFR calculated from serum cystatin C (eGFRcys) and from serum creatinine (eGFRcrea) improves CVD risk stratification over eGFRcrea alone. The term selective glomerular hypofiltration syndrome (SGHS) or shrunken pore syndrome has been proposed to define an eGFRcys:eGFRcrea ratio <1, which is hypothesized to result from a reduced glomerular filtration of 5- to 30-kDa molecules as compared with smaller molecules. SGHS may be identified in people with normal or reduced measured GFR, but the prevalence depends on the cut-off value of the eGFRcys:eGFRcrea ratio used, which is not yet standardized. SGHS is strongly associated with increased CVD and mortality risks and it may offer an opportunity to expand our understanding of the mechanisms linking GFR disorders with CVD risk (e.g. an altered plasma proteome), which may guide treatment decisions. However, muscle wasting may also contribute to a reduced eGFRcys:eGFRcrea ratio and there are open questions regarding the pathophysiology of a reduced eGFRcys:eGFRcrea ratio, the reference cut-off values of the ratio to define the syndrome and its clinical implications. We now critically review the SGHS concept, its pathophysiological basis and links to CVD and the potential consequences for clinical practice and propose a research agenda.

The complexity of kidney disease and diagnosing it - cystatin C, selective glomerular hypofiltration syndromes and proteome regulation

Estimation of kidney function is often part of daily clinical practice, mostly done by using the endogenous glomerular filtration rate (GFR)-markers creatinine or cystatin C. A recommendation to use both markers in parallel in 2010 has resulted in new knowledge concerning the pathophysiology of kidney disorders by the identification of a new set of kidney disorders, selective glomerular hypofiltration syndromes. These syndromes, connected to strong increases in mortality and morbidity, are characterized by a selective reduction in the glomerular filtration of 5-30 kDa molecules, such as cystatin C, compared to the filtration of small molecules <1 kDa dominating the glomerular filtrate, for example water, urea and creatinine. At least two types of such disorders, shrunken or elongated pore syndrome, are possible according to the pore model for glomerular filtration. Selective glomerular hypofiltration syndromes are prevalent in investigated populations, and patients with these syndromes often display normal measured GFR or creatinine-based GFR-estimates. The syndromes are characterized by proteomic changes promoting the development of atherosclerosis, indicating antibodies and specific receptor-blocking substances as possible new treatment modalities. Presently, the KDIGO guidelines for diagnosing kidney disorders do not recommend cystatin C as a general marker of kidney function and will therefore not allow the identification of a considerable number of patients with selective glomerular hypofiltration syndromes. Furthermore, as cystatin C is uninfluenced by muscle mass, diet or variations in tubular secretion and cystatin C-based GFR-estimation equations do not require controversial race or sex terms, it is obvious that cystatin C should be a part of future KDIGO guidelines.

Shrunken Pore Syndrome: A New and More Powerful Phenotype of Renal Dysfunction Than Chronic Kidney Disease for Predicting Contrast-Associated Acute Kidney Injury

Background Shrunken pore syndrome (SPS) as a novel phenotype of renal dysfunction is characterized by a difference in renal filtration between cystatin C and creatinine. The manifestation of SPS was defined as a cystatin C-based estimated glomerular filtration rate (eGFR) <60% of the creatinine-based eGFR. SPS has been shown to be associated with the progression and adverse prognosis of various cardiovascular and renal diseases. However, the predictive value of SPS for contrast-associated acute kidney injury (CA-AKI) and long-term outcomes in patients undergoing percutaneous coronary intervention remains unclear. Methods and Results We retrospectively observed 5050 consenting patients from January 2012 to December 2018. Serum cystatin C and creatinine were measured and applied to corresponding 2012 and 2021 Chronic Kidney Disease Epidemiology Collaboration equations, respectively, to calculate the eGFR. Chronic kidney disease (CKD) was defined as a creatinine-based eGFR <60 mL/min per 1.73 m² without dialysis. CA-AKI was defined as an increase in serum creatinine ≥50% or 0.3 mg/dL within 48 hours after contrast medium exposure. Overall, 649 (12.85%) patients had SPS, and 324 (6.42%) patients developed CA-AKI. Multivariate logistic regression analysis indicated that SPS was significantly associated with CA-AKI after adjusting for potential confounding factors (odds ratio [OR], 4.17 [95% CI, 3.17-5.46]; P<0.001). Receiver operating characteristic analysis indicated that the cystatin C-based eGFR:creatinine-based eGFR ratio had a better performance and stronger predictive power for CA-AKI than creatinine-based eGFR (area under the curve: 0.707 versus 0.562; P<0.001). Multivariate logistic analysis revealed that compared with those without CKD and SPS simultaneously, patients with CKD and non-SPS (OR, 1.70 [95% CI, 1.11-2.55]; P=0.012), non-CKD and SPS (OR, 4.02 [95% CI, 2.98-5.39]; P<0.001), and CKD and SPS (OR, 8.62 [95% CI, 4.67-15.7]; P<0.001) had an increased risk of CA-AKI. Patients with both SPS and CKD presented the highest risk of long-term mortality compared with those without both (hazard ratio, 2.30 [95% CI, 1.38-3.86]; P=0.002). Conclusions SPS is a new and more powerful phenotype of renal dysfunction for predicting CA-AKI than CKD and will bring new insights for an accurate clinical assessment of the risk of CA-AKI.

Prognostic Performance of Cystatin C in COVID-19: A Systematic Review and Meta-Analysis

Cystatin C is a specific biomarker of kidney function. We perform this meta-analysis to determine the association of Cystatin C with the COVID-19 severity. In this systematic review and meta-analysis, we searched PubMed, EMBASE, Cochrane library, and Web of Science for studies published until 2nd September 2022 that reported associations between Cystatin C levels and COVID-19 severity. The analysis was performed using a random-effects model to calculate pooled standard mean difference (SMD). Twenty-five studies were included in the meta-analysis. Pooled analysis showed statistically significant differences of Cystatin C levels among survive vs. decreased patients (0.998 ± 0.225 vs. 1.328 ± 0.475 mg/dL, respectively; SMD = -2.14; 95%CI: -3.28 to -1.01; p < 0.001). Cystatin C levels in COVID-19 severe vs. non-severe groups varied and amounted to 1.485 ± 1.191 vs. 1.014 ± 0.601 mg/dL, respectively (SMD = 1.81; 95%CI: 1.29 to 2.32; p < 0.001). Additionally, pooled analysis showed that Cystatin C levels in patients with acute kidney injury (AKI) was 1.562 ± 0.885 mg/dL, compared to 0.811 ± 0.108 mg/dL for patients without AKI (SMD = 4.56; 95%CI: 0.27 to 8.85; p = 0.04). Summing up, Cystatin C is a potentially very good marker to be used in the context of COVID-19 disease due to the prognosis of patients' serious condition, risk of AKI and mortality. In addition, Cystatin C could be used as a marker of renal complications in COVID-19 other than AKI due to the need to monitor patients even longer after leaving the hospital.

The predictive value of serum level of cystatin C for COVID-19 severity

To investigate the potential prognostic value of Serum cystatin C (sCys C) in patients with COVID-19 and determine the association of sCys C with severe COVID-19 illness. We performed a retrospective review of medical records of 162 (61.7 ± 13.5 years) patients with COVID-19. We assessed the predictive accuracy of sCys C for COVID-19 severity by the receiver operating characteristic (ROC) curve analysis. The participants were divided into two groups based on the sCys C cut-off value. We evaluated the association between high sCys C level and the development of severe COVID-19 disease, using a COX proportional hazards regression model. The area under the ROC curve was 0.708 (95% CI 0.594-0.822), the cut-off value was 1.245 (mg/L), and the sensitivity and specificity was 79.1% and 60.7%, respectively. A multivariable Cox analysis showed that a higher level of sCys C (adjusted HR 2.78 95% CI 1.25-6.18, p = 0.012) was significantly associated with an increased risk of developing a severe COVID-19 illness. Patients with a higher sCys C level have an increased risk of severe COVID-19 disease. Our findings suggest that early assessing sCys C could help to identify potential severe COVID-19 patients.

Potential relationship between eGFRcystatin C /eGFRcreatinine -ratio and glomerular basement membrane thickness in diabetic kidney disease

Diabetic kidney disease (DKD) is a leading cause of end-stage renal disease and renal replacement therapy worldwide. A pathophysiological hallmark of DKD is glomerular basal membrane (GBM) thickening, whereas this feature is absent in minimal change disease (MCD). According to fundamental transport physiological principles, a thicker GBM will impede the diffusion of middle-molecules such as cystatin C, potentially leading to a lower estimated GFR (eGFR) from cystatin C compared to that of creatinine. Here we test the hypothesis that thickening of the glomerular filter leads to an increased diffusion length, and lower clearance, of cystatin C. Twenty-nine patients with a kidney biopsy diagnosis of either DKD (n = 17) or MCD (n = 12) were retrospectively included in the study. GBM thickness was measured at 20 separate locations in the biopsy specimen and plasma levels of cystatin C and creatinine were retrieved from health records. A modified two-pore model was used to simulate the effects of a thicker GBM on glomerular water and solute transport. The mean age of the patients was 52 years, and 38% were women. The mean eGFRcystatin C /eGFRcreatinine -ratio was 74% in DKD compared to 98% in MCD (p < 0.001). Average GBM thickness was strongly inversely correlated to the eGFRcystatin C /eGFRcreatinine -ratio (Pearson's r = -0.61, p < 0.01). Two-pore modeling predicted a eGFRcystatin C /eGFRcreatinine -ratio of 78% in DKD. We provide clinical and theoretical evidence suggesting that thickening of the glomerular filter, increasing the diffusion length of cystatin C, lowers the eGFRcystatin C /eGFRcreatinine -ratio in DKD.

Shrunken pore syndrome - a common kidney disorder with high mortality. Diagnosis, prevalence, pathophysiology and treatment options

Invasive studies show that the glomerular sieving coefficients for 5-30 kDa plasma proteins in the human kidney may be selectively reduced compared to those for small molecules < 0.9 kDa, commonly used to measure glomerular filtration rate (GFR). Identification of this pathophysiological state, called shrunken pore syndrome (SPS), can easily and non-invasively be done by comparing estimations of GFR using cystatin C (13.3 kDa) and creatinine (0.113 kDa). SPS is present if the estimate of GFR using cystatin C is lower than 60 or 70% of the estimate using creatinine in the absence of non-renal influences on cystatin C or creatinine. All studies of SPS show that the 3- or 5-year mortality is strongly increased and high hazard ratios for mortality associated with SPS have been observed for many different patient cohorts, including cohorts with normal measured GFR, no albuminuria and no diagnosis. The prevalence of SPS in the cohorts so far investigated is between 0.2 and 36%. Proteome studies of SPS demonstrate that the high mortality associated with the syndrome might be caused by the accumulation of 10-30 kDa signalling proteins promoting development of atherosclerosis and thus suggesting use of monoclonal antibodies to reduce the levels of the most detrimental signalling proteins as a treatment option. The KDIGO recommendations for classification of chronic kidney disease (CKD) comprise determination, or estimation, of GFR and analysis of albuminuria and therefore cannot identify a large fraction of the patients with SPS. The high prevalence and mortality of SPS and the possible treatment options strongly suggest that the KDIGO recommendations should be expanded to include determination of cystatin C to be able to identify all patients with SPS.

High first-trimester maternal blood cystatin C levels despite normal serum creatinine predict pre-eclampsia in singleton pregnancies

Early biochemical identification of women at high risk for the development of pre-eclampsia (PE) is still unsatisfactory. Renal markers measured during the first trimester were analysed to predict later occurrence of PE. A nested case-control study was conducted within the prospective predictive markers for the diagnosis of preeclampsia study. Pregnant women were included at the end of the first trimester and followed up until birth. Controls were matched to PE cases. Renal markers [i.e. creatinine, cystatin C (CysC), β₂ microglobulin (B2M), β-trace protein (BTP), glomerular filtration rate estimations (eGFR) of the aforementioned markers, uric acid (UA), urea, and serum uromodulin (sUMOD)] were compared to placental growth factor (PlGF), a marker known to predict PE later in pregnancy. Reference intervals were determined for the different markers. In the 183 women (PE, n = 39; controls, n = 144), CysC, the CysC/PlGF ratio (p < .01) and UA were higher, whereas the eGFR_CysC/eGFR_Crea ratio (a marker of glomerular endothelial integrity and shrunken pore syndrome) and PlGF were lower in women who developed PE (p < .05 for all). Compromised filtration of the larger molecule CysC together with a normal creatinine, in a subset of PE cases (15.3%) was a unique, strong and independent predictor of later PE if the baseline CysC concentration was >0.85 mg/l. In conclusion, CysC and its derivatives as well as UA, indicating volume expansion, measured at the end of the first trimester are predictive of PE. Thus, women can be easily identified and followed as an early reduction in glomerular filtration quality poses a high risk for a subsequent development of PE.

Shrunken Pore Syndrome is associated with a sharp rise in mortality in patients undergoing elective coronary artery bypass grafting

Shrunken Pore Syndrome was recently suggested for the pathophysiologic state in patients characterized by an estimation of their glomerular filtration rate (GFR) based upon cystatin  C, which is lower or equal to 60% of their estimated GFR based upon creatinine, i.e. when eGFR_{cystatin  C} ≤ 60% of eGFR_creatinine. Not only the cystatin  C level, but also the levels of other low molecular mass proteins are increased in this condition. The preoperative plasma levels of cystatin  C and creatinine were measured in 1638 patients undergoing elective coronary artery bypass grafting. eGFR_{cystatin  C} and eGFR_creatinine were calculated using two pairs of estimating equations, CAPA and LMrev, and CKD-EPI_{cystatin  C} and CKD-EPI_creatinine, respectively. The Shrunken Pore Syndrome was present in 2.1% of the patients as defined by the CAPA and LMrev equations and in 5.7% of the patients as defined by the CKD-EPI_{cystatin  C} and CKD-EPI_creatinine equations. The patients were studied over a median follow-up time of 3.5 years (2.0–5.0 years) and the mortality determined. Shrunken Pore Syndrome defined by both pairs of equations was a strong, independent, predictor of long-term mortality as evaluated by Cox analysis and as illustrated by Kaplan-Meier curves. Increased mortality was observed also for the subgroups of patients with GFR above or below 60 mL/min/1.73 m². Changing the cut-off level from 60 to 70% for the CAPA and LMrev equations increased the number of patients with Shrunken Pore Syndrome to 6.5%, still displaying increased mortality.

Cystatin C-based glomerular filtration rate associates more closely with mortality than creatinine-based or combined glomerular filtration rate equations in unselected patients

BACKGROUND

Decreased glomerular filtration rate (GFR) is an important cardiovascular risk factor, but estimated GFR (eGFR) may differ depending on whether it is based on creatinine or cystatin C. A combined creatinine/cystatin C equation has recently been shown to best estimate GFR; however, the benefits of using the combined equation for risk prediction in routine clinical care have been less studied. This study compares mortality risk prediction by eGFR using the combined creatinine/cystatin C equation (CKD-EPI), a sole creatinine equation (CKD-EPI) and a sole cystatin C equation (CAPA), respectively, using assays that are traceable to international calibrators.

METHODS AND RESULTS

All patients analysed for both creatinine and cystatin C from the same blood sample tube (n = 13,054) during 2005-2007 in Uppsala University Hospital Laboratory were divided into eGFR risk categories>60, 30-60 and <30 mL/min/1.73 m(2) by each eGFR equation. During follow-up (median 4.6 years), 4398 participants died, of which 1396 deaths were due to cardiovascular causes. Reduced eGFR was significantly associated with death as assessed by all eGFR equations. The net reclassification improvement (NRI) for the combination equation compared with the sole creatinine equation was 0.10 (p < 0.001) for all-cause mortality and 0.08 (p < 0.001) for cardiovascular mortality, indicating improved reclassification. In contrast, NRI for the combination equation, compared with the sole cystatin C equation, was -0.06 (p < 0.001) for all-cause mortality and -0.02 (p = 0.032) for cardiovascular mortality, indicating a worsened reclassification.

CONCLUSIONS

In routine clinical care, cystatin C-based eGFR was more closely associated with mortality compared with both creatinine-based eGFR and creatinine/cystatin C-based eGFR.

Cystatin C is correlated with mortality in patients with and without acute kidney injury

BACKGROUND

Recent research has shown cystatin C to predict mortality and cardiovascular morbidity independent of renal function. The aim of this study was to evaluate the prognostic value of cystatin C on mortality in adult general ICU patients with acute kidney injury (AKI). We later expanded the study and included patients without signs of AKI.

METHODS

A total of 845 ICU patients were analysed for cystatin C and classified according to the RIFLE criteria. Of these, 271 patients with either creatinine >150 micromol/l, urea >25 or anuria/oliguria entered the AKI cohort. The remaining 562 patients entered the non-AKI cohort. Both cohorts were divided into quartiles according to cystatin C at entry. In the non-AKI cohort, we split the highest cystatin C quartile into two. The relationship between the different cystatin C quartiles and mortality in patients with and without AKI was estimated by hazard ratios (HR) derived from the Cox proportional hazards regression model.

RESULTS

A relationship between cystatin C and mortality was found in patients with and without AKI, being stronger in patients without AKI. In AKI patients, the HR comparing cystatin C above and below the median more than doubled from the second year on compared to the first year follow-up. After exclusion of patients in the non-AKI cohort with 'potential AKI' (creatinine >100 micromol/l or urea > 20 mmol/l), the correlation between cystatin C levels and risk of death was strengthened.

CONCLUSIONS

Cystatin C is correlated with mortality independently of renal function measured by creatinine in patients entering the general ICU.

Size and charge selectivity of the glomerular filter in early experimental diabetes in rats

Microalbuminuria is an early sign of diabetic nephropathy. The aim of the present study was to investigate whether the changes of the glomerular filtration barrier in early experimental diabetes are due to size- or charge-selective alterations. Wistar rats, made diabetic by streptozotocin (STZ) and having their blood glucose maintained at ∼20 mM for 3 or 9 wk, were compared with age-matched controls. Glomerular clearances of native albumin (Cl-HSA) and neutralized albumin (Cl-nHSA) were assessed using a renal uptake technique. Glomerular filtration rate and renal plasma flow were assessed using51Cr-EDTA and [125I]iodohippurate, respectively. In a separate set of animals, diabetic for 9 wk, and in controls, glomerular sieving coefficients (θ) for neutral FITC-Ficoll (molecular radius: 15–90 Å) were assessed using size exclusion chromatography. At 3 wk of diabetes, Cl-HSA and Cl-nHSA remained unchanged, indicating no alteration in either size or charge selectivity. By contrast, at 9 wk of diabetes, there was a twofold increase of Cl-HSA, whereas Cl-nHSA remained largely unchanged, at first suggesting a glomerular charge defect. However, according to a two-pore model, the number of large pores, assessed from both Ficoll and Cl-HSA, increased twofold. In addition, a small reduction in proximal tubular reabsorption was observed at 3 wk, which was further reduced at 9 wk. In conclusion, no functional changes were observed in the glomerular filtration barrier at 3 wk of STZ-induced diabetes, whereas at 9 wk there was a decrease in size selectivity due to an increased number of large glomerular pores.

Reduced urinary excretion of sulfated polysaccharides in diabetic rats

The aim of the present study was to further understand the changes in renal filtration that occur in the early stages of diabetes mellitus. Diabetes was induced in male Wistar rats by a single injection of streptozotocin. Glycemia, body weight, 24-h urine volume and urinary excretion of creatinine, protein and glycosaminoglycans were measured 10 and 30 days after diabetes induction. All the diabetic animals used in the present study were hyperglycemic, did not gain weight, and presented proteinuria and creatinine hyperfiltration. In contrast, the glycosaminoglycan excretion decreased. Dextran sulfates of different molecular weights (6.0 to 11.5 kDa) were administered to the diabetic rats, and to age-matched, sham-treated controls. Most of the dextran sulfate was excreted during the first 24 h, and the amounts excreted in the urine were inversely proportional to the dextran sulfate molecular weight for all groups. Nevertheless, diabetic rats excreted less and accumulated more dextran sulfate in kidney and liver, as compared to controls. These differences, which were observed only for the dextran sulfates of higher molecular weights (>7 kDa), increased with the duration of diabetes. Our findings suggest differential renal processing mechanisms for proteins and sulfated polysaccharides, with the possible involvement of kidney cells.

Early detection of acute renal failure by serum cystatin C

BACKGROUND

Acute renal failure (ARF) is associated with high mortality. Presently, no specific therapy for ARF exists. Therefore, early detection of ARF is critical to prevent its progression. However, serum creatinine, the standard marker to detect ARF, demonstrates major limitations. We prospectively evaluated whether serum cystatin C detected ARF earlier than serum creatinine.

METHODS

In 85 patients at high risk to develop ARF, serum creatinine and cystatin C were determined daily. ARF was defined according to the Risk of renal dysfunction, Injury to the kidney, Failure of kidney function, Loss of kidney function, and ESRD (RIFLE) classification when creatinine increased by >/=50% (R-criteria), by >/=100% (I-criteria), or by >/=200% (F-criteria). In analogy, ARF was detected when cystatin C increased by >/=50%, by >/=100%, or by >/=200%.

RESULTS

Forty-four patients developed ARF and 41 served as controls. In ARF by R-, I-, and F-criteria, the increase of cystatin C significantly preceded that of creatinine. Specifically, serum cystatin C increased already by >/=50% 1.5 +/- 0.6 days earlier compared to creatinine. Serum cystatin C demonstrated a high diagnostic value to detect ARF as indicated by area under the curve of the ROC analysis of 0.82 and 0.97 on the two days before the R-criteria was fulfilled by creatinine. Cystatin C detected ARF according to the R-criteria with a sensitivity of 55% and 82% on these days, respectively. Cystatin C also performed excellently, detecting ARF defined by the I- and F-criteria two days prior to creatinine, and moderately well predicting renal replacement therapy in the further course of ARF. Additionally, low T(3)- or T(3)/T(4) syndrome, glucocorticoid deficiency and excess did not affect cystatin C levels, adding to its usefulness in critically ill patients with ARF.

CONCLUSION

Serum cystatin C is a useful detection marker of ARF, and may detect ARF one to two days earlier than creatinine.