A tale of two markers

Diagnostic and prognostic value of cystatin C in acute heart failure

BACKGROUND

The accurate early diagnosis of acute kidney injury (AKI) in patients with acute heart failure (AHF) is an unmet clinical need. Cystatin C might improve the early detection of AKI.

METHODS

207 patients presenting to the emergency department with AHF were enrolled. Cystatin C was measured in plasma in a blinded fashion at presentation and serially thereafter. The potential of Cystatin C levels to predict AKI was assessed as the primary endpoint. Long-term mortality was assessed as a secondary endpoint.

RESULTS

At presentation, creatinine (140μmol/L [91-203] vs. 97μmol/L [76-132], p<0.01) and Cystatin C (2.00mg/L [1.30-3.08] vs. 1.45mg/L [1.00-1.90], p<0.01) levels were significantly higher in AKI compared to Non-AKI patients. The diagnostic accuracy for AKI quantified by the area under the receiver operating characteristic curve was mediocre and comparable for both markers (creatinine 0.68; 95%CI 0.58-78 vs. Cystatin C 0.67; 95%CI 0.58-0.76). Serial measurements of Cystatin C did not further increase the prognostic accuracy for AKI. Cystatin C levels were significantly higher in decedents than in survivors (1.90mg/L [1.30-2.70] vs. 1.30mg/L [1.0-1.6], p<0.001). The combination of Cystatin C and BNP levels significantly improved the prediction of mortality provided by either parameter alone. In multivariable regression analysis Cystatin C remained independently associated with mortality (HR 1.41; 95%CI 1.02-1.95).

CONCLUSION

Plasma Cystatin C levels do not adequately predict AKI in patients with AHF. However, in multivariable regression analysis Cystatin C predicted mortality after the adjustment for baseline renal function, AKI, BNP levels and heart failure risk factors.

Biochemistry and Clinical Role of Human Cystatin C

Low molecular-mass plasma proteins play a key role in health and disease. Cystatin C is an endogenous cysteine proteinase inhibitor belonging to the type 2 cystatin superfamily. The mature, active form of human cystatin C is a single non-glycosylated polypeptide chain consisting of 120 amino acid residues, with a molecular mass of 13,343-13,359 Da, and containing four characteristic disulfide-paired cysteine residues. Human cystatin C is encoded by the CST3 gene, ubiquitously expressed at moderate levels. Cystatin C monomer is present in all human body fluids; it is preferentially abundant in cerebrospinal fluid, seminal plasma, and milk. Cystatin C L68Q variant is an amyloid fibril-forming protein with a high tendency to dimerize. It forms self-aggregates with massive amyloid deposits in the brain arteries of young adults, leading to lethal cerebral hemorrhage. The main catabolic site of cystatin C is the kidney: more than 99% of the protein is cleared from the circulation by glomerular ultrafiltration and tubular reabsorption. The diagnostic value of cystatin C as a marker of kidney dysfunction has been extensively investigated in multiple clinical studies on adults, children, and in the elderly. In almost all the clinical studies, cystatin C demonstrated a better diagnostic accuracy than serum creatinine in discriminating normal from impaired kidney function, but controversial results have been obtained by comparing this protein with other indices of kidney disease, especially serum creatinine-based equations. In this review, we present and discuss most of the available data from the literature, critically reviewing conclusions and suggestions for the use of cystatin C in clinical practice. Despite the multitude of clinical data in the literature, cystatin C has not been widely used, perhaps because of a combination of factors, such as a general diffidence among clinicians, the absence of definitive cut-off values, conflicting results in clinical studies, no clear evidence on when and how to request the test, the poor commutability of results, and no accurate examination of costs and of its routine use in a stat laboratory.

Review article: renal function assessment in cirrhosis - difficulties and alternative measurements

BACKGROUND

Renal function in patients with cirrhosis is important prognostically, both before and following liver transplantation. Its prognostic impact is reflected by the inclusion of serum creatinine in the model for end-stage liver disease score, which is now used for recipient prioritization on liver transplantation waiting lists in the USA.

AIM

To review the accuracy of the surrogate markers for the assessment of renal function, i.e. glomerular filtration rate, particularly in patients with cirrhosis.

METHOD

We reviewed the available literature in PubMed regarding the markers for GFR evaluation and the factors which affect their accuracy in cirrhosis.

RESULTS

Although creatinine is widely available, it is an unreliable marker of glomerular filtration rate, particularly in patients with cirrhosis. Clearance of exogenous markers is considered the 'gold standard', but this methodology has many drawbacks, particularly poor applicability. Several mathematical formulae for estimated glomerular filtration rate are used to overcome some of these limitations: Cockcroft-Gault and Modification of Diet in Renal Disease formulae are the most frequently applied, but they are based on serum creatinine.

CONCLUSIONS

Due to the inaccuracy of serum creatinine and its derived formulae in estimating glomerular filtration rate, alternative serum markers, such as cystatin C, and new formulae are desirable. These need formal evaluation in patients with cirrhosis so as to have a reliable surrogate of glomerular filtration rate, and to obviate many problems that are associated with using creatinine and estimated glomerular filtration rate.

Cystatin C, an easy and reliable marker for assessment of renal dysfunction in children with liver disease and after liver transplantation

Renal dysfunction of variable severity is being increasingly recognized as a major complication of calcineurin inhibitors (CI), in some patients even necessitating renal transplantation. Close and effective monitoring of the renal function is indicated. Current methods for this monitoring are calculation of the glomerular filtration rate (GFR) based on creatinine or exogenous substances like 51Cr-EDTA. The first method is unreliable in children and the second is expensive and cumbersome. Cystatin C has been shown to be an accurate marker of glomerular filtration but has not been evaluated in a large cohort of pediatric patients before and after liver transplantation (LT). We evaluated the accuracy of cystatin C in 62 children (30 male) with LT, who had their 51Cr-EDTA measured on 40 occasions prior to LT and on 47 occasions after LT. The reciprocal of cystatin C correlated better with 51Cr-EDTA GFR (r = .78) than the reciprocal of creatinine (r = .40). Diagnostic accuracy in the identification of reduced GFR was assessed by ROC analysis. Cystatin C yielded the highest area under the ROC curve (AUC) in all groups assessed. From these data a cutoff level of cystatin C predicting 51Cr-EDTA GFR < 80 ml/min/1.73 m2 was calculated. A level of 1.06 mg/L was found to have a sensitivity of 91% and a specificity of 81%. Applying this cutoff level in our patient group would have avoided 51Cr-EDTA GFR estimation in 43 of the 87 estimations. In conclusion, the use of this simple test could be recommended as screening of renal dysfunction in children with liver disease and after LT.

Principles and Clinical Application of Assessing Alterations in Renal Elimination Pathways

Drugs and metabolites are eliminated from the body by metabolism and excretion. The kidney makes the major contribution to excretion of unchanged drug and also to excretion of metabolites. Net renal excretion is a combination of three processes - glomerular filtration, tubular secretion and tubular reabsorption. Renal function has traditionally been determined by measuring plasma creatinine and estimating creatinine clearance. However, estimated creatinine clearance measures only glomerular filtration with a small contribution from active secretion. There is accumulating evidence of poor correlation between estimated creatinine clearance and renal drug clearance in different clinical settings, challenging the 'intact nephron hypothesis' and suggesting that renal drug handling pathways may not decline in parallel. Furthermore, it is evident that renal drug handling is altered to a clinically significant extent in a number of disease states, necessitating dosage adjustment not just based on filtration. These observations suggest that a re-evaluation of markers of renal function is required. Methods that measure all renal handling pathways would allow informed dosage individualisation using an understanding of renal excretion pathways and patient characteristics. Methodologies have been described to determine individually each of the renal elimination pathways. However, their simultaneous assessment has only recently been investigated. A cocktail of markers to measure simultaneously the individual renal handling pathways have now been developed, and evaluated in healthy volunteers. This review outlines the different renal elimination pathways and the possible markers that can be used for their measurement. Diseases and other physiological conditions causing altered renal drug elimination are presented, and the potential application of a cocktail of markers for the simultaneous measurement of drug handling is evaluated. Further investigation of the effects of disease processes on renal drug handling should include people with HIV infection, transplant recipients (renal and liver) and people with rheumatoid arthritis. Furthermore, changes in renal function in the elderly, the effect of sex on renal function, assessment of living kidney donors prior to transplantation and the investigation of renal drug interactions would also be potential applications. Once renal drug handling pathways are characterised in a patient population, the implications for accurate dosage individualisation can be assessed. The simultaneous measurement of renal function elimination pathways of drugs and metabolites has the potential to assist in understanding how renal function changes with different disease states or physiological conditions. In addition, it will further our understanding of fundamental aspects of the renal elimination of drugs.