Nephrology and Clinical Chemistry: The Essential Link

<p>Even if it seems trivial at first glance, the correct interpretation of a laboratory result is not an easy task. Indeed, behind any laboratory result, different factors may be present and influence the real value of the result. These factors are generally poorly known by the clinician, which can sometimes unfortunately cause some misinterpretation of the results. In this chapter, we will present the different sources of variations that can influence an analytical result: the pre-analytical and the analytical variations. We will also discuss two essential keys to correctly interpret a laboratory result: the reference value concept and the reference change value (also called the critical difference). After reading this chapter, clinicians should have a better insight on the complexity of the biological analyses, which could help them in their clinical practice.</p>

<p>Serum creatinine is one of the most common blood tests. In this chapter, we review some historical data regarding creatinine. Different methodologies to measure creatinine in blood and urine are described. We also discuss the physiological reason for its use as a glomerular filtration rate marker. Moreover, analytical and physiological limitations will be described and discussed. The use of the creatinine clearance is also discussed.</p>

<p>Cystatin C is a low molecular weight-protein which has been proposed as a marker of renal function that could replace creatinine. Its concentration is mainly determined by glomerular filtration and is particularly of interest in clinical settings where the relationship between creatinine production and muscle mass impairs the clinical performance of creatinine. Since the last decade, numerous studies have evaluated its potential use in measuring renal function in various populations and other potential developments in clinical settings have been proposed. More recently, research on the standardization has progressed, resulting in the synthesis of an international standard. This review summarizes current knowledge about the physiology of cystatin C and about its use as a renal marker, either alone or in equations developed to estimate the glomerular filtration rate.</p>

<p>Beta-Trace Protein (BTP), also known as lipocalin prostaglandin D2 synthase, is an emerging novel marker of the Glomerular Filtration Rate (GFR). It is synthesized by a wide variety of cell types and is an important constituent of cerebral spinal fluid. The origin of serum BTP remains unclear and the biologic roles of BTP are not fully understood yet. There is only one commercially available BTP assay and higher order reference materials have not been developed. Equations to translate serum BTP levels into estimates of GFR have been developed. Whether BTP provides an incremental benefit over serum creatinine in identifying chronic kidney disease, estimating GFR, or detecting changes in GFR remains unclear. This chapter will provide an overview of the biology of BTP, the analytical aspects of its measurement and the evidence for its utility at diagnosing and following chronic kidney disease.</p>

<p>Acute Kidney Injury (AKI) is increasing to epidemic proportions. Currently available diagnostic tools are less sensitive to diagnose AKI early. Early diagnosis and risk stratification are necessary for prompt therapy and preventing progression of the disease. Finding a reliable, early, reproducible, economical and accurate biomarker for AKI is a top research priority. Many urinary and serum proteins have been intensively investigated as possible early biomarkers of AKI and some of them show great promise. This topic reviews some of the emerging biomarkers of AKI.</p>

<p>The glomerular filtration rate (GFR) is considered as the best overall index of kidney function. The GFR is a crucial component in the evaluation of patients with established chronic kidney disease (CKD). It is also an important tool for screening, diagnosis and staging kidney disease. Several creatinine-based estimation equations are available, and efforts to standardize serum creatinine assays across all laboratories have significantly helped in improving the performance of the equations. While, overall it is unlikely that one single equation will provide a precise and accurate tool to estimate kidney function in all clinical settings, understanding and using these tools in the context of patient care facilitates understanding of its applicability and limitations. This chapter will review the overall concepts of creatinine based estimation equations in various clinical settings.</p>

<p>Chronic kidney disease is persistent kidney injury, usually with reduced kidney function. It may be progressive and it carries an independent risk of cardiovascular morbidity and mortality. It is prevalent in about ten percent of western populations. Its definition relies on laboratory indices of kidney function. Its assessment in populations or individuals requires an understanding of the reliability of laboratory measurements and their conversion to numerical indices of kidney function.</p>