Nephrology and Clinical Chemistry: The Essential Link

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.

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.

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.

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.

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.

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.

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.

Renal anemia is a frequent complication of impaired renal function. It has many negative consequences on quality of life, exercise capacity, cardio-vascular events and mortality. The treatment of renal anemia has considerably been improved and facilitated by the use of erythropoiesis-stimulating agents (ESA). Such a treatment often leads to functional iron deficiency, which is the main cause of resistance to ESA therapy. It is of clinical and pharmaco-economical importance to identify functional iron deficiency. The available biological parameters are ferritin (FRT), transferrin saturation (TSAT), reticulocyte hemoglobin content (CHr), percentage of hypochromic red blood cells (%HYPO), percentage of hypochromic mature erythrocytes (%HYPOm), soluble transferrin receptors (sTfR) and, maybe, hepcidin. The more reliable parameters to predict the response to iron supplementation seem to be CHr, %HYPO and %HYPOm.

PTH determination is routinely used in Nephrology as a surrogate marker for the diagnostic and follow-up of Chronic Kidney Disease Mineral and Bone Disorders (CKD-MBD). Nevertheless, this determination is far from an easy task. Indeed, this peptide circulates as a mixture of active PTH (1-84) and multiple fragments that accumulate in CKD and interfere, with different percentages of crossreactivity, with the different kits of PTH present on the market. This has lead to very important differences in the values obtained with these kits. Unfortunately, this point had not been taken into account in the former KDIGO Guidelines that asked to maintain the PTH levels of the hemodialyzed patients between 150 and 300 pg/mL, whatever the kit. The KDIGO Guideline, in 2009, overcame this problem by asking to maintain the patients between 2 and 9 times of the upper reference range of the Laboratory. But there comes the problem of the establishment of a reference range for PTH. Indeed, for such a purpose, one should exclude any patient presenting causes of secondary hyperparathyroidism - and thus exclude patients suffering from vitamin D deficiency - which was not done by most of the Manufacturers. Thus, the upper reference range generally proposed in the inserts of the kits is generally much higher that could be expected. Finally, different interferences have been demonstrated in PTH determination and the stability of the peptide may vary according to the sample type (plasma EDTA or serum) or even with different assays. PTH measurement in patients with CKD may thus appear as an easy task as numerous automated assays are now available, but, in practice, its interpretation is not so obvious. In this Chapter, we will review the various aspects concerning the measurement of PTH in patients with CKD and especially in dialysis patients all the more that, during the past 10 years, new PTH assays became available, and new concepts concerning PTH reference values have emerged.

The skeleton is an endocrine organ regulating a multitude of metabolic functions, including carbohydrates, lipids, energy, and mineral metabolism. Each bone represents a very active functional unit, constantly remodeled in virtue of two opposed processes, bone formation and bone resorption. The equilibrium between these two processes determines the gain, loss, or balance of total bone mass. Most metabolic bone diseases show altered bone resorption/formation ratio. In Chronic Kidney Disease (CKD), secondary hyperparathyroidism is the most common complication. It is characterized by High Parathormone Levels (PTH) and increased bone resorption, which can enhance serum calcium and phosphate levels and expose to fractures, vascular calcification, arterial stiffness and increased risk of mortality. These alterations have now been redefined as CKD-MBD for Mineral and Bone Disorders (MBD) by the KDIGO (Kidney Disease Improving G lobal Outcomes). KDIGO guidelines highlight that bone biopsy remains the gold standard diagnostic test of CKD-MBD and recommend its use in order to distinguish between high and low bone turnover and to rule out mineralization troubles and trace metal deposition. However, this recommendation is difficult to follow because of its invasive method and the lack of specialized bone histology services in most parts of the world. PTH is well correlated with bone turnover and is usually used as a surrogate of bone biopsy; however, PTH clearly fails to provide information on mineralization state, which may lead to wrong diagnosis and therapeutic choices in a substantial number of patients. For these reasons, non-invasive methods for the assessment of bone turnover in CKD-MBD, such as the assessment of molecules which are either derived from bone structures itself (TRAP5b, CTX, ICP, NTX, PYD, DPD, BSP, Galactosyl hydroxylysine, sclerostin, BSAP, and osteocalcin) or closely related to bone metabolism (PTH, vitamin D, and leptin) have been proposed as specific markers of bone remodeling. This article reviews the use of these biochemical markers in the diagnosis and treatment of CKD-MBD.

Vascular calcifications constitute an important risk factor for mortality in chronic kidney disease patients. A better knowledge of physiopathologic phenomena responsible for vascular mineralization leads to emerging biological markers of vascular calcifications. In calcified arteries, presence of bone matrix as well as osteoblast and osteoclast cells suggests that vascular calcification is an active and highly regulated process. In uremic environment, vascular smooth muscle cells can transdifferentiate into osteoblast like cells. The OPG/RANK/RANKL system is clearly of central significance in controlling vascular calcifications as in bone metabolism. Converging results suggest that circulating OPG determination should be a relevant marker of calcifications. Impairment in inhibitory system such as Matrix Gla Protein and fetuin-A promotes bone matrix calcification. Finally, FGF23, an early and sensitive marker of bone and mineral disorders in chronic kidney disease patients appears as a promising marker.

The kidney plays a central role in phosphate homeostasis. It adapts urinary phosphate excretion to phosphate intake and to body needs maintaining serum phosphate concentration within the normal range. The mechanisms coupling renal phosphate excretion to intestinal phosphate absorption and bone mineralization have been unraveled over the last ten years. Fibroblast growth factor 23 (FGF23) has been identified as a major hormone involved in phosphate homeostasis. FGF23 is a circulating peptide synthesized by bone cells in response to an increase in serum phosphate concentration and to calcitriol. FGF23 inhibits renal sodium-phosphate co-transporter activity and calcitriol production by the kidney. It also controls PTH secretion. In animal models as well as in human pathology, defects of FGF23 secretion or stability induce an increase in serum phosphate concentration, calcitriol levels and are associated with tissue calcifications and early mortality. On the opposite, overproduction of FGF23 is responsible for hypophosphatemia due to renal phosphate loss and inappropriately low serum calcitriol concentration and bone demineralization. FGF23 receptor is a heterodimer composed of a FGF receptor and the transmembrane protein Klotho. The increase in plasma FGF23 concentration and the decrease in Klotho expression observed in chronic kidney diseases (CKD) seem to play a causal role in the genesis of CKD complications and mortality. The circulating level of FGF23 appears to be an important prognostic marker in CKD.

With the introduction of powerful calcineurin inhibitors in the 1980s, the occurrence of acute rejection has been dramatically decreased. Nowadays, the percentage of functioning graft one year posttransplantation is above 85%% Nevertheless, the rejection episodes still occur and even more severely than before. Degradation of kidney graft function is the striking event reflecting the ongoing process of kidney rejection. However, once serum creatinine or proteinuria starts to rise, chronic structural lesions are already present and it is usually too late for intervention. Moreover subclinical rejection can damage the allograft without affecting the kidney function. Therefore there is a clear need to identify biomarker before the detection of a degradation of the kidney function. Apart from the histological diagnosis that relies on the invasive biopsy, a need for less invasive and prognostic earlier biomarkers of acute and chronic rejection is needed. Accuracy and specificity of biomarkers, and their usefulness in prognosis or diagnosis of acute or chronic rejection will be discussed in this chapter.