**2.2 Phosphorus**

26 Chronic Kidney Disease

recommended by the American Society for Bone and Mineral Research (Parfitt et al., 1987)**,**  and investigators would supply primary measurements used to report any derived parameters. Based on all of this a new term has been proposed and coined "Chronic kidney disease – mineral and bone disorder (CKD-MBD)" willing to describe the systemic consequences of mineral metabolism disturbances in CKD patients which can no longer be considered restricted only to bone disease. CKD-MBD defines a triad of interrelated abnormalities of serum biochemistry, bone and the vasculature associated with CKD. The adverse effects of high serum phosphorus and an increase of serum calcium due to calcium overload which are present late in CKD are important component of CKD-MBD as well as vascular changes. Furthermore, to clarify the interpretation of bone biopsy results in the evaluation of CKD-MBD, it has been proposed to use three key histologic descriptors—bone turnover, bone mineralization, and bone volume (so called TMV system)— with any combination of each of the descriptors possible in a given specimen. The TMV classification scheme provides a clinically relevant description of the underlying bone pathology, as assessed by histomorphometry, which, in turn, helps to define the pathophysiology, and,

The initial evaluation of CKD-MBD should include laboratory for calcium (it has been proposed either ionized or total corrected for albumin), phosphorus, PTH, alkaline phosphatases (total or bone specific), bicarbonate, as well as imaging for soft-tissue calcification. Epidemiologic studies from the early 1990s have demonstrated that an increase in serum phosphorus and in calcium x phosphorus product are associated with poor outcomes in CKD patients. The association of elevated serum phosphorus and calcium and increased mortality in these patients has been confirmed in several recent studies. If inconsistencies exist in the biochemical markers (eg, high PTH but low alkaline phosphatases), unexplained bone pain, or unexplained fractures are present, a bone biopsy would be strongly indicated (London and Drueke, 1997; London *et al., 2003;* Neves et al.,

Serum calcium is tightly controlled in healthy individuals, within a narrow range, usually 2.2–2.6 mmol/l, with a minimal, diurnal variation. In patients with CKD, serum calcium levels fluctuate more, because of altered homeostasis and concomitant therapies. Serum calcium levels are routinely measured in clinical laboratories using colorimetric methods in automated machines. In patients with CKD stage 5D, there are additional fluctuations in association with dialysis-induced changes, hemoconcentration, and subsequent hemodilution. Moreover, predialysis samples collected from dialysis patients after the longer interdialytic interval during the weekend, as compared with predialysis samples drawn after the shorter interdialytic intervals during the week, often contain higher serum calcium levels (Tentori et al., 2008). It has been shown that the serum calcium level is a poor reflection of overall total body calcium. Only 1% of total body calcium is measurable in the extracellular compartment while the most important part of calcium is stored in the bones. Serum ionized calcium, generally 40–50% of total serum calcium, is physiologically active, while non-ionized calcium is bound to albumin or anions such as citrate, bicarbonate, and

thereby, probably to guide the therapy (Moe S. et al., 2006).

**2. CKD – MBD and biochemical abnormalities** 

2007; Bucay et al., 1998).

**2.1 Calcium** 

It has been shown that inorganic phosphorus is critical for numerous normal physiological functions, including skeletal development, mineral metabolism, cell-membrane phospholipid content and function, cell signaling, platelet aggregation, and energy transfer through mitochondrial metabolism. Owing to its importance, normal homeostasis maintains serum concentrations between 0.81–1.45 mmol/l. The terms, phosphorus and phosphate, are often used interchangeably, but strictly speaking, the term phosphate means the sum of the two physiologically occurring inorganic ions in the serum, and in other body fluids, hydrogenphosphate (HPO42) and dihydrogenphosphate (H2PO4). However, most laboratories report this measurable, inorganic component as phosphorus. Unlike calcium, a major component of phosphorus is intracellular, and factors such as pH and glucose can cause shifts of phosphate ions into or out of cells, thereby altering the serum concentration without changing the total body phosphorus. Phosphorus is routinely measured in clinical laboratories with colorimetric methods in automated machines. Serum phosphorus levels reach the lowest level in the early hours of the morning, increasing to a plateau at the afternoon, and further increasing to a peak late in the evening (Portale et al., 1987).

Hyperphosphatemia occurs as a consequence of diminished phosphorus filtration and excretion with the progression of CKD. Decreased phosphorus excretion can initially be overcome by increased secretion of parathyroid hormone (PTH), which decreases proximal phosphate reabsorption (Slatopolsky and Delmez, 1994). Hence, phosphorus levels are usually within normal range until the GFR falls below approximately 30 ml/min, or stage IV. CKD according to the National Kidney Foundation Kidney Disease Outcomes Quality Initiative (NKF–K/DOQI) classification (National Kidney Foundation: K/DOQI). In more advanced stages of CKD, the blunted urinary excretion of phosphorus can no longer keep pace with the obligatory intestinal phosphate absorption, resulting in hyperphosphatemia. Therefore, it is not surprising that the majority of patients with CKD stage 4 and stage 5 have a significant hyperphosphatemia (Block et al., 1998). It has been shown that in patients with advanced CKD high serum calcium, phosphate, and calcium-phosphate product levels are associated with unaccountably high rates of cardiovascular disease (Ganesh et al., 2001; Stevens et al., 2004; Slinin et al., 2005). Moreover, it has been shown also that these derangements in mineral metabolism could occur as well during the early stages of CKD (Slatopolsky and Delmez, 1994).

### **2.3 Parathyroid hormone**

The parathyroid gland plays an important role in the regulation of mineral homeostasis by effects trough other organs such as the kidney and bone. Fluctuation in extracellular calcium

The New Kidney and Bone Disease:

**2.4 Vitamin D (25(OH)D)** 

Chronic Kidney Disease – Mineral and Bone Disorder (CKD–MBD) 29

The parent compounds of vitamin D—D3 (cholecalciferol) or D2 (ergocalciferol)—are highly lipophilic. They are difficult to quantify in the serum or plasma. They also have a short halflife in circulation of about 24 h. These parent compounds are metabolized in the liver to 25(OH)D3 (calcidiol) or 25(OH)D2 (ercalcidiol). Collectively, they are called 25(OH)D or 25 hydroxyvitamin D. The measurement of serum 25(OH)D is regarded as the best measure of vitamin D status, because of its long half-life of approximately 3 weeks. In addition, it is an assessment of the multiple sources of vitamin D, including both nutritional intake and skin synthesis of vitamin D. There is a seasonal variation in calcidiol levels because of an increased production of cholecalciferol by the action of sunlight on skin during summer months. The gold standard of calcidiol measurement is high performance liquid chromatography (HPLC), but this is not widely available clinically. This is because HPLC is time consuming, requires expertise and special instrumentation, and is expensive. In early 1985, Hollis and Napoli developed the first radioimmunoassay (RIA) for total 25(OH)D, which was co-specific for 25(OH)D2 and 25(OH)D3. The values correlated with those obtained from HPLC analysis, and DiaSorin RIA became the first test to be approved by the Food and Drug Administration for use in clinical settings (Hollis and Napoli, 1985). Another method now carried out is liquid chromatography- tandem mass spectrometry (LC-MS/MS). Similar to HPLC, the LC-MS/MS method also has the ability to quantify 25(OH)D2 and 25(OH)D3 separately, which distinguishes it from RIA and enzyme-linked immunosorbent assay technologies. This method is very accurate and has been shown to correlate well with DiaSorin RIA (Saenger et al. 2006; Tsugawa et al., 2005). It has been suggested recently that the assays for 25(OH)D are not well standardized, and the definition of deficiency is not yet well validated. At best, clinicians should ensure that patients use the same laboratory for measurements of these levels, if carried out. The most appropriate vitamin D assays presently available seem to be those that measure both 25(OH)D2 and 25(OH)D3. Presently, approximately 20–50% of the general population has low vitamin D levels, irrespective of CKD status. However, the benefits from replacing vitamin D have not been documented in patients with CKD,

1,25(OH)2D is used to describe both hydroxylated D2 (ercalcitriol) and D3 (calcitriol) compounds, both of which have a short half-life of 4–6 h. Furthermore, in patients with earlier stages of CKD and in the general population, mild-to-moderate vitamin D deficiency, or partly treated vitamin D deficiency, is frequently associated with increased levels of 1,25(OH)2D. Thus, even accurate levels can be misleading. The serum levels of 1,25(OH)2D are uniformly low in late stages of CKD–MBD, at least in patients not treated with vitamin D derivatives (Andress et al., 2006). It has not been recommend a routine measurement of 1,25(OH)2D levels, as the assays are not well standardized, the half-life is short, and there are no data indicating that the measurement is helpful in guiding therapy or predicting

Alkaline phosphatases (ALP) are enzymes that remove phosphate from proteins and nucleotides, functioning optimally at alkaline pH. Measurement of the level of total ALP (t-

particularly if they are taking calcitriol or a vitamin D analog.

**2.5 Vitamin D (1,25(OH)2D)**

outcomes (KDIGO).

**2.6 Alkaline phosphatases** 

ion levels is sensed by the parathyroid calcium-sensing receptors (CaSRs) and subsequently regulates the synthesis and secretion of parathyroid hormone (PTH) (Felsenfeld et al., 2007). PTH acts on the bone to increase the efflux of calcium and phosphate, and acts on the kidney to reduce urinary calcium excretion, inhibit phosphate reabsorption, and stimulate the production of 1,25-dihydroxyvitamin D (1,25(OH)2D). PTH is cleaved to an 84-amino-acid protein in the parathyroid gland, where it is stored with fragments in secretory granules for release. When it is released, the circulating 1–84-amino-acid protein has a half-life of 2–4 min. The hormone is cleaved both within the parathyroid gland and after secretion into the Nterminal, C-terminal, and middle region fragments of PTH, which are metabolized in the liver and in the kidneys. Enhanced PTH synthesis/secretion occurs in response to hypocalcemia, hyperphosphatemia, and/or a decrease in serum 1,25-dihydroxyvitamin D (1,25(OH)2D), whereas high serum levels of calcium or calcitriol—and, as recently shown, of Fibroblast growth factor 23 (FGF-23)—suppress PTH synthesis/secretion. The extracellular concentration of ionized calcium is the most important determinant of the minute-to-minute secretion of PTH, which is normally oscillatory.

In patients with CKD, this normal oscillation is somehow altered. Over the past few decades there has been a progress in development of sensitive assays in order to measure PTH. Initial measurements of PTH using C-terminal assays were inaccurate in patients with CKD because of the impaired renal excretion of C-terminal fragments (and thus retention) and the measurement of these probably inactive fragments. The development of the N-terminal assay was initially thought to be more accurate but it also detected inactive metabolites. The development of a second generation of PTH assays, the two-site immunoradiometric assay—commonly called an 'intact PTH' assay—improved the detection of full-length (active) PTH molecules. In this assay, a captured antibody binds within the amino terminus and a second antibody binds within the carboxy terminus. Unfortunately, recent data indicate that this 'intact' PTH assay also detects accumulated large C-terminal fragments, commonly referred to as '7–84' fragments; these are a mixture of four PTH fragments that include, and are similar in size to, 7–84 PTH (Gao and D'Amour 2005). In parathyroidectomized rats, the injection of a truly whole 1- to 84-amino-acid PTH was able to induce bone resorption, whereas the 7- to 84-amino-acid fragment was antagonistic, explaining why patients with CKD may have high levels of 'intact' PTH but relative hypoparathyroidism at the bone-tissue level (Slatopolsky et al., 2000; Malluche et al., 2003; Huan et al., 2006). Thus, the major difficulty in accurately measuring PTH with this assay is the presence of circulating fragments, particularly in the presence of CKD. Unfortunately, the different assays measure different types and amounts of these circulating fragments, leading to inconsistent results. More recently, a third generation of assays has become available that truly detect only the 1- to 84-amino-acid, full-length molecule: 'whole' or 'bioactive' PTH assays. There are differences in PTH results when samples are measured in plasma, serum, or citrate, and depending on whether the samples are on ice, or are allowed to sit at room temperature.

PTH and vitamin D have been shown to influence cardiac and vascular growth and function experimentally in human subjects with normal renal function. Because of increased prevalence of hyperparathyroidism and altered vitamin D status in CKD, these alterations have been considered to contribute to the increased prevalence of cardiovascular disease and hypertension seen in this patient population (Slinin Y et al., 2005).
