With progression of chronic kidney disease (CKD), disorders of mineral metabolism appear. The classic sequence of events begins with a deficit of calcitriol synthesis and retention of phosphorus. As a result of this, serum calcium decreases and parathyroid hormone (PTH) is stimulated, producing in the bone the high turnover (HT) bone disease known as osteitis fibrosa while on the other extreme we find the forms of low turnover (LT) bone disease. Described later and initially associated with aluminum intoxication, these diseases are now seen primarily in older and/or diabetic patients, who in a uremic setting have relatively low levels of PTH to maintain normal bone turnover. Osteomalacia is also included in this group, which after the disappearance of aluminum intoxication is rarely observed. LT forms of hyperparathyroidism facilitate the exit of calcium (Ca) and phosphorus (P) from bone, whereas the adynamic bone limits the incorporation of Ca and P into bone tissue. Therefore, both forms facilitate the availability of Ca and P, which ends up being deposited in soft tissues such as arteries. The link between bone disease and vascular calcifications in CKD is now a well-established phenomenon. 2. Diagnostic strategies Calcium, Phosphorus They have little capacity to predict underlying bone disease, but their regular measurement is decisive for therapeutic management of the patient, especially in the dose titration stages of intestinal phosphorus binders, vitamin D analogs or calcimimetics. Ideally, Ca++ should be used, but total Ca is routinely used. It is recommended to adjust albumin levels in the event of hypoalbuminemia (for each g/dL of decrease in albumin, total serum Ca decreases 0.9 mg/dL). The following formula facilitates rapid calculation of corrected total calcium: Corrected total Ca (mg/dL) = total Ca (mg/dL) + 0.8 [4-albumina (g/dL)]. Parathyroid hormone "Intact" PTH is the biochemical parameter that best correlates with bone histology (levels measured with the Allegro assay from Nichols Institute Diagnostics, no longer available). Various assays are currently available that use antibodies against different fragments of the molecule, but they have significant intermethod variability and have not been validated. A whole PT assay (1-84) is currently unavailable. A consensus to establish uniform criteria for PTH measurement remains to be established. During the dose titration stages of intestinal phosphorus binders, vitamin D analogs or calcimimetics, more frequent measurement may be required based on clinical judgment. Calcifediol (25(OH)D3) It is important to maintain adequate levels of 25(OH)D3 (> 30 ng/mL), since they will be the substrate for production of 1- 25(OH)2 D3, and their deficiency aggravates hyperthyroidism. Determining 25(OH)D3 levels every 6-12 months is a recommended guideline. Other markers of bone turnover (osteocalcin, total and bone alkaline phosphate, free pyridolines in serum, and C-terminal telopeptide of collagen) do not improve the predictive power of PTH and therefore their systematic use is not justified. Radiologic studies Radiologic studies are of little diagnostic utility, because biochemical changes precede radiologic changes. Systematic radiologic evaluation of the skeleton in asymptomatic patients is not justified at present. They are useful as the first step in the study to detect vascular calcifications and amyloidosis due to b2-microglobulin and in symptomatic and at risk patients to detect vertebral fractures. Bone densitometry: Dual energy x-ray absorptiometry (DEXA) is the standard method to determine bone mineral density (usually in the femoral neck and vertebrae). It provides information on changes in bone mineral content, but not on the type of underlying bone disease. It is useful for follow-up of bone mass or for the study of bone mass changes in the same patient. Its value as a predictor of the risk of fracture has not been demonstrated in patients on kidney replacement therapy or with advanced chronic kidney disease. It is indicated in patients with fractures or risk factors for osteoporosis. Bone biopsy: The "gold standard" for diagnosis of bone disease. With improved knowledge of the value of noninvasive parameters, its use is infrequent.
Pathological fractures in the presence or absence of minor trauma. Symptomatic patients in the presence of incongruent clinical parameters. A typical case is the presence of unexplained hypercalcemia from systemic disease, with inconclusive serum PTH values (between 120-450 pg/mL as an estimated range). Evaluation and follow-up of cardiovascular calcifications There are no consensuated clinical practice guidelines for the evaluation and follow-up of extraosseal calcifications in CKD. The clinical tools for evaluation and follow-up of cardiovascular disease are used based on clinical judgment. The periodicity of follow-up has not been established . 3. Recommended biochemical values The biochemical values recommended in clinical practice guidelines for the evaluation of bone mineral metabolism are summarized in Figure 3. The recommended PTH values do not fully coincide with the K/DOQI guidelines. The wide variability in PTH values depending on the assays used has led us to expand the recommended PTH range in stage 3 and 4 CKD. 4. Treatment 4.1. Diet. The recommended diet for the patient with CKD is traditionally based on protein restriction and phosphorus restriction for control of mineral metabolism. A favorable circumstance is that there is a close relationship between protein and phosphorus intake. In CKD stages 3, 4 and 5, it is recommended to restrict phosphorus intake to between 0.8-1 g/day when serum levels of phosphorus and PTH are above the recommended range. This is approximately equivalent to a diet of 50-60 g of protein. This reasonable antiproteinuric strategy that also restricts phosphorus intake is nutritionally safe. What should we tell them to eat? In a practical and oversimplified way, we recommend the following daily intake: Animal proteins: 1 serving (100-120 g), dairy products: 1 serving (equivalent to 200-240 mL of milk or 2 yoghourts), bread, cereals, pastas (1 cup of pasta, rice or legumes + some bread or cookies), vegetables and fruits relatively freely, but with moderation. 4.2. Medication Vitamin D supplements should be provided if serum levels are less than 30 ng/mL. In Spain, vitamin D3 (cholecalciferol) is marketed as Vitamin D3 Berenguer 2,000 IU/mL of solution. Combinations of calcium with cholecalciferol are also available. Most of the dosage forms contain approximately 500 mg of Ca+ and 400 IU of cholecalciferol. Alternatively, calcifediol (25(OH)D3), as Hidroferol 100 mcg/mL, has been used, although the dose range is very variable and has not been established. 4.3. Phosphorus binders. Use if hyperphosphatemia occurs. Start with calcium-containing phosphorus binders (calcium carbonate or calcium acetate), which also provide calcium if dietary intake is inadequate. Do not exceed 1.5 g of Ca++ per day. The most used are calcium carbonate and calcium acetate. Calcium acetate shows a similar binding potency to calcium carbonate but with a lesser calcium overload, and thus would have certain advantages as well as its greater effect at different pH ranges. However, gastric intolerance is more frequent with this dosage form. Aluminum hydroxide may sometimes be required to control phosphoremia or the occurrence of hypercalcemia. Serum aluminum values should be maintained below 30 mcg/L. Avoid use for longer than 6 months and daily doses greater than 1.5 g. Sevelamer is associated with an increased risk of acidosis and has not been approved for use in predialysis stages. Lanthanum carbonate has been recently marketed in Spain, although its indication for use in the predialysis stage of CKD is still not approved. 4.4. Vitamin D derivatives. Indicated when PTH levels are elevated. A prerequisite for their use is that Ca and P serum levels are adequately controlled. Vitamin D derivates available in Spain are 1,25(OH)2D3 (Calcitriol)and 1a(OH)D3 (a-Calcidiol). Doses should be titrated until PTH levels are normalized. Phosphate binder doses often need to be increased because these vitamin D derivatives increase intestinal absorption of calcium and phosphorus. Low doses do not cause hypercalcemia or hyperphosphatemia and do not worsen the course of renal function. Recommended doses: Calcitriol 0.25 mcg every 48 hours and alpha-Calcidiol 0.50 mcg every 48 hours. Soon to be available on the Spanish market is the oral dosage form of paricalcitol (recommended initial dose of 1 mcg/24 h), with a lesser hypercalcemic and hyperphosphoremic effect. Clinical use of calcimimetics in the predialysis state is not yet recommended and is currently under investigation.