3.1 CKD-Mineral and Bone Disorder (CKD-MBD)

Chronic Kidney Disease-Mineral and Bone Disorder (CKD-MBD) is a systemic disorder that develops as renal function declines. Rather than viewing skeletal lesions in isolation, clinical nephrology defines CKD-MBD as an interrelated triad: biochemical abnormalities of calcium, phosphorus, parathyroid hormone (PTH), and fibroblast growth factor 23 (FGF-23); bone remodeling abnormalities (renal osteodystrophy); and extra-skeletal vascular and soft-tissue calcification. In maintenance hemodialysis, CKD-MBD is a leading driver of cardiovascular death and fractures.

Pathophysiologic Cascade of CKD-MBD

The cascade begins as declining glomerular filtration rate (GFR) impairs renal phosphorus clearance, producing transient hyperphosphatemia.

Phosphate Retention, FGF-23, and Klotho

In response to early phosphate retention, osteocytes secrete Fibroblast Growth Factor 23 (FGF-23), a 32 kDa phosphaturic hormone. Binding to the Klotho coreceptor complex in renal proximal tubules, FGF-23 downregulates sodium-phosphate cotransporters to stimulate urinary phosphate excretion. However, high circulating FGF-23 and hyperphosphatemia directly inhibit renal 1-alpha-hydroxylase (CYP27B1). This suppresses the synthesis of active 1,25-dihydroxyvitamin D [calcitriol, $1,25(\text{OH})_2\text{D}$] from its precursor 25-hydroxyvitamin D.

Calcitriol Deficiency and Parathyroid Hyperplasia

Calcitriol deficiency reduces active gastrointestinal calcium absorption and can contribute to hypocalcemia; calcium values also depend on diet, binders, dialysate and other therapies. Simultaneously, low calcitriol eliminates normal negative feedback on parathyroid chief cells. Hypocalcemia, hyperphosphatemia, and calcitriol deficiency stimulate the parathyroid glands to hypersecrete PTH, establishing Secondary Hyperparathyroidism (sHPT).

Over years, initial diffuse polyclonal chief cell hyperplasia progresses to nodular monoclonal hyperplasia. In nodular tissue, cells downregulate calcium-sensing receptors (CaSR) and vitamin D receptors (VDR). Unresponsive to calcium or vitamin D suppression, the glands transition to autonomous tertiary hyperparathyroidism, causing refractory hypercalcemia that often requires surgical parathyroidectomy.

The Renal Osteodystrophy Spectrum

Renal osteodystrophy describes the skeletal abnormalities resulting from CKD-MBD, classified primarily by bone turnover.

High-Turnover vs. Low-Turnover Bone Disease

• Osteitis Fibrosa Cystica (High-Turnover): Driven by severe sHPT with markedly elevated intact PTH (iPTH). High PTH accelerates bone resorption by osteoclasts and chaotic formation of woven collagen by osteoblasts. Histology shows marrow peritrabecular fibrosis, brown tumors (osteoclastomas), cortical thinning, bone pain, elevated bone-specific alkaline phosphatase (BSAP), and high fracture risk.
• Adynamic Bone Disease (Low-Turnover): Characterized by profound cellular quiescence and reduced bone formation without marrow fibrosis. Most often iatrogenic, resulting from excessive calcium binders or active vitamin D oversuppressing PTH. Diabetes and other patient factors also contribute; the pattern does not establish histology by itself. The inactive skeleton cannot buffer acute mineral loads, driving extra-skeletal vascular calcification.
• Osteomalacia (Low-Turnover): Characterized by accumulation of unmineralized osteoid seams. Historically caused by aluminum toxicity from aluminum-based binders or contaminated dialysate; now occasionally seen with severe calcitriol and nutritional vitamin D deficiency.

Bone Disease Comparison

Extra-Skeletal and Vascular Calcification

Assess serial calcium and phosphorus individually and together; an elevated product alone neither diagnoses calciphylaxis nor dictates treatment.

Medial Arterial and Valvular Calcification

Unlike intimal atherosclerosis, uremic calcification predominantly affects the tunica media of arteries (Mönckeberg sclerosis). Elevated phosphate and uremic toxins induce vascular smooth muscle cells to transdifferentiate into osteoblast-like cells that deposit hydroxyapatite. Medial calcification leads to arterial stiffening, loss of vascular compliance, widened pulse pressure, and severe left ventricular hypertrophy (LVH). In the heart, metastatic calcification stiffens the mitral and aortic valve rings, causing valvular stenosis, regurgitation, and cardiac conduction blocks.

Calciphylaxis (Calcific Uremic Arteriolopathy)

Calciphylaxis is uncommon but life threatening. Ischemic tissue injury, severe pain and secondary infection demand urgent coordinated care.

Pathogenesis and Risk Factors

Small-vessel calcification, thrombosis and tissue ischemia contribute to the syndrome. Risk assessment includes mineral abnormalities, obesity, diabetes, nutritional status and warfarin exposure, but none independently proves the diagnosis. A normal calcium or phosphorus result does not exclude it.

Clinical Presentation and Lesion Progression

Lesions emerge on adipose-dense tissue (thighs, abdomen, buttocks, breasts). Initial lesions present as excruciatingly painful violaceous plaques or subcutaneous nodules with livedo reticularis patterns. As microvascular thrombosis produces transmural cutaneous infarction, lesions rapidly progress into jagged, ischemic, non-healing ulcers covered with thick black necrotic eschars and surrounding violaceous borders.

Multidisciplinary Management

Calciphylaxis is a clinical emergency because ischemic ulcers cause severe pain and infection risk. Promptly report pain out of proportion to visible findings, violaceous changes, nodules or ulceration. The specialist team determines whether biopsy is useful and safe; biopsy is not a routine nursing procedure. Management includes wound assessment, infection treatment when present, adequate nutrition, pain control and correction of mineral abnormalities. Wound debridement is individualized to necrosis and infection rather than prohibited for every patient.

Review calcium loading, active vitamin D, calcimimetic therapy and warfarin with the prescriber. Nurses must not independently stop all medications or lower dialysate calcium to an unprescribed level. Sodium thiosulfate is sometimes used off label, with limited outcome evidence; monitor ordered therapy for nausea, hypotension, sodium/fluid burden and metabolic acidosis. No dose or number of dialysis sessions guarantees healing.

PTH values suggest turnover patterns but do not establish bone histology. KDIGO suggests approximately two to nine times the assay’s upper normal limit in dialysis, with marked trends prompting review. For an assay upper limit of 65 pg/mL, that approximate range is 130–585 pg/mL; it is not an instruction to force every result to 150–600. Very low PTH raises concern about low turnover; very high PTH raises concern about high turnover. When knowing the bone lesion would change treatment, the specialist may consider bone biopsy.

Sources checked 2026-10-10: KDIGO CKD-MBD