Chronic Kidney Disease-Mineral and Bone Disorder (CKD-MBD)

How kidney disease disturbs minerals and bone

Healthy kidneys keep phosphate and calcium balanced and activate vitamin D. As kidney function falls:

1. Phosphate builds up, because the kidneys cannot remove it.
2. FGF23 rises: bone releases this hormone to push phosphate out, and it suppresses vitamin D activation.
3. Active vitamin D (calcitriol) falls, so less calcium is absorbed from the gut.
4. Calcium tends to fall, and high phosphate and low vitamin D stimulate the parathyroid glands.
5. PTH rises (secondary hyperparathyroidism), pulling calcium from bone. Over time the parathyroid glands enlarge and may act independently.

The result is abnormal bone turnover and calcium-phosphate deposits in arteries, heart valves and soft tissues.

Components of CKD-MBD

Symptoms

Most people have no symptoms for years. When present:

• bone pain, often in the back, hips and legs
• joint pain and stiffness
• fractures after minor falls
• muscle weakness, especially around the hips
• severe itching, partly linked to high phosphate and PTH
• in children: poor growth and bone deformities
• calciphylaxis: rare but serious painful skin lesions from calcified small blood vessels, mostly in people on dialysis

Diagnosis and monitoring

KDIGO recommends regular blood tests from CKD stage 3a onward, more often as kidney function declines:

• serum calcium and phosphate
• PTH
• alkaline phosphatase
• 25-hydroxyvitamin D

Other tests:

• bone density (DXA) in people with CKD-MBD to assess fracture risk
• X-ray or echocardiogram, which can show vascular or valve calcification
• bone biopsy, rarely, to determine the type of bone disease before certain treatments

Trends over time guide treatment more than single results.

Treatment

Lowering phosphate

• Diet: limit high-phosphate foods, especially processed foods with phosphate additives ("PHOS" ingredients), colas, processed meats and cheese, and some fast food. Plant-based phosphorus is less absorbed. A renal dietitian helps balance protein and phosphate.
• Phosphate binders taken with meals: calcium-based (calcium acetate or carbonate, limited to avoid calcium overload), sevelamer, lanthanum and iron-based binders.
• Tenapanor, which reduces phosphate absorption in the gut, is an add-on option for some people on dialysis.
• Adequate dialysis, as longer or more frequent sessions remove more phosphate.

Managing PTH and vitamin D

• correcting low 25-hydroxyvitamin D with supplements
• active vitamin D (calcitriol, paricalcitol) for rising PTH in advanced CKD and dialysis, watching calcium levels
• calcimimetics (cinacalcet, etelcalcetide), which lower PTH in people on dialysis
• parathyroidectomy for severe secondary or tertiary hyperparathyroidism that does not respond to medicines

Bone health

• fall prevention and exercise
• osteoporosis medicines in selected patients, chosen carefully because some are not suitable in advanced CKD
• kidney transplant, which corrects many abnormalities, though bone issues may persist

Prevention and outlook

Starting dietary and medical management early, from CKD stage 3, slows the progression of CKD-MBD. Vascular calcification is a major contributor to the high rate of heart disease in kidney disease, so controlling phosphate and avoiding excess calcium matter. Kidney transplant offers the best long-term correction.

See also chronic kidney disease and vitamin D deficiency.

Frequently asked questions

What is CKD-MBD?

CKD-MBD, or chronic kidney disease-mineral and bone disorder, is the combination of abnormal calcium, phosphate, PTH and vitamin D levels, bone disease and blood vessel calcification that develops as kidney function declines.

Why is phosphorus high in kidney disease?

The kidneys normally remove excess phosphorus from the blood. When kidney function falls, phosphorus builds up. High phosphorus drives parathyroid hormone up, weakens bones and contributes to calcium deposits in blood vessels.

What foods should I limit for high phosphorus?

Limit processed foods with phosphate additives, dark colas, processed meats, many cheeses, fast food and packaged baked goods. Check ingredient lists for words containing "PHOS". A renal dietitian can help you get enough protein while controlling phosphorus.

What do phosphate binders do?

Phosphate binders are medicines taken with meals and snacks that bind phosphorus from food in the gut so it passes out in the stool instead of being absorbed. They work only when taken with food.

What is renal osteodystrophy?

Renal osteodystrophy is the bone disease caused by chronic kidney disease. Bones may turn over too fast or too slowly, or be poorly mineralized, making them weaker and more prone to fractures and pain. It is part of CKD-MBD.