Bone disease

Chronic Kidney Disease

Bone disease

The kidneys are an integral part of mineral metabolism activating vitamin D (to produce 1,25 dihydroxycholecalciferol) and modulating phosphate balance.

With the onset of chronic kidney disease, a relative deficiency of 1,25 vitamin D and phosphate retention may result in secondary hyperparathyroidism and hyperphosphataemia. Such abnormalities may occur early in the progression of CKD- consequently the national guidance is to check PTH levels in patients with CKD 3 or worse. However this guidance is not practical - locally we would therefore suggest the following:

For CKD stage 3 that is non progressive check calcium, phosphate and ALP with the 6 monthly monitoring bloods - if these are abnormal, refer for advice via the choose and book system - we do not necessarily need to see the patient. We are also considering the advisability of suggesting a replacement dose of vitamin D to all patients with CKD 3.

For those who require treatment, medication falls into three categories - phosphate binders, vitamin D analogues and calcimimetics.

Phosphate binders are designed to bind phosphate in the diet. Most patients on dialysis will need phosphate binders, which are prescribed with food, and dosed to lower phosphate to below 1.8 mmol/L. The renal dieticians generally advise patients on timings and doses and also do a great deal of the monitoring. There are three classes of phosphate binders

	Calcium containing (calcichew or phosex). These are the most commonly used. Note, please avoid Calcichew D3, since the vitamin D content is redundant. The main side effect is hypercalcemia, and they may have a role in the pathogenesis of vascular calcification in CKD. Typical doses are 2-3 tablets per meal.
	Aluminium hydroxide (alucaps). This remains the most effective phosphate binder, but aluminium toxicity limits its usage. Patients receiving alucaps have regular aluminium level checks.
	Renagel. This is a relatively new non calcaemic phosphate binder. It is used to minimise calcium exposure, but is often required in large doses to be effective. It should be used with care in patients with bowel disorders and can result in bowel obstruction.

For vitamin D analogues the most commonly prescribed is oral 1-alpha. Doses range from 0.25 mcg/alt day to 2 mcg/day. Hypercalcaemia, especially in conjunction with calcium containing phosphate binders, is a common side effect.

Cinacalcet is the first of a new class of 'calcimimetics'. This is designed to target the parathyroid gland and suppress PTH production. It has recently undergone appraisal from NICE and is reserved for refractory hyperparathyroidism in established renal failure.

These three classes of agents often require careful adjustment in established dialysis patients, with close monitoring.