CSR Bone and Mineral Metabolism in CKD (CKD-MBD) 2 — Questions and Answers
Question 1: A hemodialysis patient has serum calcium of 10.8 mg/dL and serum phosphorus of 7.2 mg/dL. What is the calcium-phosphorus product and what does it indicate?
- 18 mg²/dL² — within safe limits, no intervention needed
- 77.8 mg²/dL² — elevated, indicating increased risk of vascular and soft-tissue calcification (Correct answer)
- 77.8 mg²/dL² — within acceptable range for dialysis patients
- 12.5 mg²/dL² — low, requires calcium supplementation
Correct answer: 77.8 mg²/dL² — elevated, indicating increased risk of vascular and soft-tissue calcification
Ca × P = 10.8 × 7.2 = 77.8 mg²/dL², which exceeds the historical target of <55 mg²/dL² and is associated with metastatic calcification and cardiovascular mortality.
Question 2: Which phosphate binder is calcium-free and non-aluminum-based, making it preferred for patients with hypercalcemia or vascular calcification?
- Calcium carbonate
- Aluminum hydroxide
- Sevelamer carbonate (Correct answer)
- Calcium acetate
Correct answer: Sevelamer carbonate
Sevelamer carbonate is a polymer-based binder that contains neither calcium nor aluminum, reducing the risk of calcium loading and hypercalcemia while also providing an LDL-lowering effect.
Question 3: Lanthanum carbonate must be chewed with meals to be effective. What is its mechanism of phosphate-lowering action?
- It alkalinizes the intestinal lumen to precipitate phosphate salts
- It binds dietary phosphate in the GI tract, forming insoluble lanthanum phosphate that is excreted in stool (Correct answer)
- It inhibits sodium-phosphate co-transporters in the intestinal mucosa
- It increases renal tubular phosphate excretion
Correct answer: It binds dietary phosphate in the GI tract, forming insoluble lanthanum phosphate that is excreted in stool
Lanthanum ions released from lanthanum carbonate bind dietary phosphate in the GI tract to form insoluble lanthanum phosphate complexes that are excreted without systemic absorption of significant amounts of lanthanum.
Question 4: Cinacalcet (Sensipar) is a calcimimetic used in secondary hyperparathyroidism. What is its mechanism of action?
- It blocks phosphate absorption in the gastrointestinal tract
- It activates the calcium-sensing receptor (CaSR) on parathyroid cells, increasing sensitivity to calcium and suppressing PTH secretion (Correct answer)
- It provides exogenous active vitamin D to suppress parathyroid gland activity
- It chelates excess phosphorus in the bloodstream
Correct answer: It activates the calcium-sensing receptor (CaSR) on parathyroid cells, increasing sensitivity to calcium and suppressing PTH secretion
Cinacalcet is an allosteric activator of the CaSR on parathyroid chief cells, making them perceive higher calcium levels and thereby reduce PTH secretion without raising serum calcium.
Question 5: Which vitamin D analog used in CKD management is synthetic, binds preferentially to the vitamin D receptor (VDR) in parathyroid tissue, and is associated with a lower risk of hypercalcemia compared to calcitriol?
- Ergocalciferol (vitamin D2)
- Alfacalcidol (1-alpha-hydroxyvitamin D3)
- Paricalcitol (19-nor-1,25-dihydroxyvitamin D2) (Correct answer)
- Cholecalciferol (vitamin D3)
Correct answer: Paricalcitol (19-nor-1,25-dihydroxyvitamin D2)
Paricalcitol selectively activates VDR with reduced intestinal calcium and phosphorus absorption effects relative to calcitriol, making it useful for treating secondary hyperparathyroidism with a lower risk of hypercalcemia.
Question 6: What dietary counseling is MOST effective for reducing phosphorus intake from food additives in a CKD patient?
- Advise the patient to read ingredient labels and avoid foods containing words ending in '-phosphate' or '-phosphoric acid' (Correct answer)
- Instruct the patient to avoid all animal protein to eliminate phosphorus
- Recommend soaking vegetables in water to leach out phosphorus
- Suggest limiting potassium-rich foods, as they are always co-packaged with phosphate additives
Correct answer: Advise the patient to read ingredient labels and avoid foods containing words ending in '-phosphate' or '-phosphoric acid'
Inorganic phosphate additives (e.g., sodium phosphate, disodium phosphate) used as preservatives and emulsifiers in processed foods are nearly 100% bioavailable; teaching patients to identify '-phosphate' terms on ingredient lists is the most effective counseling strategy.
Question 7: A peritoneal dialysis patient takes sevelamer with meals yet has persistently elevated serum phosphorus. Which assessment question should the renal dietitian ask FIRST?
- 'Are you taking your phosphate binders WITH each meal and snack, not before or after?' (Correct answer)
- 'Have you been drinking more water lately?'
- 'Are you exercising regularly?'
- 'Have you recently reduced your sodium intake?'
Correct answer: 'Are you taking your phosphate binders WITH each meal and snack, not before or after?'
Phosphate binders must be co-ingested with food to physically bind dietary phosphate in the GI tract; timing errors (taking binders before or after meals) are the most common reason for treatment failure before adherence is assumed.
CSR Bone and Mineral Metabolism in CKD (CKD-MBD) 2
A hemodialysis patient has serum calcium of 10.8 mg/dL and serum phosphorus of 7.2 mg/dL.
What is the calcium-phosphorus product and what does it indicate?