How to manage hypercalcemia with a low BUN/creatinine ratio?

Management of Hypercalcemia with Low BUN/Creatinine Ratio

Immediate hydration with intravenous normal saline is the first-line treatment for hypercalcemia with a low BUN/creatinine ratio, as this indicates volume depletion that is worsening the hypercalcemic state.

Initial Assessment

The lab values show:

• Calcium: 10.4 mg/dL (above high normal range of 8.7-10.2 mg/dL)
• BUN: 7 mg/dL (normal range 6-20 mg/dL)
• Creatinine: 0.85 mg/dL (normal range 0.57-1.00 mg/dL)
• BUN/creatinine ratio: 8 (below low normal range of 9-23)
• eGFR: 93 mL/min/1.73 (normal >59 mL/min/1.73)

Diagnostic Approach

1. Calculate corrected calcium:

   • Using the formula: Corrected calcium (mg/dL) = Total calcium (mg/dL) + 0.8 × [4 - Serum albumin (g/dL)] 1
   • With albumin of 4.7 g/dL: Corrected calcium = 10.4 + 0.8 × [4 - 4.7] = 10.4 - 0.56 = 9.84 mg/dL
   • This indicates the hypercalcemia is milder than initially apparent

2. Evaluate the low BUN/creatinine ratio:

   • A low ratio (8) suggests volume expansion or overhydration
   • This is unusual with hypercalcemia, which typically causes dehydration

3. Key diagnostic tests:

   • Serum intact parathyroid hormone (PTH) level - most important initial test 2
   • 25-OH vitamin D and 1,25(OH)₂D levels
   • PTH-related protein (PTHrP) if malignancy suspected
   • 24-hour urine calcium and creatinine to calculate calcium:creatinine clearance ratio

Treatment Algorithm

Step 1: Immediate Management

• Begin IV normal saline hydration at 200-300 mL/hour if symptomatic or calcium >12 mg/dL 2
• For this patient with mild hypercalcemia (10.4 mg/dL) and normal kidney function, oral hydration may be sufficient

Step 2: Determine Underlying Cause

Based on the low BUN/creatinine ratio, consider these possibilities:

1. Familial Benign Hypocalciuric Hypercalcemia (FBH):

   • Check calcium:creatinine clearance ratio - low ratio (<0.01) suggests FBH 3
   • Family history of hypercalcemia would support this diagnosis
   • No treatment needed if confirmed, as this is a benign condition

2. Primary Hyperparathyroidism with Volume Expansion:

   • Check PTH level - elevated or inappropriately normal PTH with hypercalcemia suggests primary hyperparathyroidism 4
   • Treatment: parathyroidectomy if criteria met, otherwise observation

3. Vitamin D Supplementation with Overhydration:

   • Check 25-OH vitamin D levels
   • Treatment: discontinue supplements if levels elevated

4. Immobilization Hypercalcemia:

   • Recent history of immobilization or bed rest? 5
   • Treatment: mobilization, hydration, consider bisphosphonates if severe

Step 3: Specific Management Based on Severity

For mild hypercalcemia (as in this case):

• Ensure adequate hydration
• Discontinue medications that may contribute (thiazide diuretics, calcium supplements, excessive vitamin D)
• Monitor calcium levels regularly
• Treat underlying cause

For moderate to severe hypercalcemia (calcium >12 mg/dL):

• IV hydration with normal saline
• Consider loop diuretics after volume repletion
• IV bisphosphonates (zoledronic acid or pamidronate) 4
• Calcitonin for rapid but short-term calcium reduction

Monitoring and Follow-up

• Recheck calcium, BUN, and creatinine in 1-2 weeks
• Monitor hydration status
• Adjust treatment based on response and identified cause

Pitfalls to Avoid

1. Don't assume all hypercalcemia requires aggressive treatment - mild asymptomatic hypercalcemia may only need monitoring and treatment of underlying cause

2. Don't use loop diuretics before adequate hydration - this can worsen dehydration and hypercalcemia

3. Don't miss familial hypocalciuric hypercalcemia - unnecessary parathyroidectomy in these patients doesn't resolve hypercalcemia and should be avoided 3

4. Don't overlook medication causes - review all medications, supplements, and over-the-counter products

5. Don't forget to correct calcium for albumin level - as demonstrated above, the true calcium level may be different from the measured value 1