What laboratory tests should be ordered for the workup of hypercalcemia from an internal medicine perspective?

Hypercalcemia Laboratory Workup

Measure intact parathyroid hormone (PTH) as the single most critical first test to differentiate PTH-dependent from PTH-independent hypercalcemia. 1, 2

Essential Initial Laboratory Tests

First-Line Testing

• Intact PTH level is the most important branch point in the diagnostic algorithm and should be obtained promptly to avoid delaying appropriate workup and treatment 1, 2
• Serum calcium (total calcium) to confirm and quantify hypercalcemia 2
• Serum albumin to assess whether calcium correction is needed, though albumin-adjusted calcium has poor diagnostic accuracy in hospitalized patients 3, 4
• Ionized calcium should be measured when available, as it is the reference standard and superior to albumin-adjusted total calcium for diagnosing true hypercalcemia 3

Interpreting PTH Results

• Elevated or inappropriately normal PTH (in the setting of hypercalcemia) indicates PTH-dependent causes, most commonly primary hyperparathyroidism (PHPT), which accounts for approximately 45% of hypercalcemia cases 1, 2
• Suppressed PTH (<20 pg/mL) points toward PTH-independent etiologies and triggers evaluation for malignancy, granulomatous disease, endocrinopathies, medications, or vitamin D toxicity 1, 2

Additional Essential Tests

• Serum phosphorus has diagnostic value in differentiating causes, as hyperparathyroidism typically causes hypophosphatemia 4
• Serum chloride is one of the most useful ancillary tests, with elevated chloride favoring hyperparathyroidism 4
• 25-hydroxyvitamin D level should be measured because vitamin D deficiency can increase PTH by approximately 20% and confound interpretation 1
• Creatinine/BUN to assess renal function, as chronic kidney disease affects PTH interpretation and calcium handling 5

Secondary Laboratory Tests Based on PTH Results

If PTH is Elevated or Normal (PTH-Dependent)

• Alkaline phosphatase to assess bone turnover and complications 4
• 24-hour urine calcium to exclude familial hypocalciuric hypercalcemia if PTH is only mildly elevated 2
• Bone mineral density testing to evaluate for osteoporosis in confirmed PHPT 1

If PTH is Suppressed (PTH-Independent)

• PTH-related peptide (PTHrP) if malignancy is suspected, as this is the most common mechanism of hypercalcemia of malignancy 2, 6
• 1,25-dihydroxyvitamin D (calcitriol) if granulomatous disease or lymphoma is suspected 2, 7
• Serum and urine protein electrophoresis if multiple myeloma is considered 2
• Thyroid function tests (TSH, free T4) to exclude hyperthyroidism 2

Additional Useful Tests

• Venous pH has diagnostic value, as metabolic acidosis may suggest malignancy 4
• Hemoglobin/hematocrit can provide supportive information, particularly in male patients 4
• Chloride-to-phosphate ratio has been proposed but adds limited value beyond individual measurements 4

Critical Caveats

• Albumin-adjusted calcium formulas are poorly sensitive (50% sensitivity) for detecting hypocalcemia and do not improve diagnostic accuracy for hypercalcemia compared to unadjusted total calcium 3
• Ionized calcium measurement should be performed to confirm dyscalcemia when total calcium is abnormal or in the lower range of normal 3
• Age-related GFR decline physiologically raises PTH in elderly patients, requiring careful interpretation 1
• Avoid relying on multiple ancillary tests when PTH measurement provides the primary diagnostic information; albumin, PTH, and chloride are the only three tests that statistically improve diagnostic separation between hyperparathyroidism and malignancy 4