What is the most likely diagnosis and recommended management for a patient with urea 7.5 mmol/L, phosphate 0.79 mmol/L, anion gap 14 mmol/L, hemoglobin 174 g/L, RDW (red cell distribution width) 11.4 %, and calcium 2.57 mmol/L?

Diagnosis: Hemoconcentration with Possible Tumor Lysis Syndrome

The most likely diagnosis is hemoconcentration (evidenced by elevated hemoglobin 174 g/L) with possible early tumor lysis syndrome or other cause of metabolic acidosis, given the normal anion gap of 14 mmol/L, low phosphate 0.79 mmol/L, and elevated calcium 2.57 mmol/L.

Laboratory Analysis

Hemoglobin Elevation

• Hemoglobin 174 g/L is significantly elevated above the normal range of 120-150 g/L for non-pregnant adults 1
• This degree of elevation suggests hemoconcentration from volume depletion, dehydration, or a primary polycythemic process
• The normal RDW CV of 11.4% indicates uniform red cell size, making hemolysis or nutritional deficiencies less likely

Urea and Renal Function

• Urea 7.5 mmol/L is at the upper limit of normal (normal range 2.5-7.5 mmol/L) 1
• In pregnancy, normal urea ranges from 2.4-4.2 mmol/L depending on trimester 1, but this patient's context is unclear
• This borderline elevation combined with hemoconcentration suggests prerenal azotemia from volume depletion

Anion Gap Assessment

• Anion gap of 14 mmol/L is within normal limits (typically 8-16 mmol/L with potassium included) 1
• A normal anion gap with these other abnormalities argues against advanced renal failure, which typically presents with anion gap >20 mmol/L when GFR falls below 20 mL/min 2
• The absence of elevated anion gap makes diabetic ketoacidosis, lactic acidosis, and advanced uremia less likely 3

Phosphate and Calcium

• Phosphate 0.79 mmol/L is low (normal range approximately 0.8-1.5 mmol/L)
• Calcium 2.57 mmol/L is elevated (normal range 2.1-2.5 mmol/L) 1
• This combination of hypophosphatemia with hypercalcemia is unusual and clinically significant 4
• The elevated calcium-to-phosphate ratio suggests possible malignancy-related hypercalcemia, primary hyperparathyroidism, or other causes of PTH-mediated or PTH-independent hypercalcemia 5

Differential Diagnosis Priority

Most Likely: Hemoconcentration with Hypercalcemia

• The constellation of elevated hemoglobin, borderline-high urea, normal anion gap, and hypercalcemia with hypophosphatemia points toward volume depletion with an underlying calcium disorder
• Hypercalcemia of malignancy must be ruled out, particularly lung cancer or multiple myeloma, which can present with both hypercalcemia and potential volume depletion 5

Less Likely: Early Chronic Kidney Disease

• While urea is at the upper limit, the normal anion gap argues against significant renal impairment 2, 3
• In CKD with GFR <20 mL/min, anion gap typically exceeds 20 mmol/L and phosphate is elevated, not decreased 2

Ruled Out: Tumor Lysis Syndrome

• Classic TLS presents with hyperphosphatemia (not hypophosphatemia), hypocalcemia (not hypercalcemia), and elevated anion gap 6, 4
• This patient's laboratory pattern is the opposite of TLS

Recommended Management Algorithm

Immediate Actions

1. Obtain additional critical laboratories:

   • Serum creatinine to assess renal function definitively
   • Ionized calcium to confirm hypercalcemia 5
   • Intact PTH level (most important initial test for hypercalcemia) 5
   • Complete metabolic panel including sodium, potassium, bicarbonate
   • Albumin level to correct calcium and assess nutritional status 1

2. Assess volume status clinically:

   • Orthostatic vital signs
   • Mucous membrane moisture
   • Skin turgor
   • Urine output and concentration

3. If severe hypercalcemia confirmed (>3.0 mmol/L or symptomatic):

   • Initiate IV crystalloid fluids immediately 5
   • Consider loop diuretics after volume repletion 5
   • Administer bisphosphonates (or denosumab if renal impairment present) 5

Diagnostic Workup

• If PTH is suppressed: Measure PTHrP to evaluate for malignancy-related hypercalcemia 5
• If PTH is elevated or inappropriately normal: Consider primary hyperparathyroidism
• Chest imaging to evaluate for lung malignancy 5
• Serum and urine protein electrophoresis if multiple myeloma suspected 5

Critical Pitfalls to Avoid

• Do not assume normal renal function based on urea alone—creatinine is essential, as urea can be falsely low with malnutrition or falsely elevated with volume depletion 1
• Do not aggressively treat hypophosphatemia without addressing hypercalcemia first, as phosphate supplementation in the setting of hypercalcemia risks metastatic calcification when calcium-phosphate product exceeds 55 mg²/dL² 4
• Do not overlook malignancy—the combination of hypercalcemia with volume depletion carries a poor prognosis if malignancy-related, with median survival of approximately 1 month in lung cancer patients 5