Laboratory Findings in CKD Stage 4 with Metaphyseal Fraying
The answer is B. Hyperphosphatemia. A 10-year-old child with CKD stage 4 presenting with metaphyseal fraying (a radiographic sign of rickets/renal osteodystrophy) will most characteristically demonstrate hyperphosphatemia in laboratory testing, along with elevated PTH, low vitamin D, and typically low-normal or low calcium 1, 2, 3.
Understanding the Pathophysiology
Metaphyseal fraying indicates active bone disease (renal osteodystrophy/rickets) in the context of CKD-mineral and bone disorder (CKD-MBD). This radiographic finding reflects impaired bone mineralization and high bone turnover from secondary hyperparathyroidism 1, 2.
The Characteristic Laboratory Pattern in CKD Stage 4:
- Hyperphosphatemia develops when GFR falls below 30 mL/min/1.73 m² (CKD stage 4), as the kidneys can no longer adequately excrete phosphorus despite compensatory mechanisms 1, 4
- Elevated PTH (secondary hyperparathyroidism) occurs much earlier—beginning when GFR falls below 60 mL/min/1.73 m² (CKD stage 3)—even before serum phosphorus becomes overtly elevated 1, 2, 5
- Low or low-normal calcium results from decreased calcitriol synthesis, hyperphosphatemia, and skeletal resistance to PTH 5, 6
- Low vitamin D (25-OH vitamin D and 1,25-dihydroxy vitamin D) due to impaired renal conversion and substrate deficiency 1, 2, 5
Why Each Answer Choice is Correct or Incorrect
B. Hyperphosphatemia (CORRECT)
Hyperphosphatemia is the hallmark laboratory finding in CKD stage 4 with bone disease. Serum phosphorus levels typically exceed the normal reference range for age when GFR declines to 20-30 mL/min/1.73 m² 1, 4. The KDOQI guidelines specifically note that phosphorus restriction to 80% of the dietary reference intake (DRI) is indicated when both PTH is elevated AND serum phosphorus exceeds the normal reference range for age in CKD stages 3-5 1.
The presence of metaphyseal fraying indicates that the hyperphosphatemia and elevated PTH have progressed to cause radiographically evident bone disease 1, 2.
A. Hypokalemia (INCORRECT)
Hyperkalemia, not hypokalemia, is the electrolyte concern in CKD stage 4. Potassium intake should be limited in children with CKD stages 2-5 who have or are at risk of hyperkalemia 1. Hypokalemia would be unexpected unless the child has specific tubular disorders or is receiving diuretics.
C. High Vitamin D (INCORRECT)
Vitamin D levels are LOW, not high, in CKD stage 4. Both 25-OH vitamin D (calcifediol) and 1,25-dihydroxy vitamin D (calcitriol) are typically deficient 1, 2, 5. The kidneys lose their ability to convert 25-OH vitamin D to the active 1,25-dihydroxy form, and substrate deficiency of 25-OH vitamin D is common 5. The KDOQI guidelines recommend maintaining 25-OH vitamin D levels above 30 ng/mL 1, 5.
D. High Calcium (INCORRECT)
Calcium is typically LOW or low-normal, not high, in CKD stage 4 with active bone disease. Hypocalcemia results from decreased intestinal calcium absorption (due to low calcitriol), hyperphosphatemia (which complexes with calcium), and skeletal resistance to PTH 5, 6. The guidelines emphasize avoiding hypercalcemia and maintaining calcium in the age-appropriate normal range 1, 2, 3.
Complete Laboratory Profile Expected
In a child with CKD stage 4 and metaphyseal fraying, you would expect:
- Elevated serum phosphorus (>5.5 mg/dL for age) 1, 3
- Elevated PTH (progressively increasing, typically >2-3 times upper normal limit) 1, 2, 7
- Low or low-normal serum calcium (corrected for albumin) 2, 5, 6
- Low 25-OH vitamin D (<30 ng/mL) 1, 5
- Elevated alkaline phosphatase (reflecting high bone turnover) 2, 7
- Possible metabolic acidosis (low serum bicarbonate/total CO₂) 1, 2
Clinical Significance
The combination of hyperphosphatemia with metaphyseal fraying indicates inadequate control of CKD-MBD and requires immediate intervention 1, 2, 3. Treatment priorities include:
- Dietary phosphorus restriction to 80% of DRI (given elevated phosphorus and PTH) 1, 3
- Phosphate binders (starting with calcium-based if calcium is not elevated) 1, 3
- Vitamin D supplementation (both nutritional vitamin D to achieve 25-OH vitamin D >30 ng/mL, and potentially active vitamin D analogs for PTH control) 1, 2, 5
- Monitoring calcium closely to avoid hypercalcemia during treatment 2, 3
The presence of radiographic bone changes indicates that biochemical abnormalities have been present long enough to cause structural damage, emphasizing the importance of early detection and treatment of CKD-MBD before such complications develop 1, 2.