What lab findings are expected in a pediatric patient with symptoms of rickets?

Laboratory Findings in Pediatric Rickets

In a pediatric patient with rickets symptoms, you will find: low 25-hydroxyvitamin D (or normal in phosphopenic rickets), low or normal calcium, low phosphate, and high alkaline phosphatase (option b is closest to the typical presentation). 1

Biochemical Hallmarks by Rickets Type

The specific laboratory pattern depends critically on whether the rickets is calcipenic (nutritional) versus phosphopenic (hereditary):

Phosphopenic Rickets (X-Linked Hypophosphatemia - Most Common Hereditary Form)

• Phosphate: Low (hallmark finding) 1
• Alkaline phosphatase (ALP): Elevated (reliable biomarker of rickets activity) 1
• Calcium: Normal or low-normal range 1
• 25-hydroxyvitamin D: Normal 1
• PTH: Upper limit of normal or slightly elevated 1
• 1,25-dihydroxyvitamin D: Low or inappropriately normal 1
• Urinary calcium: Low 1

Calcipenic Rickets (Nutritional - Vitamin D or Calcium Deficiency)

• Phosphate: Low or normal 2, 3
• Alkaline phosphatase: High 2, 4, 5
• Calcium: Low 2, 6, 5
• 25-hydroxyvitamin D: Low (<30 nmol/L) 2, 4, 5
• PTH: High (secondary hyperparathyroidism) 2, 6, 4, 5

Key Diagnostic Algorithm

Step 1: Check ALP - if elevated, suspect active rickets 1

Step 2: Measure serum phosphate:

• If low phosphate with normal 25(OH)D → suspect phosphopenic rickets (XLH or other hereditary forms) 1
• If low phosphate with low 25(OH)D → suspect nutritional rickets 2, 4

Step 3: Assess calcium and PTH:

• Low calcium + high PTH → calcipenic rickets (nutritional) 2, 6, 4
• Normal/low-normal calcium + normal/slightly elevated PTH → phosphopenic rickets 1

Step 4: Calculate TmP/GFR (tubular maximum reabsorption of phosphate) to confirm renal phosphate wasting in phosphopenic rickets 1

Critical Pitfalls to Avoid

• Serum phosphate may be normal in the first 3-4 months of life even in XLH, delaying diagnosis 1
• ALP elevation is universal across all rickets types when bone is undertreated - use bone-specific ALP in adults as ~50% of total ALP comes from liver 1
• Do not rely on FGF23 levels for routine diagnosis or monitoring as they vary by assay and are influenced by treatment 1
• Vitamin D insufficiency is extremely common (40% of infants aged 8-24 months) but frank rickets with fractures is rare 1
• Urinary calcium is low in active rickets and increases as rickets heals - this helps distinguish treatment adequacy 1