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

Laboratory Findings in Pediatric Rickets

In a pediatric patient with rickets symptoms, you will find elevated alkaline phosphatase (ALP), low phosphate, and either low 25-hydroxyvitamin D (in nutritional rickets) or normal 25-hydroxyvitamin D with renal phosphate wasting (in phosphopenic rickets like X-linked hypophosphatemia). 1

Core Laboratory Panel

The initial diagnostic workup should include serum phosphate, calcium, alkaline phosphatase, parathyroid hormone (PTH), 25(OH) vitamin D, 1,25(OH)₂ vitamin D, and creatinine, along with spot urine for phosphate, calcium, and creatinine to calculate tubular maximum reabsorption of phosphate per glomerular filtration rate (TmP/GFR) 2, 3.

Biochemical Patterns by Rickets Type

Nutritional (Calcipenic) Rickets

• Low 25-hydroxyvitamin D is the hallmark finding 1, 4
• Low or low-normal calcium (can be as low as 1.20 mmol/L) 5, 6
• Low or normal phosphate 4
• Elevated alkaline phosphatase (can exceed 800 IU/L) 5, 6, 4
• Elevated PTH due to secondary hyperparathyroidism 6, 4
• Low urinary calcium due to decreased intestinal absorption 1

Phosphopenic Rickets (X-Linked Hypophosphatemia)

• Low phosphate below age-related reference range is the defining feature 2, 1
• Normal 25-hydroxyvitamin D distinguishes this from nutritional rickets 2, 1
• Normal or low-normal calcium (typically lower normal range) 2, 1
• PTH at upper limit of normal or slightly elevated (not as high as in calcipenic rickets) 2, 1
• Low or inappropriately normal 1,25(OH)₂ vitamin D in the setting of hypophosphatemia 2, 1
• Elevated alkaline phosphatase when rickets is active 2, 1
• Renal phosphate wasting confirmed by elevated TmP/GFR (fractional excretion >15%) 2, 3
• Low urinary calcium 2, 1

Critical Diagnostic Algorithm

Step 1: Check alkaline phosphatase—elevation confirms active rickets across all types 2, 1, 7

Step 2: Measure serum phosphate and 25(OH) vitamin D simultaneously:

• Low phosphate + low 25(OH)D = nutritional rickets 1, 4
• Low phosphate + normal 25(OH)D = phosphopenic rickets (XLH or hereditary forms) 2, 1

Step 3: If phosphopenic pattern, calculate TmP/GFR to confirm renal phosphate wasting 2, 3

Step 4: Exclude Fanconi syndrome by checking urine for abnormal bicarbonate, amino acid, glucose, and uric acid losses plus low molecular mass proteinuria 2, 3

Essential Pitfalls to Avoid

• Serum phosphate may be normal in the first 3-4 months of life even in X-linked hypophosphatemia, potentially delaying diagnosis 1, 3
• Always use age-related reference ranges for phosphate as normal values are higher in infants and children 2, 3
• Do not rely on FGF23 for routine diagnosis—levels vary by assay and are influenced by phosphate intake and vitamin D therapy, making them most informative only in untreated patients 2, 1, 3
• Urinary calcium is low in active rickets and increases as rickets heals, helping distinguish treatment adequacy 1
• Nutritional rickets and XLH can coexist—if phosphate doesn't normalize after vitamin D/calcium supplementation, consider underlying phosphopenic rickets 2
• In vitamin D-dependent rickets type II, 1,25(OH)₂ vitamin D is paradoxically elevated (not low), distinguishing it from type I where it's low 5

Bone-Specific Considerations

In children, total ALP represents 80-90% bone-specific ALP and can be used reliably 2. ALP elevation is universal across all rickets types when bone is undertreated and serves as the most reliable biomarker of rickets activity 2, 1.