What is the normal value for alkaline phosphatase (ALP)?

Normal Values for Alkaline Phosphatase (ALP)

The normal range for alkaline phosphatase (ALP) in adults is typically 20-200 nmol/L (5-50 mg/L), though this varies by laboratory, age, and sex. 1

Age and Sex Variations in ALP

• ALP levels increase by approximately 20% between the 3rd and 8th decade of life 2
• Reference ranges vary between laboratories, with some reporting separate upper limit of normal (ULN) values for different sex and age groups 1
• Laboratory-specific reference ranges should always be consulted when interpreting results

Factors Affecting ALP Interpretation

Physiological Factors

• Age: Higher in children/adolescents (due to bone growth) and elderly
• Sex: Slightly different reference ranges for males and females
• Pregnancy: Elevated due to placental production

Laboratory Considerations

• Different assay methods may yield different reference ranges
• Laboratories report ULN values that may vary significantly 1
• Some laboratories use absolute values while others use multiples of ULN

Clinical Significance of ALP Levels

Normal ALP

• Indicates normal bone metabolism and liver function
• Should be interpreted alongside other liver function tests and clinical context

Elevated ALP

• Significant elevations are usually considered when:
  • ≥2× ULN for cholestatic patterns
  • ≥5× ULN for more concerning liver injury 2
• Extremely high levels (>1,000 IU/L) are commonly associated with:
  • Sepsis (can occur with normal bilirubin)
  • Malignant biliary obstruction
  • Advanced liver disease 3, 4

Low ALP

• May indicate hypophosphatasia, a rare inherited disorder 5, 6
• Found in approximately 9% of patients attending osteoporosis clinics 6
• Can be misdiagnosed as osteoporosis, leading to inappropriate treatment

ALP Isoenzymes

• Total ALP includes several isoenzymes from different tissues:

  • Liver
  • Bone
  • Intestine
  • Placenta
  • Kidney

• Bone-specific ALP:

  • Normal range: 5.3-19.5 μg/L 5
  • Important for diagnosing specific bone disorders
  • May be low even when total ALP is normal in some cases of hypophosphatasia 5

Practical Considerations

• When interpreting ALP results, always check the laboratory's specific reference range
• Consider age, sex, and clinical context when evaluating ALP values
• For suspected bone disorders, bone-specific ALP may provide more specific information
• Inflammation can affect plasma ALP levels but minimally affects red blood cell concentrations 1

Remember that ALP values should always be interpreted in the context of other laboratory findings and the patient's clinical presentation.