Normal Alkaline Phosphatase Levels in Adults
Normal alkaline phosphatase (ALP) levels in adults typically range from 55-150 U/L in men and 60-180 U/L in women, though these values vary by laboratory and analytical method used. 1, 2
Standard Reference Ranges
The most recent patient-derived reference intervals for adults (>18 years old) are:
These ranges were statistically derived from large patient populations using modified Hoffmann approaches and represent the most current evidence-based values. 2
Important Laboratory Considerations
Upper limit of normal (ULN) varies significantly among laboratories due to differences in reference populations and analytical variation among commercial assays, making it essential to use laboratory-specific reference ranges when interpreting results. 3
- Most central laboratories report ULN for ALP around 115-120 U/L, though some report different values for sex and age groups 3
- Absolute laboratory values should always be reported and analyzed alongside multiples of ULN 3
Age-Related Variations
ALP levels are physiologically higher during childhood due to bone growth, with values substantially elevated compared to adults:
- Neonates (<15 days): 110-295 U/L 2
- Infants (15 days-1 year): 80-400 U/L 2
- Children (1-10 years): 90-290 U/L 2
- Adolescents (10-15 years): 75-300 U/L 2
- Late adolescence (15-18 years): 55-180 U/L 2
Special Populations
Postmenopausal women may have slightly elevated ALP due to increased bone turnover, with mean increases of approximately 77% compared to premenopausal women. 4 This elevation may be of bone origin related to osteoporosis and should be confirmed with GGT or ALP isoenzyme fractionation. 3
Pregnancy causes physiologically elevated ALP due to placental production, making standard adult reference ranges inappropriate for pregnant women. 1
Clinical Significance of Deviations
Low ALP (Hypophosphatasia)
ALP levels below 30-40 U/L in adults should raise suspicion for hypophosphatasia, a rare genetic disorder caused by ALPL gene mutations. 5, 6 Approximately 50% of adults with unexplained low ALP carry an ALPL mutation. 5
Elevated ALP Classification
The severity of ALP elevation guides diagnostic urgency:
- Mild elevation: <5× ULN 1, 2
- Moderate elevation: 5-10× ULN 3, 1
- Severe elevation: >10× ULN (requires expedited workup) 3, 1
Common Pitfalls
ALP of bone origin can confound interpretation in patients with osteoporosis or bone metastases, particularly in postmenopausal women. 3 Measuring GGT concurrently confirms hepatobiliary origin when elevated, while normal GGT suggests bone or other non-hepatic sources. 1
Patients with liver disease can have normal or even elevated ALP despite underlying hypophosphatasia, as demonstrated in cases where alcohol-induced hepatitis temporarily raised ALP from 23 U/L to 204 U/L in a patient with genetically confirmed hypophosphatasia. 6 This highlights that concomitant liver disease can mask the diagnosis of hypophosphatasia.
In patients with chronic kidney disease (GFR <45 mL/min/1.73 m²), calcium and phosphate values are typically normal in 87-92% of cases, making routine screening of limited value unless GFR is <30 mL/min/1.73 m². 3