What is the normal serum alkaline phosphatase level in a healthy adult?

Normal Alkaline Phosphatase Levels

Normal serum alkaline phosphatase (ALP) levels in healthy adults range from approximately 55–140 U/L in males and 60–147 U/L in females, though these values vary significantly by age, sex, race/ethnicity, and laboratory methodology. 1

Age-Specific Reference Ranges

Adults (>18 years)

• Males: 55–140 U/L 1
• Females: 60–147 U/L 1
• The upper limit of normal varies by race/ethnicity, with Hispanic individuals showing the highest values (123.2–123.8 U/L), followed by African Americans (109.9–116.3 U/L), whites (97.1–109.6 U/L), and Asian American/Pacific Islanders having the lowest (93.8–95.3 U/L) 2

Children and Adolescents

• Neonates (<15 days): 110–250 U/L (males), 120–295 U/L (females) 1
• Infants (15 days–1 year): 80–400 U/L (males), 90–380 U/L (females) 1
• Children (1–10 years): 105–280 U/L (males), 90–290 U/L (females) 1
• Early adolescence (10–13 years): 75–300 U/L (males), 90–300 U/L (females) 1
• Mid-adolescence (13–15 years): 80–300 U/L (males), 60–175 U/L (females) 1
• Late adolescence (15–18 years): 55–150 U/L (males), 60–180 U/L (females) 1

ALP levels demonstrate a tetraphasic pattern with two peaks—one during infancy and another during puberty—reflecting periods of active bone growth 3. Girls reach peak levels at ages 10–11 years, while boys peak at ages 12–13 years, after which levels decline 3.

Important Clinical Considerations

Physiologic Elevations

• Childhood and adolescence: ALP levels are physiologically 2–3× adult values due to active bone growth, with bone-specific ALP representing 80–90% of total ALP in children 4
• Pregnancy: Mild elevations occur in the second and third trimesters due to placental production, while other liver enzymes (ALT, AST, bilirubin) remain normal 5

Laboratory Methodology

• Different assay methods and buffers produce variable results, even when using the same analytical technique 3
• Absolute laboratory values should always be reported alongside multiples of the upper limit of normal (ULN) to account for inter-laboratory variation 6

Source Determination

• In adults, approximately 50% of circulating ALP originates from bone and 50% from hepatocytes, whereas in children, 80–90% is bone-derived 4
• Measuring gamma-glutamyl transferase (GGT) or obtaining ALP isoenzyme fractionation confirms hepatobiliary origin when ALP is elevated, as GGT elevation indicates liver source while normal GGT suggests bone or other non-hepatic sources 6, 4

Abnormally Low ALP

• Persistently low ALP (<40 U/L on repeated testing) may indicate hypophosphatasia, a rare genetic disorder, though most cases represent benign variants 7
• Low ALP with otherwise normal liver function tests does not indicate liver injury and generally does not warrant extensive investigation 8