What is alkaline phosphatase (ALP)?

What is Alkaline Phosphatase (ALP)?

Alkaline phosphatase (ALP) is a group of enzymes found throughout your body that help break down phosphate compounds at an alkaline pH, producing inorganic phosphate. It's primarily found in your liver, bones, intestines, and placenta, and is one of the most frequently tested enzymes in medicine 1.

Key Facts About Alkaline Phosphatase

• Location: ALP is a membrane-bound glycoprotein found in multiple tissues 2
• Function: Catalyzes the hydrolysis of phosphate monoesters at basic (alkaline) pH levels 2
• Types: There are four main isozymes (different forms) of ALP:
  • Intestinal ALP
  • Placental ALP
  • Germ cell ALP
  • Tissue non-specific ALP (found in liver, bone, and kidney) 2

What ALP Levels Tell Your Doctor

ALP is measured as part of a standard liver function panel. When your ALP levels are abnormal, it helps your doctor understand what might be happening in your body:

• Elevated ALP levels may indicate:

  • Liver disease, especially conditions affecting bile flow 3
  • Bone disorders where there is increased bone cell activity 1
  • Pregnancy (normal physiological increase) 4
  • Growing children (normal during growth spurts) 4
  • Certain cancers, especially with bone or liver metastasis 5

• Low ALP levels may be associated with:

  • A rare genetic condition called hypophosphatasia 1
  • Malnutrition
  • Certain types of cancer 5

How ALP Helps Diagnose Medical Conditions

Your doctor uses ALP along with other tests to identify specific patterns:

• Liver disease pattern: When ALP is elevated along with gamma-glutamyl transferase (GGT), it suggests liver disease, particularly affecting the bile ducts 3, 6

• Bone disease pattern: When bone-specific ALP is elevated, it may indicate conditions like Paget's disease, healing fractures, or bone tumors 6

• Pattern of liver injury: ALP helps classify the type of liver damage:

  • Cholestatic pattern: When ALP is ≥2 times the upper limit of normal (particularly with elevated GGT) 3
  • Hepatocellular pattern: When ALT/ALP ratio ≥5 3
  • Mixed pattern: When ALT/ALP ratio is between 2 and 5 3

Important Things to Know

• Normal ALP levels vary by age, sex, and laboratory
• ALP levels can be elevated due to certain medications, including glucocorticoids and anticonvulsants 4
• ALP is often elevated in pregnant women and growing children as a normal finding 4
• Isolated elevation of ALP may require further testing to determine which isozyme (bone, liver, etc.) is elevated 7

Remember that ALP is just one piece of information your doctor uses. It's typically interpreted alongside other tests to get a complete picture of your health.