Alkaline Phosphatase (ALP) Test

A blood sample will be taken from a vein in your arm.

Tell your doctor about all medications you’re taking. Some drugs can affect ALP levels, though significant increases are rare.

What the test measures

Alkaline phosphatase (ALP) is an enzyme – a protein that speeds up chemical reactions in your body. It’s found in highest amounts in your liver and bones, with smaller amounts in the placenta (during pregnancy) and intestines.

Different body tissues produce different forms of ALP, called isoenzymes. Liver ALP is different from bone ALP, which is different from intestinal ALP. This test measures total ALP from all sources.

In the liver, ALP is found in cells lining the bile ducts – tiny tubes that carry bile from the liver to the intestines. When bile ducts are blocked or inflamed, ALP levels rise.

In bones, ALP is produced by cells called osteoblasts that build new bone tissue. Higher bone activity means higher ALP levels.

What your results mean

Raised ALP levels are usually due to a liver or bone disorder. Your doctor will look at other test results to work out which.

High ALP from liver disease

If other liver tests are also abnormal – particularly bilirubin, GGT, or ALT – this suggests the high ALP is coming from your liver.

Very high ALP levels (much higher than AST or ALT) typically indicate bile duct problems, a condition called cholestasis. This means bile flow is blocked or reduced.

Common causes of high liver ALP include:

• Bile duct blockage: gallstones, tumours, scars from previous surgery
• Primary biliary cholangitis: autoimmune liver disease
• Primary sclerosing cholangitis: scarring of bile ducts
• Liver cancer or cancer spreading to the liver
• Some medications

In hepatitis, ALP is usually only mildly elevated compared to AST and ALT, which are much higher.

High ALP from bone disease

If calcium and phosphate tests are abnormal, this suggests the high ALP is coming from bone.

Causes of high bone ALP include:

• Paget’s disease of bone: bones become enlarged and deformed
• Osteomalacia: bone softening due to vitamin D deficiency
• Cancer spreading to bone
• Healing fractures
• Overactive parathyroid glands (hyperparathyroidism)

Normal high ALP (physiological)

High ALP doesn’t always mean disease. Normal causes include:

• Children and teenagers: growing bones produce lots of ALP; levels are often very high during growth spurts
• Pregnancy: the placenta produces ALP, especially in the third trimester
• Young infants: temporary benign increases can occur

After eating, ALP can temporarily increase slightly in some people.

Normal or low ALP

Normal ALP is reassuring but doesn’t rule out all liver or bone problems. Low ALP is uncommon and can occasionally indicate zinc or magnesium deficiency, malnutrition, or certain rare genetic conditions.

If ALP is unclear, your doctor may request ALP isoenzyme testing to determine whether the ALP is coming from liver, bone, or another source.

If your ALP is high

Your doctor will first check whether the high ALP is normal for your situation (child, pregnancy, recent meal). If not, they’ll investigate the cause.

To identify the source:

• Check other liver tests: bilirubin, GGT, ALT, AST
• Check bone markers: calcium, phosphate, vitamin D
• ALP isoenzyme test: specifically identifies which type of ALP (liver, bone, intestinal, placental)

If liver disease suspected:

• Liver ultrasound or CT scan (to check bile ducts, look for blockages or tumours)
• MRCP (MRI of bile ducts)
• Autoimmune antibody tests (for primary biliary cholangitis)
• Sometimes liver biopsy

If bone disease suspected:

• Bone X‑rays or bone scan
• DEXA scan (bone density)
• Parathyroid hormone (PTH) test
• Vitamin D test

Treatment depends entirely on the underlying cause. Bile duct blockages may need ERCP or surgery, vitamin D deficiency needs supplements, Paget’s disease has specific medications, and so on.

Monitoring treatment

If you have liver or bone disease, ALP is useful for monitoring your response to treatment. For example, if you have cancer that has spread to liver or bone, decreasing ALP levels usually indicate the treatment is working.

Your doctor may request regular ALP tests to track disease progression or treatment response.

If your ALP is normal

Normal ALP is reassuring and suggests your liver bile ducts and bones are healthy. However, some liver diseases (like hepatitis) don’t cause high ALP, so your doctor may still request other liver tests if liver disease is suspected.

Several factors can affect ALP levels:

• Age: children and teenagers have much higher levels than adults due to bone growth
• Pregnancy: levels increase, especially in third trimester
• Recent meals: eating can temporarily increase ALP in some people
• Medications: some antibiotics, antihistamines, and drugs for psychiatric problems or epilepsy can increase ALP (though significant increases are rare)
• Blood group: people with blood groups B or O may have higher ALP after eating fatty meals
• Healing fractures: temporary elevation during bone repair

ALP is almost always tested alongside other tests. It’s rarely measured on its own.

Liver function tests (often tested together):

• ALT (alanine aminotransferase) – liver cell damage
• AST (aspartate aminotransferase) – liver cell damage
• GGT (gamma-glutamyl transferase) – helps confirm high ALP is from liver
• Bilirubin – bile pigment, raised when bile flow blocked
• Albumin – protein made by liver

Bone profile (often tested together):

• Calcium
• Phosphate
• Vitamin D
• Parathyroid hormone (PTH)

Specialized tests:

• ALP isoenzyme test – identifies whether ALP is from liver, bone, placenta, or intestines
• Other bone markers – for detailed bone assessment

The name ​‘alkaline phosphatase’ comes from the fact that this enzyme works best in alkaline (non-acidic) conditions and breaks down phosphate-containing compounds.

In the liver, ALP is concentrated in cells lining the bile ducts. When bile ducts are damaged, inflamed, or blocked, these cells release more ALP into the bloodstream. This makes ALP particularly useful for detecting bile duct problems (cholestasis).

In bones, ALP is produced by osteoblasts – cells that build new bone. Active bone formation (whether normal growth in children or abnormal bone changes in adults) produces high ALP levels.

Different forms (isoenzymes) of ALP from different tissues can be separated and measured individually if needed. This is helpful when total ALP is high, but the source is unclear.