Biomarker 101: Alkaline phosphatase (ALP)

ALP comes from two quite different places in the body, which is why it is never interpreted on its own.

What ALP is

Alkaline phosphatase is an enzyme found circulating in the blood, derived mainly from the liver and from bone. It plays a role in liver metabolism, protein breakdown and bone development.

It exists in several forms that differ by the tissue they come from — the placenta produces its own during pregnancy, for instance. Routine laboratories report a total figure without separating them, which is part of why context matters so much when reading a result.

Why it's measured

ALP appears in both liver panels and bone assessments, so it is often included in a general health screen. A raised result may point toward the liver or bile ducts, or toward bone, and further testing is what distinguishes between them.

What the numbers mean

Reference ranges vary between laboratories and change considerably with age — children and adolescents run much higher than adults because their bones are actively growing. Read your result against the range on your own report.

Several ordinary factors shift ALP. It rises in pregnancy, particularly in the third trimester, because of placental production. It can increase transiently after a fatty meal, which is why fasting is often preferred. Smokers tend to run slightly higher. There are also documented differences between ethnic groups.

What moves ALP

Raised ALP most often reflects either a liver or bile duct issue — blocked bile ducts, gallstones, or liver inflammation — or bone activity, including healing fractures, conditions affecting bone turnover, overactive parathyroid glands, and vitamin D deficiency. Untreated coeliac disease can also raise it.

Low ALP is uncommon. It has been associated with low phosphate, zinc or magnesium, an underactive thyroid, severe anaemia, some hormonal medications, and rare inherited conditions.

What ALP won't tell you

• It doesn't distinguish liver from bone. That requires further testing, often GGT — raised alongside ALP it points to the liver, normal it points toward bone.
• It isn't specific. A raised total ALP is a starting point for investigation, not an answer.
• Context changes everything. The same number means different things in a growing teenager, a pregnant woman and a sixty-year-old.
• Symptoms may be absent. Many people with raised ALP feel entirely well.

The bottom line

ALP straddles the liver and the skeleton, which makes it useful as a signal and useless as a standalone answer. Its value is in prompting the right next question.