Alkaline Phosphatase (ALP) – Understanding Your Blood Test Result

Section Editors: Christopher McCudden Ph.D., DABCC, FADLM, FCACB
October 20, 2026

Alkaline phosphatase, usually shortened to ALP, is an enzyme found mainly in two places in the body. One is the lining of the bile ducts in the liver, and the other is the bone-building cells of the skeleton. ALP is measured as part of a liver panel and a comprehensive metabolic panel. It is reported in units per liter (U/L), sometimes written IU/L, which means the same thing.

This article explains what the ALP test measures and why a high result must be traced to either the liver or the bones before you can interpret it. It also covers the situations in which a high ALP is completely normal, what low results mean, and what your doctor is likely to do next.

The reference range that applies to your result is the one printed on your laboratory report, not the typical ranges shown here. Reference ranges vary between laboratories based on the equipment used, the population tested, and individual factors such as age, sex, and pregnancy status. Always compare your result to the reference range printed on your own report, and discuss any abnormal result with your doctor.

What does the ALP test measure?

The ALP test measures the total alkaline phosphatase activity in your blood. Several tissues make their own slightly different version of the enzyme. In healthy adults, the liver and bone versions together account for about 95 percent of the total, with small contributions from the intestine and the kidneys.

In the liver, ALP sits in the lining of the bile ducts and spills into the blood when bile flow slows or is blocked. In bone, osteoblasts, the cells that lay down new bone, make it. Anything that speeds up bone formation raises ALP, whether that is healthy growth, a healing fracture, or a disease.

Why a high ALP has to be traced to the liver or to bone

A standard ALP test measures all the versions of the enzyme together and reports a single number. A high result therefore tells your doctor that ALP is raised, but not where it comes from, and the list of possible causes differs completely for each source. The first step with any unexplained high ALP is to find out which one it is.

The usual way to settle this is to look at gamma-glutamyl transferase (GGT), another enzyme often measured on the same panel. GGT comes from the liver and bile ducts, not bone, so the two results are easy to interpret together.

ALP and GGT both high. The extra ALP almost certainly comes from the liver or bile ducts.
ALP high with a normal GGT. The extra ALP most likely comes from bone, and a liver workup is usually not needed.

If the answer is still unclear, the lab can separate liver and bone forms directly with a test called ALP isoenzyme analysis. This is ordered less often, because GGT answers the question in most people.

How is the ALP test performed?

ALP is measured on a routine blood sample, most often as part of a panel. A technologist collects blood from a vein, usually in your arm. You may be asked to fast beforehand because, in some people, ALP rises for several hours after a fatty meal.

That rise comes from the intestinal form of the enzyme, and it is most noticeable in people with blood group O or B. This is why a mildly high ALP on a sample taken after a meal is often repeated on a fasting sample before anything else is done.

What do ALP results mean?

Your ALP result is read against the reference range printed on your own report. ALP ranges differ between laboratories more than most, and they depend heavily on age.

A normal ALP in adults. Many laboratories use a range of approximately 40 to 130 U/L (0.67 to 2.17 µkat/L). Some extend the upper limit to around 150 U/L, particularly for adults over 50.
A normal ALP in children and teenagers. Growing bones release large amounts of the enzyme, so children commonly range from about 150 to 420 U/L (2.5 to 7.0 µkat/L). During the teenage growth spurt, levels can reach about 500 U/L (8.3 µkat/L) in a perfectly healthy child.
A normal ALP in pregnancy. The placenta makes its own ALP, and levels rise through the third trimester. They return to the usual range within weeks of delivery.
A high ALP. A result above the range means the enzyme is being released from the liver, the bones, or occasionally another source. GGT is the usual next step to find out which.
A low ALP. This is much less common, and a mildly low result on its own is rarely investigated.

What conditions and factors can affect ALP?

Because ALP comes from more than one tissue, its causes fall into separate groups. Your doctor will usually decide which group applies before considering any specific cause.

Liver and bile duct causes, which usually raise GGT as well, include:

Blockage of the bile ducts. A gallstone in the duct is the most common cause, and a narrowing or a tumor can block the duct in the same way. Doctors call slowed bile flow cholestasis.
Damage to the small bile ducts inside the liver. Primary biliary cholangitis and primary sclerosing cholangitis both raise ALP, often before any symptoms appear.
Medications. Many drugs can slow bile flow and raise ALP, which usually settles once the medication is stopped.
Conditions that spread through the liver. Cancer that has spread to the liver can raise ALP, as can inflammatory conditions such as sarcoidosis.

Bone causes, which leave GGT normal, include:

A healing fracture. ALP rises for weeks to months as new bone forms, which is expected and harmless.
Vitamin D deficiency. When bone cannot mineralize properly, osteoblasts work harder, raising ALP. This is one of the most common causes of bone disease in adults.
Paget disease of bone. This condition causes disorganized, excessive bone remodeling in older adults, and it can raise ALP to several times the upper limit.
An overactive parathyroid gland. Excess parathyroid hormone speeds up bone turnover and raises ALP along with calcium.
Cancer that has spread to bone. Breast and prostate cancer in particular can raise ALP when they involve the skeleton.

Causes that are normal and need no treatment include:

Growth in children and teenagers. This is by far the most common cause of high ALP in anyone under 18.
Pregnancy. The placental enzyme raises ALP in the third trimester.
Transient hyperphosphatasemia of childhood. In children under about five, ALP can briefly rise to several times the upper limit, often after a viral illness. The child is well, with no liver or bone disease, and the level returns to normal within about four months.
Inherited high ALP. A few families have a lifelong high ALP with no associated disease.

A low ALP is rare. It can occur with severe undernutrition or an underactive thyroid gland. More rarely, it reflects an inherited bone condition called hypophosphatasia, usually suspected when bone or dental problems accompany low ALP.

What other tests are ordered with ALP?

ALP is interpreted alongside other results, and the combination quickly narrows the possible causes.

GGT. This is the most important companion test because it shows whether ALP comes from the liver or bone.
The other liver tests. ALT, AST, and bilirubin show whether liver cells are injured and whether bile is backing up.
Calcium, parathyroid hormone, and vitamin D. These are ordered when the ALP appears to be coming from bone.
An ultrasound of the liver and bile ducts. This is usually the first imaging test when ALP is coming from the liver.
ALP isoenzyme analysis. This separates the liver and bone forms directly when GGT does not settle the question.

What happens after an ALP test?

For mildly elevated ALP in an adult, the usual first steps are to repeat the test on a fasting sample and check GGT. In children and teenagers, doctors interpret a high ALP against age-specific ranges and usually explain it with growth.

If the ALP comes from the liver, your doctor will usually arrange a bile duct ultrasound and blood tests for common causes of bile duct disease. If it comes from bone, the usual next steps are a vitamin D level and checks of calcium and parathyroid hormone. Bone imaging follows if those do not explain the result.

Report some symptoms regardless of the number. Yellowing of the eyes or skin and persistent itching both point to the bile ducts. New bone pain that is present at rest or at night should also be mentioned to your doctor.

Questions to ask your doctor

What was my ALP, and what is the reference range at this laboratory?
Was GGT measured, and was it normal?
Do you think the ALP is coming from my liver or from my bones?
Was this sample taken after a meal, and should it be repeated on a fasting sample? sample
Could any of my medications be raising my ALP?
Should my vitamin D level be checked?
Do I need an ultrasound or any other imaging?
How does this compare with my previous ALP results?
For my child, is this result normal for their age and stage of growth?
What symptoms should prompt me to contact you before my next appointment?