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.
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.
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.
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.
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.
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.
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:
Bone causes, which leave GGT normal, include:
Causes that are normal and need no treatment include:
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.
ALP is interpreted alongside other results, and the combination quickly narrows the possible causes.
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.