Alkaline phosphatase in the blood

The test measures the total activity of alkaline phosphatase in your blood, reported in u/l. Total is the important word. Every tissue makes a slightly different variant of the same enzyme, an isoenzyme, and the standard test adds those variants together. They are not measured separately.

In adults two isoenzymes matter. The liver isoenzyme sits mainly in the epithelium of the small bile ducts.

The bone isoenzyme comes from osteoblasts, the cells that build new bone. There is also an intestinal form, and during pregnancy the placenta makes a variant of its own.

That explains why the normal value moves so much with age and sex. The more active the bone, the higher the result. A growing child does not have a liver problem because the number sits above an adult ceiling.

The Dutch reference values come from NUMBER, the harmonisation project run by the NVKC. For men between 19 and 50 the band runs from 45 to 128 u/l. For women in the same age group it runs from 38 to 123 u/l.

Above fifty both bands rise again. Laboratories set their own limits, so the values on your own report always take precedence.

The main job of this result is not to prove something. It is to ask a question: did this rise come from the liver or from bone. While that question is open, the number means little on its own.

Gamma-GT usually answers it. That enzyme sits in the liver and bile ducts, but not in bone. Two patterns follow. If alkaline phosphatase and gamma-GT are both raised, a liver or bile-duct cause becomes likely.

If alkaline phosphatase is raised while gamma-GT stays normal, that points towards bone or towards an ordinary physiological explanation.

That is the one rule of thumb every reader should take from this page. It also explains why a doctor rarely acts on this enzyme alone.

The liver side is a different type from ALAT and ASAT. Those two leak out of damaged liver cells. Alkaline phosphatase rises mainly when bile flow is obstructed.

Gallstones blocking the bile duct belong here, as does cholestasis caused by medication. Amoxicillin with clavulanic acid is the classic example. In that pattern direct bilirubin often rises alongside.

The bone side has its own list. A healing fracture raises the value temporarily. Vitamin D deficiency and osteomalacia do the same, because bone turnover speeds up.

An overactive thyroid has the same effect. Paget's disease of bone classically gives a strongly raised value with an otherwise normal panel.

Alkaline phosphatase is a standard part of almost every liver panel. Most people therefore meet the result without ever having asked for it. It was simply on the list.

Requesting it deliberately happens in a few situations. With symptoms that may point to a bile-duct problem, such as persistent itching, yellowing of the skin or the whites of the eyes, or pale stools.

With bone pain that does not fit a known injury. And when monitoring a liver or bone condition that is already known.

The test is almost never done on its own, and that is deliberate. Without gamma-GT beside it a raised value cannot be placed. So request it together with the other liver values.

There is a misunderstanding about fasting. For this test alone you usually do not need to fast. There is still a reason to do so.

The intestinal form of the enzyme rises for some hours after a fatty meal in people with blood group O or B who are secretors. That is a benign explanation for a mildly raised result. A fasting sample removes that noise.

If other values are measured at the same time, follow the instructions for that test.