Alkaline phosphatase: what your value means

The test measures how much alkaline phosphatase is active in your blood, in units per litre (u/l). Alkaline phosphatase is an enzyme almost all your tissues make, each in its own form. The test adds all those forms together and does not measure them separately, and that is where the whole ambiguity comes from: your alkaline phosphatase value is a sum of sources. In adults two really count, the cells that build new bone and the small bile ducts in the liver.

You will see two spellings on reports. AF is the Dutch abbreviation and ALP the international one; they mean the same test.

The normal value depends heavily on your age and your sex, and that is not sloppiness on the lab's part. The more active your bones, the higher the number, so a teenager in the growth spurt sits well above an adult ceiling and that is normal. According to NUMBER, the national harmonisation project of the Dutch Society for Clinical Chemistry, the band for a man aged 19 to 50 runs from 45 to 128 u/l and for a woman of that age from 38 to 123 u/l. Between 51 and 65 it is 46 to 126 for men and 48 to 142 for women, and above eighty 45 to 145 for men and 45 to 146 for women.

Look closely at those last figures. A value of 145 falls inside the band for a man of eighty and outside it for a man of thirty; age changes what the same number means. Your own laboratory may use slightly different limits, and the reference on your own report always takes precedence.

The meaning of alkaline phosphatase is not in the number itself but in the question it asks: did this come from your liver or from your bones? Until you know that, the result gets you very little.

Your GP almost always resolves it the same way. They look at your gamma-GT, an enzyme that sits in your liver and bile ducts but not in bone, which turns it into a kind of switch. If alkaline phosphatase and gamma-GT are both high, your doctor looks towards your liver or bile ducts. If only alkaline phosphatase is raised and gamma-GT is normal, it probably comes from your bones, or it is not a problem at all.

That last case happens more often than people expect. Raised values belong with growth and with pregnancy, a healing fracture lifts the result, and so can a fatty meal shortly before the draw.

The second step is often simpler than you expect. Many GPs simply repeat the test after a few weeks, because a one-off mild abnormality says little and a second measurement shows whether it stays. If it disappears, the story usually ends there.

What your result does not say is how serious something is: the height of the number does not track the seriousness of the cause. So read it alongside your ALT and your direct bilirubin, because those two tell you whether the liver itself is involved.

Usually you did not request this test yourself: alkaline phosphatase sits in almost every liver panel as standard, so it simply came out with the rest. Asking for it deliberately makes sense with itching without a rash, with yellowing of your skin or eyes, with dark urine or pale stools, and with bone pain that does not fit an injury you know about.

Never request it on its own. Without gamma-GT beside it you cannot place a raised result and you end up with a number that points nowhere, so have the other liver values drawn at the same time.

Do you need to fast? For this test alone, usually not, but fasting is still smart here. The intestinal form of the enzyme rises for a few hours after a fatty meal, most strongly in people with blood group O or B, and that is a harmless explanation for a mildly raised result that you would rather not have in your measurement. If other values are drawn at the same time, follow the instructions for that test.