Alkaline Phosphatase

ALP (Alkaline Phosphatase) is an enzyme produced by several tissues around the body, but two sources matter most: the liver (specifically the cells lining the bile ducts) and bone (specifically the cells that build new bone, called osteoblasts). Smaller amounts come from the intestines and placenta. Because ALP has two main origins, interpreting a raised result depends entirely on working out which source is responsible, and that is where other markers on your panel become essential.

A useful way to think about ALP is as a combined readout from two systems. In the liver, it is concentrated in the tiny ducts that drain bile from the liver to the digestive tract. If those ducts are blocked, inflamed, or pressed upon, the cells lining them are stressed and release more ALP into the blood. In bone, ALP is released when bone-building activity is high: during childhood and teenage growth, while a fracture heals, and in conditions such as Paget's disease. In pregnancy, the placenta releases it too. This is why ALP is naturally much higher in growing children and in pregnant women, and why a raised ALP should not be assumed to come from the liver without further thought.

The standard way to decide the source is to look at ALP alongside GGT. GGT is a liver and bile duct enzyme and is not made by bone. So a raised ALP with a raised GGT strongly suggests a liver or bile duct origin, while a raised ALP with a normal GGT points to bone as the source. Other factors can also influence ALP: vitamin D deficiency often causes mild elevations as the body tries to mobilise bone to maintain calcium levels, eating a fatty meal shortly before a blood test can transiently raise ALP from the intestinal form, and certain medications (some antibiotics, anti-epileptics, and hormonal contraceptives) can nudge it up.

If your ALP is out of range, your GP will use the rest of your panel, GGT, bilirubin, calcium, vitamin D, and liver enzymes, to work out whether it reflects liver or bile duct activity, normal bone turnover, or something worth investigating further. A low ALP is uncommon and can occasionally reflect zinc deficiency, malnutrition, hypothyroidism, or (rarely) an inherited condition called hypophosphatasia.