High Alkaline Phosphatase (ALP): Liver or Bone?

The ACG (American College of Gastroenterology) guideline defines cholestatic injury as a disproportionate rise of alkaline phosphatase compared with ALT and AST. If the rise in ALP is judged to be of hepatic origin, the guideline recommends testing for primary biliary cholangitis (PBC) and primary sclerosing cholangitis (PSC). The crucial prior question, however, is whether the liver is involved at all.

GGT is an enzyme found in the bile ducts and liver, but not in bone. A raised ALP with normal GGT therefore favours a bone source (or, less often, a placental one), while a raised ALP together with raised GGT favours a hepatobiliary source. 5'-nucleotidase, less often measured, can play a similar role. When the result remains unclear, the laboratory can measure ALP isoenzymes (for example by electrophoresis or heat inactivation) or a specific bone marker, bone-specific alkaline phosphatase.

When ALP rises together with GGT, the commonest mechanism is impaired bile outflow (cholestasis). It can result from an obstruction in the large bile ducts (duct stones, a stricture, a tumour of the pancreatic head or bile ducts) or from disease of the small ducts and hepatocytes. Assessment usually starts with abdominal ultrasound, which shows dilated bile ducts, stones or focal liver lesions.

PBC and PSC deserve attention because in these diseases ALP is the central parameter. If a raised ALP has no obvious explanation (stones, drugs) and ultrasound shows no obstruction, the doctor usually orders anti-mitochondrial antibodies and further biliary work-up, in line with the ACG guideline. We write about fatty liver in a separate article, and about the assessment of fibrosis in the last text of this series.

Bone is the second main source of ALP. The enzyme is produced by osteoblasts, so it rises wherever bone remodelling or new bone formation is accelerated. The commonest causes in adults are fracture healing, Paget disease, osteomalacia (impaired bone mineralisation, most often due to vitamin D deficiency), hyperparathyroidism and cancer metastases to bone.

This last point has practical meaning: Paget disease should be considered when ALP is raised and liver tests (including GGT) are not — because in exactly such conditions the source is bone. Characteristic symptoms can include bone pain, deformities, thickening or enlargement of the skull and hearing loss, although the disease is often asymptomatic and found by chance from a raised ALP. Diagnosis requires imaging (radiographs, bone scan).

A second important scenario is vitamin D deficiency. In a review by Minisola and colleagues (2021) it was noted that osteomalacia related to vitamin D deficiency passes through three stages; in the first, blood calcium and phosphate are normal and ALP, PTH and 1,25(OH)2D are raised, in the second calcium and often phosphate begin to fall while ALP and PTH rise further, and in the third there is hypocalcaemia and hypophosphataemia. Symptoms are diffuse bone and muscle pain, muscle weakness and characteristic fractures. That is, however, a severe scenario: in a large analysis by Bolland and colleagues (2021), covering more than 42,000 25(OH)D measurements, biochemical osteomalacia (low adjusted calcium, high PTH and ALP >150 IU/L) was very rare — 0.23% of people with 25(OH)D below 25 nmol/L. A small rise in ALP with low vitamin D therefore does not automatically mean osteomalacia.

ALP does not always mean disease. In children and adolescents ALP activity is many times higher than in adults because bones are growing intensively, and it changes with age and sex. Zierk and colleagues (2017) built, from more than 361,000 measurements, percentile charts of ALP activity from birth to age 18 that make it possible to separate physiological from pathological changes — a result that would be raised in an adult may be normal in a child. For this reason a child's result must always be interpreted against norms for age and sex.

A special phenomenon is benign transient hyperphosphatasaemia of infants and young children. In a retrospective study from Israel (Shkalim Zemer et al., 2023) of 382 children, ALP exceeded 1000 U/L (on average about 2557 U/L), 87% of the children were up to 24 months old, and in many cases there had been a preceding infection, such as gastroenteritis or otitis media. The phenomenon resolves spontaneously, and the authors recommend that, when a high ALP is found incidentally in an otherwise healthy child with no other suspicion, the test be repeated within a few months instead of running extensive investigations.

A practical lesson for pregnant women follows from this study: a rise in ALP in the third trimester is normal, because part of the enzyme comes from the placenta. At the same time an excessive rise may point to cholestasis of pregnancy or other complications, particularly if accompanied by marked itching (especially of the palms and soles) — a reason for urgent consultation with whoever is managing the pregnancy. Always interpret an ALP result in pregnancy against norms for gestational age, not for non-pregnant people. Likewise, ALP can be raised for several weeks or months while a fracture heals.

A common situation: ALP is slightly raised while ALT, AST, bilirubin and GGT are normal, you have no symptoms, and you are not pregnant or a growing child. Is this a reason to worry? The answer comes from a classic observational study of hospital patients, which has its limitations but shows the scale of the ambiguity.

What follows from this, and what does not? The study concerned hospitalised people, who are far more burdened with illness than a typical patient with a result from an outpatient clinic; its percentages cannot be carried over to healthy adults with an incidental, slightly raised result. The qualitative conclusions are nevertheless useful: an isolated raised ALP is often transient (nearly half resolved within a few months), the higher the result the greater the chance that it persists and that a specific disease lies behind it, and a reasonable strategy is history, examination and a check after 1–3 months.

In the general population a higher ALP was also linked with higher mortality: in an NHANES analysis (Yan et al., 2023; 34,147 adults) the highest ALP group had an HR of 1.30 for all-cause death and 1.39 for cardiovascular death compared with the lowest, with GGT, vitamin D and CRP identified as possible mediators. These are observational data — they do not prove that ALP is causal, and they give no basis for conclusions about an individual result, but they are one reason why a raised ALP should not be dismissed without explanation.

Alkaline phosphatase is an enzyme with two faces, and its result therefore always needs context. A good first question is not 'do I have a diseased liver?' but 'where does this enzyme come from?' — and the answer is often given by one simple extra test, GGT. When the result is incidental and slightly raised, do not dismiss it but do not panic either: a calm history, a few supplementary tests and a check after weeks or months resolve most such situations. If the rise persists or is accompanied by symptoms, that is a signal to move to broader work-up — of the liver, the bile ducts or the bones.