Alkaline Phosphatase (ALP) Test

ALP reference ranges

What does high ALP mean?

Causes of raised ALP

Why Gamma-GT Settles the Question

The reason a gamma-GT is the next test rather than a scan is anatomical. GGT is concentrated in the cells lining the liver and bile ducts, and is essentially absent from bone. So the two enzymes rising together points at the biliary system, while ALP rising alone leaves bone as the remaining candidate. It is a cheap way of halving the problem before anyone books imaging.

The pattern is definitional in cholestasis, where bile production, secretion or flow is obstructed: ALP and GGT rise disproportionately to ALT and AST.² That word matters. Liver enzymes commonly move together to some degree, and it is the shape of the rise, rather than any single enzyme crossing a threshold, that separates a blockage in the drainage from injury to the liver cells themselves.

When ALP is raised and GGT is normal, the follow-up shifts to bone chemistry — calcium, phosphate, vitamin D and parathyroid hormone — because the common causes there are vitamin D deficiency, Paget disease and hyperparathyroidism, each of which those tests can distinguish.

Low ALP Deserves More Attention Than It Gets

A result below the range is easy to dismiss, and the approach to it is genuinely less well established than for a high result.³ It can follow acute severe illness, and persistently low values may reflect drug treatment — antiresorptive bone drugs among them — or malnutrition, vitamin and mineral deficiencies, or endocrine disease.³

The reason not to ignore it is hypophosphatasia, an inherited fault in the enzyme itself and the commonest genetic cause of a low ALP. In adults it does not usually look dramatic: skeletal pain, stress fractures, dental problems, and calcium crystal deposits causing joint symptoms.³ It is easy to attribute all of that to something else.

One caution on confirming it: sequencing the responsible gene does not always settle the matter, and a substantial proportion of people with the clinical picture return normal results on standard testing.³ A negative genetic test does not close the question by itself.

Questions to ask your doctor

• Is my raised ALP from the liver or bone?
• Should I have a GGT to help distinguish the cause?
• Do I need a liver ultrasound or bone scan?

Frequently Asked Questions

Working out where a raised ALP is coming from

Alkaline phosphatase is produced in several tissues, chiefly liver and bone, so a raised result is a question rather than an answer. The usual next step is a gamma-GT. If both ALP and GGT are raised, the source is almost certainly hepatobiliary. If ALP is raised and GGT is normal, bone becomes the likely origin, and a calcium, phosphate, vitamin D and parathyroid hormone panel follows. Some laboratories can also separate ALP into its bone and liver isoenzymes directly when the picture stays unclear.

Physiological causes that are not disease

• Growth: children and adolescents normally run ALP several times the adult upper limit during growth spurts, needing no investigation.
• Pregnancy: the placenta produces its own ALP and levels rise substantially in the third trimester.
• Healing fractures, where bone turnover stays high for weeks to months.
• A recent fatty meal in people of certain blood groups, which raises intestinal ALP modestly.

What a low ALP can mean

Low ALP attracts less attention but is occasionally informative. It accompanies malnutrition, zinc or magnesium deficiency, severe anaemia, underactive thyroid, and bisphosphonate treatment. Persistently low ALP with fractures, early loss of baby teeth or bone pain can point to hypophosphatasia, a rare inherited disorder of the enzyme itself, worth considering because bisphosphonates are contraindicated in it.

How the result is followed up

An isolated mild elevation in someone who is well is often simply repeated after a few weeks, since transient rises are common. A persistent rise with a raised GGT usually leads to ultrasound of the liver and bile ducts to look for obstruction. A bone-pattern rise leads to assessment of calcium metabolism and, depending on age and symptoms, imaging for Paget's disease or metastatic disease. Because ALP falls slowly after the cause resolves, a gradually declining result is usually reassuring rather than a sign of ongoing disease.

References

The clinical information on this page is drawn from peer-reviewed sources indexed by the US National Library of Medicine. Links go to the source so you can read it yourself.

1. Serum Alkaline Phosphatase: Clinical and Laboratory Perspectives. In: StatPearls. Treasure Island (FL): StatPearls Publishing. NCBI Bookshelf NBK459201
2. Siddique A, Kowdley KV. Approach to a patient with elevated serum alkaline phosphatase. Clin Liver Dis. 2012;16(2):199–229. PMID 22541695 · doi:10.1016/j.cld.2012.03.012
3. Riancho JA. Diagnostic approach to patients with low serum alkaline phosphatase. Calcif Tissue Int. 2023;112(3):289–96. PMID 36348061 · doi:10.1007/s00223-022-01039-y