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Alkaline phosphatase (ALP) is a group of enzymes measured routinely in the basic metabolic panel and in liver tests. If you were told yours is “high,” the most common explanation is a problem of the liver and bile ducts, but ALP can also rise from bone or from normal situations such as pregnancy and growth. For that reason, a high ALP is not a diagnosis in itself but a signal that needs to be interpreted.

The first and most useful step is to clarify where it comes from. When the source is the liver, ALP is accompanied by a raised GGT as well; together they define what is called the cholestatic pattern. If the GGT is normal, the ALP most likely comes from bone rather than the liver.

What is alkaline phosphatase?

These are enzymes whose job is to split phosphate esters in an alkaline environment. The ALP that circulates in the blood comes mainly from three tissues:

  • Liver (specifically, the cells lining the bile ducts).
  • Bone, from the activity of bone-forming cells.
  • Intestine.

A fourth source is the placenta, which explains its rise during pregnancy. The result is reported in international units per litre (IU/L), and the normal range depends on age and sex.

Why does it rise? The cholestatic pattern

In the liver, ALP is concentrated in the membrane of the cells that form the bile ducts. When bile flow is hindered, a situation we call cholestasis, both the production of ALP and its passage into the blood increase. The retained bile salts appear to be responsible for that rise.

This is the hallmark of bile duct disease: raised ALP together with raised GGT, often with normal or only slightly elevated liver enzymes. If cholestasis progresses, bilirubin may also rise, and jaundice, dark urine and itching may appear.

How is a liver origin confirmed?

Separating liver ALP from bone ALP by electrophoresis is possible but cumbersome and rarely used in practice. The simple, standard way to confirm the source is to measure gamma-glutamyl transferase (GGT):

  • If the GGT is high alongside the ALP, the source is almost certainly the liver or bile ducts.
  • If the GGT is normal, it is worth looking for a bone cause (or a normal physiological rise) before investigating the liver.

Normal (physiological) rises

Not every high ALP means disease. It rises predictably in:

  • Growth: in children and adolescents, bone-derived ALP can reach three times the adult value. This is normal.
  • Pregnancy: placental ALP can double the levels toward the end of pregnancy.
  • Age and sex: it tends to be higher after age 60.

Liver causes of high alkaline phosphatase

Once a liver origin is confirmed (with a raised GGT), the causes are organised by where the problem sits along the bile ducts.

Bile duct obstruction (extrahepatic cholestasis)

Any blockage of the duct that carries bile to the intestine raises ALP. The most frequent causes are gallstones in the bile duct and tumours that compress or obstruct it, such as pancreatic cancer or cholangiocarcinoma.

Disease of the bile ducts inside the liver (intrahepatic cholestasis)

Here the damage is in the small bile ducts. The main ones are:

  • Primary biliary cholangitis (formerly called primary biliary cirrhosis), an autoimmune disease that mainly affects women and whose most typical abnormality is precisely a persistently raised ALP.
  • Primary sclerosing cholangitis, which inflames and narrows the bile ducts.
  • Medications. Many drugs can cause liver injury of the cholestatic pattern, among them some antibiotics, anabolic steroids and oral contraceptives. It almost always resolves once the drug is stopped.

Infiltrative diseases

Less often, processes that “infiltrate” the liver raise ALP markedly with normal bilirubin: metastases, lymphoma, sarcoidosis, tuberculosis and some infections.

What should you do if yours is high?

First, do not be alarmed: an isolated high ALP with the rest of the tests normal often has no major significance. Your doctor will confirm the source with the GGT, review the medications you take and, if warranted, order an abdominal ultrasound to look at the bile ducts. The workup is guided by the whole picture of your tests and history, not by a single value.

See also

References

  1. Kwo PY, Cohen SM, Lim JK. ACG Clinical Guideline: Evaluation of Abnormal Liver Chemistries. Am J Gastroenterol. 2017;112(1):18-35.
  2. Tanaka A, Ma X, Takahashi A, Vierling JM. Primary biliary cholangitis. Lancet. 2024;404(10457):1053-1066.
  3. Gulamhusein AF, Hirschfield GM. Primary biliary cholangitis: pathogenesis and therapeutic opportunities. Nat Rev Gastroenterol Hepatol. 2020;17(2):93-110.
  4. Allison R, Guraka A, Shawa IT, et al. Drug induced liver injury - a 2023 update. J Toxicol Environ Health B Crit Rev. 2023;26(8):442-467.
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