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Blood Test Tracker

Alkaline Phosphatase (ALP)

An enzyme made mostly in your liver, bile ducts and bones. Its blood level gives clues about those tissues.

AI-verified against cited sources. Awaiting expert reviewAlso called ALP, ALK PHOS, ALKPLOINC 6768-6

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What it is

Alkaline phosphatase (ALP) is an enzyme found in many tissues, with the most in your liver, bile ducts and bones. Smaller amounts come from the intestines, kidneys and, during pregnancy, the placenta. The ALP in a blood sample is a mix from all of these sources. Levels are normally higher in growing children and teens, and they rise during pregnancy.

Why it matters

ALP is part of routine liver and bone checks. A high level can be a sign that bile flow from the liver is slowed or blocked, or that bones are turning over faster than usual. Many ordinary things also raise it, such as growth, pregnancy and healing bones, so the number alone can't say where it comes from. Your clinician reads it together with other tests, such as GGT, ALT, AST, bilirubin, calcium and vitamin D.

If it’s low

A level below your lab's range is less common and often isn't a problem. It is sometimes linked to low zinc, poor nutrition or, rarely, an inherited condition that affects bones and teeth. Some medicines can also lower ALP.

Common causes

  • Low zinc, low protein or poor overall nutrition
  • Thyroid disease, in some people
  • Vitamin B12 deficiency (pernicious anemia)
  • Wilson disease, a rare copper-handling condition
  • Hypophosphatasia, a rare inherited condition that affects bones and teeth
  • Some medicines, including estrogen-containing birth control

What can help

  • Ask your clinician whether a low result should be repeated, since a single low value is often not meaningful
  • If it stays low, ask whether checking zinc, thyroid hormones or nutrition makes sense for you

If it’s high

A level above your lab's range is common, and it often has a harmless explanation, such as growth in children and teens, pregnancy, a healing fracture or a recent fatty meal. It can also be linked to bile duct or liver conditions, or to bone conditions with fast bone turnover. The test alone can't tell which, so clinicians look at your other results.

Common causes

  • Normal growth in children and teens
  • Pregnancy, mostly in the third trimester, when the placenta makes ALP
  • A healing bone fracture
  • A recent fatty meal (a small rise in some people)
  • Medicines and supplements that affect the liver or bile flow
  • Slowed or blocked bile flow, such as a gallstone in the bile duct, primary biliary cholangitis or primary sclerosing cholangitis
  • Liver conditions such as hepatitis or cirrhosis, or liver congestion from heart failure
  • Bone conditions such as Paget disease of bone, vitamin D deficiency with soft bones, or cancer that has spread to bone
  • Less often, infiltrating liver conditions such as sarcoidosis, an overactive parathyroid gland, or some cancers

What can help

  • Ask your clinician whether a GGT test (or an ALP isoenzyme test) could show if the ALP comes from the liver or from bone
  • Review your medicines and supplements with your clinician
  • If your vitamin D is low, ask about checking it and your calcium, since low vitamin D can raise ALP

Usual ranges

What labs print as the reference range, by who was tested. Ranges differ a little between labs, so the range on your own report is the one your result is judged against.

The usual range depends on sex and age. Choose women or men to see the one that applies.

Children and teens 7 ranges

See ALP by age and sex, with the U.S. survey numbers where they exist.

If this applies to you

ALP means a little more for some people. Open the one that fits you.

Fatty liver or liver condition

ALP comes mainly from the bile ducts in the liver, and also from bone. BSG includes it in the standard first liver panel with bilirubin, albumin, ALT and GGT.

Blood tests for liver health

How common is your level?

NHANES describes the US civilian, non-institutionalized population, not a 'healthy' reference group. People with disease, on medication, fasting or not (except fasting-subsample markers), smokers and so on are all included. A percentile says where a value sits among Americans of that sex and age, not whether it is normal.

How ALP varies with age in the U.S.Shaded: middle 90% and middle 50% of people. Line: the median. U/L. Point at any age, or focus the chart and use the arrow keys, to read both sexes in words.

Use the left and right arrow keys to read each age. Shift moves five years.

Reference data derived from CDC/NCHS National Health and Nutrition Examination Survey (NHANES), cycles 2015-2016, 2017-March 2020 (pre-pandemic), August 2021-August 2023. Analysis by bloodtesttracker.com; not endorsed by CDC, NCHS or HHS.

Explore ALP by age and sex: every percentile as a table, with a download.

What can affect your result

  • Age: levels are highest during childhood growth and the teen growth spurt, then settle in adulthood
  • Sex: Mayo's adult intervals are somewhat higher for men than for women
  • Older age in women: some labs list higher upper limits for women over about 50
  • Pregnancy: levels rise, mostly in the third trimester
  • Meals: a fatty meal can cause a small rise in some people
  • Medicines: many raise it, and a few (such as some birth control pills) can lower it
  • Tube type: EDTA, citrate and oxalate tubes aren't suitable for this test
  • Lab and method: intervals differ between labs, and results from different methods shouldn't be compared directly
  • A fatty meal can cause a small rise in some people, because intestinal ALP can enter the blood after a fatty meal. ALP is usually drawn with a fasting panel for this reason.
  • Plasma collected in EDTA, citrate or oxalate tubes is not suitable. These bind the zinc and magnesium that ALP needs, and labs reject the sample (Labcorp, ARUP).
  • Hemolyzed samples can be rejected by labs (Labcorp, ARUP).
  • Pregnancy, growth in children and teens, and many medicines change ALP without liver or bone disease.

ALP has a half-life of about a week, so it rises late in a bile duct blockage and falls slowly, over weeks, after the cause is fixed.

When to talk to a clinician

Contact your clinician promptly if a high ALP comes with yellow skin or eyes, dark urine, pale stools, itching all over, fever with belly pain, or new bone pain. Labcorp notes that a rise of tenfold can occur with a blocked bile duct, so a result many times your lab's upper limit deserves a prompt call. Call 911 or your local emergency number for severe belly pain, confusion or vomiting blood.

Questions you could ask

  • How does my ALP fit with my other liver and bone results?
  • Is the range printed on my report the right one for my age and sex?
  • Could my ALP be coming from my liver or my bones, and would a GGT or isoenzyme test help?
  • Could any of my medicines or supplements be affecting it?
  • Should we check my vitamin D and calcium, and when should we recheck the ALP?
  • Is this low result worth repeating?

Questions people ask

Why is my ALP high when my other liver tests are normal?

ALP also comes from bone and, in pregnancy, from the placenta, so a raised ALP with normal liver tests often points away from the liver. Growth, a healing fracture, low vitamin D and a recent fatty meal are common explanations. A GGT test can help show whether the liver is the source, since GGT is not made by bone.

My child's ALP is above the adult range. Is that a problem?

Children and teens normally have higher ALP because their bones are growing. Labs use age- and sex-specific ranges for this reason. For example, Mayo's range for ages 10 to 12 is 129–417 U/L, far above the adult range. Ask your clinician to compare the result with the range for your child's age.

Does pregnancy raise ALP?

Yes. The placenta makes its own ALP, so levels usually rise during pregnancy, mostly in the third trimester. A rise on its own is expected, and your obstetric clinician can tell you how to read your result.

Can ALP be too low?

Low ALP is less common and is often not a concern. It is sometimes linked to low zinc, poor nutrition, thyroid disease or, rarely, hypophosphatasia, an inherited condition that affects bones and teeth. If your result stays low, ask your clinician whether it needs a closer look.

Do I need to fast before an ALP test?

ALP is usually drawn with other fasting tests, because a fatty meal can cause a small rise in some people. Follow the instructions you were given for your whole blood draw. If you ate recently, it is worth telling your clinician.

Show the science and sources

Units

  • 1 U/L = 1 U/L
  • 1 U/L = 0.01667 µkat/L

Open the ALP unit converter

Is a change real?

ALP naturally varies about 5.3% from test to test in the same healthy person. Between two results from the same lab, a change of roughly 17% or more is unlikely to be day-to-day noise or lab variation (95% reference change value). Source: The European Biological Variation Study (EuBIVAS): a summary report.

For clinicians and researchers

ALP (EC 3.1.3.1) is a zinc- and magnesium-dependent membrane-bound hydrolase with an alkaline pH optimum. Serum activity is the sum of tissue-nonspecific ALP from liver, bone and kidney (similar molecules) plus intestinal and placental forms. Intestinal ALP can rise after a fatty meal in some people (ABO and secretor status). In cholestasis, rising bile salt concentrations increase ALP synthesis and release from the bile duct cell surface, and with a circulating half-life of about a week, ALP rises late in obstruction and falls slowly after it resolves. GGT, 5'-nucleotidase or isoenzyme separation by electrophoresis (liver, bone and intestinal fractions) can show the source. Persistently low values can reflect hypophosphatasia (ALPL gene variants), zinc deficiency or malnutrition. Mayo reports a colorimetric method and ARUP an enzymatic assay. Chelating anticoagulants (EDTA, citrate, oxalate) are unacceptable. Reference intervals depend on age, sex, pregnancy, lab and method (for example 40–129 U/L for adult men at Mayo and 44–121 U/L for all adults at Labcorp), and Labcorp advises against mixing methods in serial monitoring.

LOINC 6768-6 (primary), 77141-0, 1783-0. Specimen: serum-or-plasma.

Sources

  1. Alkaline Phosphatase, Total and Isoenzymes, Serum (test ID ALKP), Mayo Clinic Laboratories
  2. Comprehensive Metabolic Panel, Serum (test ID CMAMA), Mayo Clinic Laboratories
  3. Liver Profile, Serum (test ID LIVPR), Mayo Clinic Laboratories
  4. Alkaline Phosphatase (test 001107), Labcorp
  5. Pediatric Testing Reference Ranges, Labcorp (2022)
  6. Alkaline Phosphatase, Serum or Plasma (test 0020005), ARUP Laboratories
  7. ABIM Laboratory Test Reference Ranges (January 2026), American Board of Internal Medicine (2026)
  8. Guidelines on the management of abnormal liver blood tests, Gut (British Society of Gastroenterology) (2018)
  9. Liver enzyme alteration: a guide for clinicians, Canadian Medical Association Journal (2005)
  10. Alkaline Phosphatase, MedlinePlus, National Library of Medicine

AI-verified against cited sources. Awaiting expert review. Last reviewed 2026-10-03. How we build and check these pages.

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