Alkaline Phosphatase (ALP) Test
The ALP test measures serum alkaline phosphatase, a group of tissue-specific isoenzymes expressed predominantly in liver (biliary epithelium) and bone (osteoblasts). It is the principal marker of cholestatic liver injury, interpreted with GGT to distinguish hepatobiliary from bone sources, and a key component of the liver panel used to classify hepatocellular versus cholestatic disease patterns.
1
Biomarkers
Blood (serum)
Sample
1–2 business days
Turnaround
Preferred
Fasting
What is the ALP test?
The Alkaline Phosphatase (ALP) Test measures serum activity of a group of isoenzymes expressed predominantly in liver, bone, intestine, and placenta. It is a cornerstone of the liver panel, ordered to detect and characterise cholestatic injury, distinguish hepatocellular from biliary disease, and investigate unexplained elevations that may point to bone pathology or other sources. Practitioners order ALP alongside aminotransferases, GGT, and bilirubin to classify the pattern of liver injury: a disproportionate rise in ALP relative to ALT signals cholestasis — from intra- or extrahepatic obstruction, infiltrative disease, or drug-induced cholestatic hepatitis — while a parallel rise in both suggests mixed or severe hepatocellular injury. An isolated ALP elevation, confirmed by a concurrent rise in GGT, points to the liver, whereas a normal GGT with high ALP suggests bone disease. The test is also used to monitor known cholestatic or metabolic bone conditions and to screen asymptomatic patients with incidental findings on routine panels.
What does the ALP test measure?
Alkaline phosphatase is a membrane-bound enzyme that hydrolyses phosphate esters at alkaline pH. It is expressed in multiple tissues, with the highest concentrations in liver (canalicular membrane of hepatocytes and bile-duct epithelium), bone (osteoblasts), intestine, and placenta. The circulating enzyme reflects a mixture of isoenzymes, predominantly hepatic and bone in most clinical contexts. Because ALP is excreted via bile, any obstruction or injury to the biliary tree causes retention and elevation in serum. In bone, ALP is released by osteoblasts during active mineralisation, so growing children and conditions with increased bone turnover show marked elevations.
ALP is the principal marker of cholestatic liver injury and biliary-tract obstruction, complementing aminotransferases to distinguish hepatocellular from cholestatic disease patterns. An isolated ALP elevation prompts consideration of both hepatobiliary and bone causes, so practitioners typically pair ALP with GGT (which rises only in hepatic disease) and correlate with calcium, phosphate, and bone-specific markers. Persistently high ALP in the absence of liver or bone disease may reflect intestinal or other isoenzyme sources, warranting isoenzyme fractionation.
Why is the ALP test ordered?
- Evaluation of suspected cholestatic liver disease or biliary obstruction in patients with jaundice, pruritus, pale stools, or dark urine
- Classification of liver-panel abnormalities, interpreted with aminotransferases, GGT, and bilirubin to distinguish hepatocellular from cholestatic patterns
- Investigation of isolated ALP elevation, paired with GGT to differentiate hepatobiliary from bone sources
- Screening and monitoring of infiltrative liver diseases including primary biliary cholangitis, primary sclerosing cholangitis, and sarcoidosis
- Assessment of metabolic bone disease — Paget disease, osteomalacia, or rickets — and monitoring bisphosphonate therapy or bone metastases
- Routine component of comprehensive metabolic panels, where incidental elevations trigger targeted follow-up
- Monitoring during pregnancy, when placental ALP contributes to physiological elevation in the third trimester
- Evaluation of suspected malignancy with liver or bone involvement, including metastatic disease
Sample collection and turnaround
Sample type
Blood (serum)
Fasting required
Preferred
Collection method
Venipuncture at patient service centre
Turnaround
1–2 business days
Collection notes
Phlebotomy via LabCorp patient service centre. Fasting is preferred to minimise transient elevations from intestinal ALP, though not strictly required. Separate serum promptly and store refrigerated if processing is delayed.
Specimen requirements
Minimum 0.5 mL serum (gel separator acceptable). ALP is relatively stable refrigerated for several days; avoid prolonged room-temperature storage and repeated freeze-thaw cycles.
What can affect ALP results?
Children and adolescents have ALP values two to three times higher than adults owing to osteoblastic activity during skeletal growth; elderly individuals may also show mild elevation with age-related bone turnover.
The placenta produces a distinct ALP isoenzyme, so third-trimester values can be 1.5–2 times the non-pregnant upper limit; this is physiological and resolves postpartum.
Intestinal ALP can rise transiently after eating, particularly in blood group O or B secretors, so fasting specimens reduce this source of variability.
Phenytoin, carbamazepine, rifampicin, and isoniazid can induce hepatic ALP; anabolic steroids and oestrogens may cause cholestatic injury with ALP elevation.
Hypothyroidism can reduce bone turnover and lower ALP; hyperthyroidism increases bone resorption and may modestly raise it.
ALP is relatively stable at room temperature but prolonged storage without refrigeration or freeze-thaw cycles can alter measured activity; separate serum promptly and store refrigerated if analysis is delayed.
What do ALP results mean?
Elevated
Cholestatic liver disease: intra- or extrahepatic biliary obstruction (gallstones, strictures, malignancy), primary biliary cholangitis, primary sclerosing cholangitis, drug-induced cholestasis, infiltrative disease (sarcoidosis, amyloidosis). Bone disease: Paget disease (often markedly elevated), osteomalacia, rickets, healing fractures, bone metastases. Physiological: childhood and adolescence, third-trimester pregnancy. Other: intestinal isoenzyme (post-prandial, inflammatory bowel disease), malignancy (hepatocellular or cholangiocarcinoma). Pair with GGT to localise — concordant rise confirms hepatobiliary source, isolated ALP with normal GGT suggests bone.
Normal
A normal ALP does not exclude liver disease — hepatocellular injury without cholestasis (such as steatotic liver disease or uncomplicated viral hepatitis) may show normal ALP with elevated aminotransferases. Interpret in the context of the full liver panel and the clinical picture rather than as a stand-alone marker.
Reduced
Low ALP is uncommon and of uncertain clinical significance in most contexts. Causes include hypophosphataemia, hypothyroidism, malnutrition, zinc deficiency, and rare genetic conditions such as hypophosphatasia. It is not routinely investigated unless accompanied by clinical features of bone disease or metabolic disturbance.
Note: ALP is expressed by multiple tissues, so an isolated elevation requires correlation with GGT, bone markers, clinical history, and imaging. Reference limits are age- and sex-dependent; paediatric and adolescent ranges differ markedly from adult values. Physiological elevations in growth and pregnancy are normal. Isoenzyme fractionation or bone-specific markers (such as procollagen type I N-terminal propeptide) can clarify the source when the origin is ambiguous.
Frequently asked questions
Both rise in cholestatic liver disease, but GGT is liver-specific while ALP is also expressed in bone. A high ALP with normal GGT points to a bone source; concordant elevation of both confirms hepatobiliary disease.
How to access the ALP test
The ALP reference page is free and open to all practitioners. Create a free practitioner account to see pricing, save tests to your list, and generate requisition forms. Verification takes 1–2 business days.
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Published: October 7, 2026
Last reviewed: October 7, 2026
- Sharma U, et al. Hypophosphatasia. Lancet Diabetes Endocrinol. (2014)
- Kaplan MM, Gershwin ME. Primary biliary cirrhosis. N Engl J Med. (2005)
- EASL Clinical Practice Guidelines: management of cholestatic liver diseases. J Hepatol. (2009)
- Bone markers: biochemical and clinical perspectives (consensus on reference intervals and clinical interpretation). (2017)
- Kwo PY, et al. ACG clinical guideline: evaluation of abnormal liver chemistries. Am J Gastroenterol. (2017)
This content is for licensed healthcare practitioners only and does not constitute medical advice. Clinical decisions should be based on the full clinical picture and not on any single test result.
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