ALP (Alkaline Phosphatase) Blood Test: Normal Ranges, Causes & What Your Results Mean

What Is Alkaline Phosphatase (ALP)?

Alkaline phosphatase (ALP) is an enzyme found throughout your body, with the highest concentrations in the liver, bones, kidneys, and intestines. Enzymes are biological catalysts - they speed up chemical reactions that would otherwise happen too slowly to sustain life. ALP's primary role involves breaking down proteins and facilitating the transfer of phosphate groups, which is essential for energy metabolism, cell signalling, and bone mineralisation.

Although ALP is present in many tissues, it is the liver and bone isoforms that are most clinically relevant. When your doctor measures your ALP level, they are looking at the combined activity of all these isoforms. A raised result prompts the question: is it coming from the liver, the bones, or somewhere else entirely?

ALP is one of the standard markers included in a liver function test (LFT) panel, alongside ALT, AST, GGT, bilirubin, and albumin. Together, these markers help build a picture of how well your liver is working and whether there is any obstruction to bile flow.

Why Is ALP Tested?

ALP testing serves as a screening and diagnostic tool across several clinical areas:

• Liver disease screening: Elevated ALP is one of the hallmarks of cholestatic liver disease - conditions where bile flow is obstructed. This includes gallstones blocking the bile duct, primary biliary cholangitis, and primary sclerosing cholangitis.
• Bone disease investigation: Because osteoblasts (bone-building cells) produce ALP, elevated levels can indicate increased bone turnover, as seen in Paget's disease, osteomalacia, bone metastases, and healing fractures.
• Monitoring treatment: Serial ALP measurements help track the response to treatment in conditions like Paget's disease and certain cancers.
• Paediatric growth assessment: ALP is naturally higher in children and adolescents due to active bone growth. Unusually low levels in children can indicate a rare condition called hypophosphatasia.
• Pregnancy monitoring: The placenta produces its own isoform of ALP, leading to a physiological rise in the third trimester.
• Investigating unexplained symptoms: Fatigue, itching (pruritus), jaundice, bone pain, or unexplained fractures may all prompt an ALP test.

Normal ALP Ranges

ALP is measured in units per litre (U/L) or international units per litre (IU/L). Reference ranges vary by age, sex, and laboratory method, but the following are representative of most UK laboratories:

Group	Normal Range (U/L)
Adult men	40–129 U/L
Adult women	35–104 U/L
Children (1–9 years)	150–350 U/L
Adolescents (10–17 years)	100–500 U/L (peaks during growth spurts)
Pregnancy (third trimester)	Up to 2–3× upper normal limit
Elderly (over 70)	Slightly higher than adult range

It is important to recognise that children and teenagers normally have ALP levels that would be considered very high in adults. This is entirely due to active bone growth and is not a cause for concern. Similarly, a modest rise in the third trimester of pregnancy reflects placental ALP production and is physiological.

Post-menopausal women may also show slightly elevated ALP compared to pre-menopausal women, reflecting increased bone turnover associated with declining oestrogen levels.

What Do High ALP Levels Mean?

Elevated ALP is one of the most common abnormalities found on routine blood tests. The clinical significance depends on the degree of elevation and the source of the enzyme.

Liver-related causes

• Bile duct obstruction: Gallstones, tumours (pancreatic or bile duct cancer), or strictures can block bile flow, causing ALP to rise dramatically - often to 3–10 times the upper normal limit.
• Primary biliary cholangitis (PBC): An autoimmune condition that progressively destroys the small bile ducts within the liver. ALP is typically the first liver enzyme to rise.
• Primary sclerosing cholangitis (PSC): Chronic inflammation and scarring of the bile ducts, often associated with inflammatory bowel disease.
• Drug-induced cholestasis: Certain medications, including some antibiotics, oral contraceptives, and anti-epileptics, can impair bile flow and raise ALP.
• Infiltrative liver disease: Sarcoidosis, amyloidosis, and liver metastases can elevate ALP.
• Hepatitis: Viral or autoimmune hepatitis can raise ALP, though ALT and AST typically rise more prominently.

Bone-related causes

• Paget's disease of bone: A condition causing disorganised bone remodelling. ALP can reach extremely high levels (10–25 times normal).
• Osteomalacia and rickets: Caused by vitamin D deficiency, leading to defective bone mineralisation.
• Bone metastases: Cancers that spread to bone (breast, prostate, lung) stimulate osteoblast activity and raise ALP.
• Healing fractures: A transient rise in ALP during fracture healing is normal and expected.
• Hyperparathyroidism: Excess parathyroid hormone increases bone turnover.

Other causes

• Pregnancy (third trimester - placental isoform)
• Heart failure (hepatic congestion)
• Chronic kidney disease
• Lymphoma and other malignancies
• Hyperthyroidism

Differentiating the source

When ALP is elevated, your doctor may request a GGT test. If GGT is also elevated, the source is most likely the liver. If GGT is normal but ALP is high, the source is most likely bone. ALP isoform electrophoresis can provide definitive confirmation if needed.

What Do Low ALP Levels Mean?

Low ALP is less common than elevated levels but can be clinically significant:

• Hypophosphatasia: A rare inherited condition where deficient ALP activity impairs bone mineralisation. It ranges from severe (lethal in infancy) to mild (dental problems and stress fractures in adults).
• Zinc and magnesium deficiency: ALP is a zinc-dependent enzyme, so deficiency in these minerals can reduce its activity.
• Malnutrition and protein deficiency: Severe malnutrition reduces the production of many proteins and enzymes, including ALP.
• Hypothyroidism: An underactive thyroid can modestly suppress ALP levels.
• Pernicious anaemia: Vitamin B12 deficiency-related anaemia has been associated with low ALP.
• Wilson's disease: This rare copper metabolism disorder can present with low ALP, especially during acute liver failure - a paradoxical finding that can be diagnostically useful.
• Post-cardiac surgery: A temporary drop in ALP after cardiac bypass has been documented.

If your ALP is persistently below the reference range, your doctor should check your zinc, magnesium, vitamin B12, and thyroid function as a minimum.

How to Improve Your ALP Levels

Management depends on whether your ALP is too high or too low, and on the underlying cause.

If ALP is elevated (liver-related)

• Reduce alcohol consumption: Alcohol can worsen cholestatic liver conditions and impair overall liver function.
• Review medications: Speak to your GP about whether any of your medications might be contributing to elevated ALP.
• Maintain a healthy weight: Non-alcoholic fatty liver disease (NAFLD) is increasingly common and can affect bile flow.
• Eat a balanced diet: Focus on vegetables, fruits, whole grains, and lean protein. Reduce saturated fat and refined sugar.

If ALP is elevated (bone-related)

• Optimise vitamin D: Ensure adequate vitamin D intake (10 µg/day as recommended by the NHS, or higher if deficient). Vitamin D deficiency is extremely common in the UK, particularly during winter months.
• Ensure adequate calcium intake: Aim for 700 mg/day from dietary sources (dairy, fortified plant milks, green leafy vegetables).
• Weight-bearing exercise: Regular walking, running, dancing, or resistance training supports healthy bone remodelling.
• Treat underlying conditions: Paget's disease, hyperparathyroidism, and bone metastases all require specific medical treatment.

If ALP is low

• Check zinc and magnesium: Supplementing these minerals (if deficient) can help restore normal ALP activity. Good dietary sources include shellfish, meat, pumpkin seeds, and dark chocolate for zinc; nuts, seeds, and green leafy vegetables for magnesium.
• Address thyroid dysfunction: If hypothyroidism is present, thyroid hormone replacement should normalise ALP over time.
• Investigate hypophosphatasia: If levels are persistently very low, genetic testing may be warranted.

When Should You Get Tested?

Consider having your ALP levels checked if you experience:

• Unexplained fatigue or weakness
• Jaundice (yellowing of the skin or eyes)
• Itchy skin (pruritus) without an obvious rash
• Dark urine or pale stools
• Bone pain or unexplained fractures
• Abdominal pain, particularly in the upper right area
• Nausea, loss of appetite, or unexplained weight loss
• Known liver disease, bone disease, or cancer

For routine health screening, ALP is included in standard liver function panels and should be checked annually as part of a comprehensive blood test, especially if you are over 40, consume alcohol regularly, or have a family history of liver or bone disease.

Which Lola Health Tests Include ALP?

ALP is a core component of liver function testing and is included in several Lola Health panels:

• Peak Insights (70 Biomarkers) - Our most comprehensive panel with full liver function, bone markers, and more.
• Vital Check (56 Biomarkers) - Broad health screen including liver and bone health markers.
• Core Health (45 Biomarkers) - Essential health markers including ALP and the full liver panel.
• Liver Health Blood Test — Focused liver function assessment.

All Lola Health tests use a venous blood draw at one of our nationwide clinics, ensuring laboratory-grade accuracy. Your results are reviewed by a doctor and returned within 4 working days.