Alkaline phosphatase (ALP) blood test: what high and low levels mean

A high ALP tells you that liver, bile-duct or bone cells are producing more of the enzyme than usual. How worried to be depends on three things: how high it is, your age and situation, and what the other liver tests show. A result up to about 1.5 times the upper limit with a normal GGT, normal bilirubin and normal ALT is very often bone-related or physiological and is frequently followed up with a simple repeat. A result more than two to three times the upper limit, especially with a raised GGT or bilirubin, usually means the bile ducts are inflamed or partly blocked and needs an ultrasound.

In growing children, pregnant women and people healing a fracture, a high ALP is expected and does not need to be chased. Outside those groups, a persistently high ALP is a sign to find a cause rather than a diagnosis in itself, and most causes are treatable once identified.

Common causes

1. 1.

   Bile-duct blockage or cholestasis

   Gallstones, narrowing of the ducts, primary biliary cholangitis, primary sclerosing cholangitis or a tumour pressing on the ducts all slow bile flow. The duct-lining cells respond by making much more ALP, often three times normal or higher, usually with a high GGT and sometimes a high bilirubin.

2. 2.

   Fatty liver and other liver disease

   Non-alcoholic fatty liver, alcohol-related liver disease, hepatitis and cirrhosis can all raise ALP, usually more mildly than a blocked duct and alongside raised ALT or AST. Liver infiltration by sarcoidosis, lymphoma or metastases can push ALP high with relatively normal ALT.

3. 3.

   Bone turnover: healing fractures, Paget's disease and bone metastases

   Osteoblasts release ALP whenever bone is being built or remodelled quickly. A healing fracture raises it for weeks; Paget's disease of bone can raise it several-fold; cancers that spread to bone (prostate, breast) raise bone ALP with a normal GGT.

4. 4.

   Vitamin D deficiency and osteomalacia

   Too little vitamin D means poorly mineralised bone, and the body responds with more osteoblast activity. ALP rises while calcium and phosphate drift low and parathyroid hormone rises; this pattern is common and easily missed.

5. 5.

   Pregnancy

   The placenta produces a distinct ALP isoenzyme from the second trimester onward. Levels of two to three times the non-pregnant upper limit in the third trimester are normal and fall within weeks of delivery.

6. 6.

   Normal growth in children and teenagers

   Bone growth produces large amounts of ALP, so children and adolescents routinely run two to three times adult values. This is physiological and needs no work-up unless other liver tests are abnormal.

7. 7.

   Medications

   Anti-seizure drugs (phenytoin, carbamazepine, phenobarbital), some antibiotics, estrogens and oral contraceptives, methotrexate, allopurinol and certain antidepressants can raise ALP by inducing liver enzymes or causing mild cholestasis. Anabolic steroids and some herbal products can do the same.

8. 8.

   Overactive parathyroid or thyroid

   Both hyperparathyroidism and hyperthyroidism speed up bone turnover, which raises bone ALP. Calcium is often high in hyperparathyroidism, which helps separate it from other bone causes.

9. 9.

   Heart failure and infections

   Congestion of the liver in right-sided heart failure, and infections such as mononucleosis, cytomegalovirus or sepsis, can raise ALP temporarily along with other liver enzymes.

10. 10.

    A recent fatty meal or blood group effect (lab artefact)

    Intestinal ALP rises after eating, particularly in people with blood group O or B. The effect is modest, typically within the normal range or just above it, but can explain a borderline non-fasting result.

Symptoms that can go with it

• Often none at all, particularly with mild or bone-related elevations
• Yellowing of the skin or whites of the eyes (jaundice)
• Itching, especially at night, which is typical of blocked or inflamed bile ducts
• Pale stools and dark urine
• Pain under the right ribs, nausea or loss of appetite
• Bone pain, bone deformity or an unexplained fracture
• Fatigue and muscle weakness (common with vitamin D deficiency)

A low ALP is unusual and is often overlooked because reports highlight high values. A mildly low result in someone who feels well is frequently normal variation. A persistently low result, particularly with fatigue, bone or dental problems, or signs of poor nutrition, deserves a closer look at zinc, magnesium, protein intake and thyroid function. Very low values (well under 20–30 U/L) in a child with early tooth loss or bone weakness, or an adult with repeated stress fractures, raise the possibility of hypophosphatasia, a rare inherited condition that is now treatable.

Common causes

1. 1.

   Zinc deficiency

   ALP needs zinc as a cofactor, so when zinc is low the enzyme is made but works poorly. This is one of the most common correctable causes of a low ALP and is seen with poor diet, bariatric surgery, inflammatory bowel disease and heavy alcohol use.

2. 2.

   Malnutrition, very low protein intake or anorexia

   The liver needs adequate protein and energy to make enzymes. Prolonged under-eating, restrictive dieting or illness-related weight loss lowers ALP along with albumin and other proteins.

3. 3.

   Hypothyroidism

   Thyroid hormone drives bone turnover. When it is low, osteoblasts slow down and bone ALP falls; the result usually corrects once thyroid treatment is established.

4. 4.

   Magnesium deficiency

   Magnesium is also required for ALP activity. Low levels from diuretics, proton-pump inhibitors, alcohol or poor absorption can lower ALP, often alongside low calcium and potassium.

5. 5.

   Hypophosphatasia

   A rare genetic deficiency of the tissue-nonspecific ALP enzyme. It causes very low ALP with high vitamin B6 (pyridoxal phosphate) in the blood, soft or fragile bones, and early loss of baby teeth in children.

6. 6.

   Vitamin B12 or folate deficiency (pernicious anemia)

   Severe megaloblastic anemia is associated with lower ALP, probably through reduced bone-cell activity; the anemia and large red cells (high MCV) usually give the diagnosis.

7. 7.

   Medications

   Bisphosphonates and denosumab for osteoporosis deliberately suppress bone turnover and lower bone ALP. Oral contraceptives, clofibrate and some statins can also lower it slightly.

8. 8.

   Wilson disease (rare)

   In acute liver failure from Wilson disease, ALP can be strikingly low while bilirubin is very high, a pattern specialists use as a clue. This is rare and comes with obvious illness, not an incidental finding in a well person.

Symptoms that can go with it

• Usually none; a mildly low ALP is often an incidental finding
• Fatigue, poor appetite or unexplained weight loss
• Hair thinning, poor wound healing or frequent infections (zinc deficiency)
• Cold intolerance, constipation and dry skin (hypothyroidism)
• Bone or joint pain, repeated fractures, or early loss of baby teeth in children (hypophosphatasia)