What Does Low Alkaline Phosphatase Mean on a Blood Test?

Low alkaline phosphatase means the level of the enzyme ALP in your blood is below the reference range used by the laboratory. It is much less common than a high result. Possible causes include zinc or magnesium deficiency, malnutrition, an underactive thyroid and, rarely, an inherited bone condition called hypophosphatasia.

This guide is for adults and parents reviewing a low ALP result that has already been reported. It explains what the result can and cannot tell you and helps you prepare questions. It cannot identify the cause of your result. If your result was high rather than low, see our separate guide to high alkaline phosphatase.

A low ALP result on its own is not usually an emergency. Some situations are worth raising with a clinician sooner rather than waiting for a routine review:

• A child who loses baby teeth early. Cleveland Clinic describes one or more baby teeth falling out before age 5 as a possible sign of childhood hypophosphatasia.
• Unexplained fractures, repeated stress fractures in the feet, or thigh bone pain, especially with early loss of adult teeth.
• Ongoing bone or joint pain, muscle weakness, or a change in how a child walks or grows.
• Signs of significant undernutrition, such as unplanned weight loss or difficulty eating enough.
• Any sudden, severe symptoms such as difficulty breathing, collapse or confusion need emergency care through local services, whatever the blood test showed.

ALP is an enzyme found in many tissues. Most of the ALP in blood comes from the liver and bones. It is often measured as part of a routine liver panel, so a low result is frequently found by chance.

There is no single normal range. Cleveland Clinic notes that ranges vary between laboratories, with one common range of 44 to 147 IU/L and some organisations using 30 to 120 IU/L. MedlinePlus gives 40 to 129 IU/L as one example. Levels also vary with age and sex. Children and teenagers who are still growing usually have different ranges from adults, so always use the range printed on your own report.

MedlinePlus and Cleveland Clinic list several conditions that can be associated with a low ALP level. A low result does not mean you have any of them; it is one clue the clinician weighs alongside your history and other results.

• Zinc deficiency.
• Magnesium deficiency.
• Malnutrition or protein deficiency, including from restricted eating or conditions that affect absorption.
• Hypothyroidism (an underactive thyroid gland).
• Pernicious anaemia, a cause of vitamin B12 deficiency.
• Wilson disease, a rare inherited condition that affects how the body handles copper.
• Hypophosphatasia, a rare inherited condition that affects bones and teeth.

A single result just below the reference range is different from a level that is clearly and repeatedly low. Ranges are set so that a small share of healthy people fall outside them, and laboratories use different methods and ranges, so a borderline result may simply need to be seen in context.

What usually matters more is the pattern: whether earlier results were also low, how far below the range the level is, the person's age, and whether there are symptoms or other abnormal results. A clinician may suggest repeating the test or checking related results, such as thyroid function or nutrition-related tests, before drawing any conclusion. A low result should not be ignored when it comes with bone, teeth or growth problems.

Medicines can affect blood test results. Cleveland Clinic gives birth control pills as an example of a medicine that can affect ALP results, and MedlinePlus notes that many medicines can interfere with blood tests in general.

Bring a complete list of prescribed medicines, over-the-counter products and supplements to your appointment, including recent changes. Do not stop or change any medicine to alter a result. MedlinePlus advises talking to your provider before changing anything; your clinician or pharmacist can say whether a medicine is relevant.

Hypophosphatasia (HPP) is a rare inherited condition caused by changes in the ALPL gene. These changes reduce the activity of a form of alkaline phosphatase that is needed to deposit minerals such as calcium and phosphorus into bones and teeth. According to MedlinePlus Genetics, severity ranges from serious forms that appear before birth to milder forms in adulthood, and the mildest form affects only the teeth.

In children, early loss of baby teeth is often one of the first signs. Adults may have softened bones (osteomalacia), repeated foot or thigh fractures, chronic bone pain and early loss of adult teeth.

Cleveland Clinic explains that people with HPP typically have low ALP for their age, but that other conditions can also lower ALP, so this test cannot confirm the diagnosis. Specialists may also look at a form of vitamin B6 called PLP, which is usually high in HPP, X-rays and, where available, genetic testing. Most people with a low ALP result do not have hypophosphatasia.

Take the original report with the result, unit, reference range and date. Earlier results help show whether the level has always been low or has changed.

• Is my result only slightly below the range or well below it, and is the range right for my age and sex?
• Have my earlier ALP results also been low, and does this need a repeat test?
• Could my diet, weight loss, a digestive condition or a thyroid problem explain this?
• Should zinc, magnesium, thyroid function or other tests be checked, and why?
• Could any of my medicines or supplements be affecting the result?
• Given my teeth, bone and fracture history, is a rare bone condition worth considering, and who would assess that?
• Who will contact me with the plan, and by when?

On SameCase, patients and verified doctors share de-identified cases. Reading how other people with a low ALP result described their history, which follow-up tests were discussed and what questions they asked can help you organise your own notes before an appointment.

Other people's cases cannot tell you what your result means or replace an assessment by a clinician who knows your history. We suggest using them only to sharpen your questions.