Low ALP on a Blood Test: Causes

Alkaline phosphatase is an enzyme found in highest amounts in the liver and bones, with smaller amounts in the intestines and the placenta. Lab Tests Online UK explains that in the liver it sits in the cells lining the bile ducts, and in bone it is made by osteoblasts, the cells that build new bone. A test of total ALP measures the activity of all of these forms together.

It is rarely ordered on its own. ALP is almost always requested as part of liver function tests or a bone profile, which is why a low result often appears without anyone having looked for it. Different laboratories print different reference ranges, and ranges differ by age and sex, so the flag on your report is relative to that laboratory.

Lab Tests Online UK states that low ALP is uncommon and can occasionally indicate zinc or magnesium deficiency, malnutrition, or certain rare genetic conditions. It does not give a long list, and the honest position in the sources read is that the commonest explanations are nutritional and the rarest is genetic.

A 2019 abstract from the Society for Endocrinology describes hypophosphatasia as a very rare systemic musculoskeletal disease characterised by low tissue non-specific alkaline phosphatase. A 2016 case report from the same society explains that it results from loss-of-function mutations in the ALPL gene, which impairs skeletal mineralisation, and that adults typically present in middle age with loss of adult teeth or recurrent non-healing metatarsal fractures.

The British Society for Rheumatology adds that patients usually present with musculoskeletal symptoms including chronic pain, pseudogout, early-onset osteoarthritis, calcific peritendonitis, fractures and dental loss. It says that if a patient has an unexplained persistently low ALP, a referral to a specialist centre should be considered.

The word to hold on to is persistently. A single low reading, especially in someone with a poor diet, a recent illness or no symptoms, is a very different situation from a low ALP on repeated tests over years in a person with fractures or dental problems.

The Glasgow study abstract reported that a search of one health board over two years found 16,280 results below 36 IU/L, and that 1,143 adults had two such results at least 30 days apart. Of these, 706 had conditions known to be associated with low ALP and were excluded, as were 129 aged over 80, leaving 312 people with persistently low ALP and no explanation. Four of them (1.3 percent) already had a diagnosis of hypophosphatasia.

The authors concluded that there are a sizeable number of adults with persistently low ALP, that fractures and non-specific joint pains were the most common symptoms and that awareness needs to improve. It does not mean that every person with a low result has the condition. It shows that low ALP deserves a thought rather than being dismissed as an irrelevant flag.

Start with the context. Tell your doctor about fractures, bone or joint pain, dental problems, diet, weight loss and whether the result has been low before. Lab Tests Online UK says the clinician may look at other liver and bone results and may request an ALP isoenzyme test if the source is unclear.

Where the result is a one-off in a person who feels well, a repeat test in the weeks that follow is the usual route. If it stays low and there are musculoskeletal symptoms such as fractures, bone or joint pain or dental loss, the British Society for Rheumatology advises considering referral to a specialist centre. For hypophosphatasia, treatment options are limited and specialist-led, and enzyme replacement is licensed for paediatric-onset disease with limited evidence for adults.