Low DAO Levels and What a DAO Blood Test Can Tell You

Low DAO levels on a blood test can feel like an answer after a long search for the cause of your symptoms. Diamine oxidase (DAO) is the main enzyme that breaks down histamine from food, and reduced DAO activity is the leading explanation for histamine intolerance. But a DAO blood test is not a definitive diagnostic test. Experts disagree about how reliable it is, results do not always match symptoms, and labs use different methods and cut-offs. This guide explains what a low DAO result can and cannot tell you, the possible causes of low DAO activity, and sensible next steps to take with your doctor.

What DAO does

DAO is made mainly by the cells lining the small intestine. It is released into the gut, where it breaks down histamine in food before it is absorbed. A widely cited review in The American Journal of Clinical Nutrition describes an imbalance between histamine intake and the body’s ability to break it down, mainly through reduced DAO activity, as the basis of histamine intolerance. When DAO activity is low, more histamine from food may reach the bloodstream, causing symptoms such as headaches, flushing, digestive upset, a blocked nose or hives.

How the DAO blood test works

The test measures DAO activity in a blood sample. Commercial kits use different laboratory methods, and labs report results in different units, often as units per millilitre. Many labs give ranges that suggest histamine intolerance is unlikely, possible or likely. Because methods and cut-offs vary, a result from one lab cannot always be compared directly with another. Always look at the reference range printed on your own report.

In research, a value below 10 U/mL has been used to define low DAO activity. A 2019 study in Intestinal Research used this cut-off to identify patients with recurring digestive complaints and reduced DAO, most of whom reported bloating, diarrhoea or abdominal pain.

How reliable is the test?

This is the most important point to understand. A 2020 review of histamine intolerance in Biomolecules described the evidence for the diagnostic value of blood DAO testing as “neither abundant nor conclusive”. Several studies found no significant link between symptoms and blood DAO values, and one reported large variation between repeated measurements. The authors concluded that the test should not be used in routine practice until it is better validated.

There are good reasons why a blood test might not reflect what happens in the gut:

• DAO does its main work in the gut, and blood levels may not mirror activity in the intestinal lining.
• DAO in the blood varies with pregnancy, recent meals, medicines and other factors.
• A deficiency caused only by gut disease may not show up in a blood test, as the Biomolecules review notes.
• Different kits and labs give different results.

In practice, this means a low result can support a diagnosis of histamine intolerance when it fits with your symptoms and response to diet, but it cannot prove it. A normal result does not rule it out.

What the number on your report means

Most reports show your result next to a reference range, sometimes with bands such as normal, reduced and low. A result in the low band suggests your blood DAO activity is below what the lab considers typical. It does not tell you how much histamine you can tolerate, how severe your symptoms will be, or which foods affect you. Two people with the same number can have very different experiences.

How doctors usually diagnose histamine intolerance

Because no single test is reliable enough, the Biomolecules review notes that there is no agreed, validated diagnostic pathway. In practice, diagnosis usually involves:

1. Ruling out other conditions that cause similar symptoms, including food allergy, coeliac disease, inflammatory bowel disease and mast cell disorders.
2. Reviewing medicines that may affect DAO.
3. Identifying two or more typical symptoms.
4. Seeing whether symptoms improve on a low histamine diet, and return when histamine-rich foods are reintroduced.

A DAO blood test may be used alongside these steps, but the response to diet is usually the most informative part. Our guide to histamine intolerance test options covers the other tests that are sometimes offered, including which ones are not useful.

Other tests that are sometimes used

The Biomolecules review describes several other approaches that have been studied. None is yet a standard, validated test.

• Histamine skin prick test. A small amount of histamine is pricked into the skin and the wheal is watched. Some studies found that the wheal took longer to fade in people with symptoms, and one found a link with lower blood DAO. It is still experimental.
• Intestinal biopsy. Measuring DAO activity in gut tissue taken during an endoscopy may reflect gut DAO more directly, but it is invasive and still needs validation.
• Oral histamine challenge. Swallowing a measured dose of histamine under supervision. Results have not been reproducible within the same person, it carries risks, and there is no standard protocol, so it is not used routinely.
• Genetic tests. Tests for a few common variants in the AOC1 gene can be done from a cheek swab. They only reflect genetic causes, and their diagnostic value is unproven.
• Tests to rule out other conditions. Skin prick or blood tests for food allergy, and a tryptase blood test to look for mast cell disorders, help exclude other causes.

Possible causes of low DAO activity

Genetic differences

DAO is made from instructions in a gene called AOC1. Some common variants in this gene have been linked with lower DAO activity. Genetic tests for a few of these variants are available, but their value for diagnosis is still unproven. Our article on the DAO gene and histamine intolerance explains more.

Gut conditions

Because DAO is made in the lining of the small intestine, conditions that inflame or damage that lining can reduce it. Lower DAO activity has been found in people with Crohn’s disease and ulcerative colitis, and coeliac disease can damage the cells that make DAO. Treating the underlying condition may help DAO activity recover over time. Our guides to histamine intolerance and gut health and SIBO and histamine intolerance discuss these overlaps.

Medicines

Laboratory studies have found that some medicines can strongly inhibit DAO, including the antimalarial chloroquine, clavulanic acid (found in some antibiotic combinations), verapamil and cimetidine. Many other medicines appear on lists of possible DAO inhibitors, mostly based on laboratory data. The Biomolecules review estimates that around 20% of people in Europe regularly take medicines that may inhibit DAO. Our guide to medications that may block DAO covers this. Never stop or change a prescribed medicine because of a DAO result. Talk to your doctor about whether any medicine might be relevant and whether there are alternatives.

Alcohol

Alcohol and its breakdown products are thought to interfere with histamine breakdown, and alcohol can irritate the gut lining.

Nutrients

DAO is a copper-containing enzyme, so severe copper deficiency could in theory reduce its activity. Vitamin B6 has been linked to DAO in older laboratory research, but its role in people is not clear, and vitamin C is not a DAO cofactor. Our article on DAO enzyme foods explains what diet can and cannot do.

Pregnancy

The placenta makes large amounts of DAO, so blood DAO rises sharply during pregnancy. A DAO test during pregnancy is not comparable with results at other times.

Low DAO without symptoms, and symptoms without low DAO

Some people with low DAO results have no symptoms, perhaps because their diet is naturally low in histamine or because blood levels do not reflect gut activity. Others have clear histamine-type symptoms with normal DAO results. In some of these people, the cause may be something else entirely, such as a mast cell disorder, food allergy, irritable bowel syndrome or another condition. Our comparison of histamine intolerance vs MCAS explains one important difference.

What to do if your DAO result is low

1. Discuss the result with your doctor. Look at it together with your symptoms, history and any other test results, rather than acting on the number alone.
2. Rule out other conditions. Ask whether tests for coeliac disease, food allergy, anaemia or inflammatory bowel disease are appropriate, especially if you have digestive symptoms, weight loss or blood in your stool. Coeliac testing needs you to be eating gluten.
3. Review medicines with your doctor. Ask whether any of your medicines might affect DAO and whether alternatives exist. Do not stop anything on your own.
4. Try a structured low histamine diet. Follow it carefully for two to four weeks while keeping a histamine food diary. Our elimination diet plan explains how. If symptoms improve and return with reintroduction, that supports the diagnosis more strongly than the blood test alone.
5. Reintroduce foods to find your threshold. The goal is the widest diet you tolerate, not permanent restriction. See how long to stay on a low histamine diet.
6. Consider DAO supplements with your doctor. Some people use a DAO enzyme supplement with meals. The evidence is limited, and a low blood DAO result does not prove a supplement will help.
7. Treat any underlying gut condition. If a gut condition is found, treating it may improve DAO activity and symptoms over time.

Why the diet trial matters more than the number

Because the blood test is unreliable, the most useful evidence usually comes from your own body. A well-run diet trial has three parts. First, a short elimination phase of two to four weeks on fresh, low histamine food. Second, a clear assessment of whether symptoms improved, using diary scores rather than memory. Third, careful reintroduction of foods one at a time, to see whether symptoms return. If symptoms improve on the diet and come back with high histamine foods, that is strong practical support for histamine intolerance, whatever the DAO number says. If nothing changes, histamine is probably not the main problem, even if your DAO result was low. A dietitian can help make the trial both reliable and nutritionally safe.

Should you repeat the test?

It is a common question after a borderline or surprising result, and people often hope a second test will settle things.

Repeating a DAO test is rarely useful for day-to-day management, because results vary and do not reliably track symptoms. Your response to diet and the course of your symptoms are usually better guides. Your doctor may suggest repeating it in specific situations, for example after treatment of a gut condition, but there is no standard schedule.

Before a DAO test

If you and your doctor decide to do a DAO test, ask the lab or clinic how to prepare. Instructions vary, but some labs ask you not to take DAO enzyme supplements beforehand, and some medicines and recent meals may affect results. Do not stop prescribed medicines for a test unless your doctor tells you to. Tell the person ordering the test if you are pregnant, because pregnancy raises DAO levels a great deal.

Questions to ask your doctor

• How does this result fit with my symptoms and history?
• Which other conditions should we rule out, and how?
• Could any of my medicines be relevant, and are there alternatives?
• Would a supervised low histamine diet trial be a sensible next step?
• Would a referral to a dietitian help me plan the diet safely?
• Is there any reason to repeat the test, and when?

Making sense of the result emotionally

After years of unexplained symptoms, a low result can bring relief, and that is understandable. It can also bring worry about a long-term condition. It may help to remember that histamine intolerance is usually manageable with diet and practical habits, that most people find foods they can enjoy again, and that a single blood test is only one piece of the picture. If health worries are affecting your mood or daily life, mention this to your doctor.

Tests to be cautious about

Several tests marketed for food intolerance are not reliable. IgG food intolerance blood tests do not diagnose food intolerance or allergy and can lead to unnecessary restriction. Hair analysis and many home test kits are also unvalidated. Spending money on these can delay proper assessment. If you are offered a test, ask what it measures, how it has been validated, and how the result would change your care.

Living well with low DAO activity

A low DAO result does not mean a lifetime of very strict eating. Most people find a level of histamine they can tolerate, especially once they know which foods and situations push them over the edge. Practical habits help: eat fresh food, freeze leftovers on the day you cook them, read labels for vinegar, yeast extract and soy sauce, be careful with alcohol, and plan ahead for eating out. Our histamine intolerance food list and guide to hidden histamine in food make this easier.

When to see a doctor

Call emergency services straight away if you or someone else has swelling of the lips, tongue or throat, difficulty breathing or wheezing, a hoarse voice, feeling faint or collapse. These are signs of anaphylaxis, which needs an adrenaline (epinephrine) injection and urgent care. A severe reaction is not explained by low DAO and needs assessment for allergy.

See your doctor to interpret any DAO result, and promptly if you have unexplained weight loss, blood in your stool, persistent diarrhoea, anaemia, night sweats or symptoms that are getting worse. These need investigation for other causes. Talk to your doctor before starting supplements if you are pregnant or breastfeeding or take regular medicines.

Questions about low DAO results

What does a low DAO level mean?

It suggests reduced activity of the enzyme that breaks down histamine from food, which may support a diagnosis of histamine intolerance. The test is not reliable enough to diagnose it on its own, so the result should be read alongside your symptoms and response to diet.

What is a normal DAO level?

There is no single normal range, because labs use different methods and units. Use the reference range on your report. In research, values below 10 U/mL have been used to define low activity.

Can a normal DAO result rule out histamine intolerance?

No. Blood DAO may not reflect activity in the gut, and some people with typical symptoms have normal results. Response to a low histamine diet is often more informative.

What causes low DAO?

Possible causes include genetic differences in the AOC1 gene, gut conditions such as inflammatory bowel disease or coeliac disease, some medicines and alcohol. Often no clear cause is found.

Can low DAO levels be raised?

If a gut condition is reducing DAO, treating it may help. There is no proven food or supplement that raises DAO activity in people who are otherwise well. DAO enzyme supplements add enzyme during meals but do not raise your own levels.

Should I stop my medicines if my DAO is low?

No. Never stop a prescribed medicine because of a DAO result. Ask your doctor whether any medicine might be relevant and whether an alternative is available.

Medical disclaimer: This content is for informational purposes only and does not constitute medical advice. Always consult a qualified healthcare provider before starting any supplement regimen.