Diamine oxidase (DAO) has garnered increasing interest in the fields of gastroenterology, allergy, and integrative medicine, primarily due to its central role in histamine metabolism. Histamine intolerance, a condition marked by the body's inability to adequately degrade histamine, is often associated with a deficiency or dysfunction of DAO. With rising awareness of this clinical entity, testing for DAO levels has become an important diagnostic and management tool. However, the complexities surrounding DAO testing—from methodology and interpretation to clinical relevance—demand a nuanced and informed approach.

At Creative Enzymes, we offer with this article a thorough and evidence-based exploration of DAO testing. We also supply premium DAO enzymes for a wide range of research, diagnostic, and commercial applications.

Understanding DAO: A Brief Overview

Diamine oxidase is an extracellular enzyme primarily synthesized in the epithelial cells of the small intestine, though it is also present in the kidneys, thymus, and placenta. Its principal physiological function is the degradation of histamine and other diamines such as putrescine and cadaverine. DAO acts by catalyzing the oxidative deamination of histamine, converting it into imidazole acetaldehyde, which is then further metabolized. The typical reaction catalyzed by DAO is:

R-CH₂-NH₂ + H₂O + O₂ → R-CHO + NH₃ + H₂O₂

In healthy individuals, DAO serves as the first line of defense against dietary histamine. When histamine levels exceed the metabolic capacity of DAO—either due to increased intake, decreased DAO activity, or both—histamine may accumulate and trigger a wide array of symptoms, including gastrointestinal disturbances, dermatological reactions, respiratory issues, and neurological symptoms such as headaches or migraines.

The Role of DAO Testing in Clinical Practice

Given the key function of DAO in histamine metabolism, assessing DAO levels can be useful in diagnosing histamine intolerance (HIT) and guiding management strategies. However, DAO testing is not without controversy, and understanding its utility requires consideration of both the pathophysiology and the limitations of current diagnostic tools.

When Is DAO Testing Considered?

DAO testing may be considered in the following clinical scenarios:

• Chronic or recurrent gastrointestinal symptoms (e.g., bloating, diarrhea) with no clear cause
• Unexplained migraines or headaches
• Dermatological issues such as chronic urticaria or pruritus
• Suspected food intolerances with a pattern suggestive of histamine sensitivity
• Coexisting disorders that may impair DAO activity, such as inflammatory bowel disease, celiac disease, or irritable bowel syndrome

DAO testing is most useful when performed in conjunction with a detailed clinical history, dietary assessment, and, if indicated, an elimination diet trial.

DAO Testing Methodologies

Currently, the most commonly available DAO tests are serum DAO activity assays, although other emerging techniques are being explored. Understanding the differences among these methods is critical for appropriate interpretation.

Figure 1. An example of DAO activity and histamine concentration testing in the blood. (www.imd-berlin.de)

Serum DAO Activity Assay

The standard test measures DAO activity in the blood, reported in Histamine Degrading Units per milliliter (HDU/mL). that degrades 1 pmol/ml (0.11 ng/ml) of histamine. The typical reference ranges are:

• > 80 HDU/mL: Normal DAO activity
• 40–80 HDU/mL: Borderline or reduced activity
• < 40 HDU/mL: Indicative of DAO deficiency

These thresholds are not universally standardized, and interpretation should be contextualized based on laboratory-specific reference ranges and the patient's clinical picture.

DAO Concentration Tests (ELISA)

Some laboratories may offer enzyme-linked immunosorbent assay (ELISA)-based tests to measure the concentration of DAO protein rather than its enzymatic activity. While these tests are simpler to perform, they do not provide information about functional capacity and are therefore less clinically informative.

Genetic Testing

Polymorphisms in the AOC1 gene, which encodes the DAO enzyme, have been associated with reduced enzyme activity. Genetic testing can identify such variants, but it is generally considered supplementary due to its limited predictive value in isolation.

Factors Influencing DAO Levels

Physiological Factors

• Age and Sex: DAO activity can vary based on age and hormonal status; for example, pregnancy is associated with significantly increased DAO levels due to placental production.
• Nutritional Status: Deficiencies in vitamin B6, copper, and vitamin C—cofactors for DAO—can reduce enzyme activity.

Pathological Conditions

• Inflammatory bowel diseases: Conditions such as Crohn's disease and ulcerative colitis damage the intestinal mucosa, impairing DAO synthesis.
• Celiac disease: Similar to IBD, untreated celiac disease may reduce DAO levels due to villous atrophy.

Medications

Several drugs can inhibit DAO activity or interfere with histamine metabolism:

• Non-steroidal anti-inflammatory drugs (NSAIDs)
• Antidepressants (e.g., monoamine oxidase inhibitors)
• Antihypertensives (e.g., verapamil)
• Histamine-releasing medications (e.g., muscle relaxants, contrast media)

Patients undergoing DAO testing should ideally avoid these medications for a specified period, under medical supervision.

Interpreting DAO Test Results

DAO test results should be interpreted with caution and always in the context of clinical findings. A low DAO level does not definitively confirm histamine intolerance, nor does a normal level rule it out. Consider the following principles:

• Low DAO activity (<40 HDU/mL) suggests an impaired ability to degrade histamine and, in the presence of compatible symptoms, supports a diagnosis of HIT.
• Borderline levels (40–80 HDU/mL) are equivocal and may warrant further evaluation, including dietary trials or repeat testing.
• Normal DAO activity (>80 HDU/mL) may still be seen in some patients with histamine sensitivity, particularly if their symptoms are mediated by other pathways (e.g., mast cell activation).

Additionally, the test does not assess the degradation of histamine from endogenous sources, such as that released during allergic reactions or mast cell degranulation.

DAO Testing in the Diagnostic Workup of Histamine Intolerance

Histamine intolerance is considered a diagnosis of exclusion. No single test is definitive. The diagnostic process typically involves:

• Detailed medical and dietary history
• Symptom correlation with histamine-rich food intake
• Trial of a low-histamine diet
• DAO testing to support the diagnosis
• Exclusion of other conditions (e.g., food allergies, celiac disease, mastocytosis)

A positive response to a low-histamine diet, especially when supported by low DAO activity, provides strong circumstantial evidence of histamine intolerance.

Clinical Applications of DAO Testing

• Guiding Dietary Recommendations: DAO testing helps identify patients who may benefit from histamine-restricted diets. It can also help in stratifying patients likely to benefit from DAO enzyme supplementation, which is most effective in those with low DAO activity.
• Monitoring Treatment Efficacy: Repeated DAO testing, though not commonly practiced, may be useful in certain scenarios, such as after treatment of an underlying gut disorder or following dietary/nutritional interventions aimed at boosting DAO activity.
• Supporting Research and Personalized Medicine: DAO testing contributes valuable data to research in food intolerance, migraines, and gastrointestinal health. It also supports a more personalized approach to treatment based on enzyme activity levels.

Limitations and Criticisms of DAO Testing

Despite its utility, DAO testing is not without limitations:

• Lack of standardization: Different labs use different assays, leading to variability in results.
• Unclear cutoffs: The threshold values for diagnosing DAO deficiency are not universally agreed upon.
• Questionable correlation with symptoms: Some individuals with low DAO levels remain asymptomatic, while others with normal levels may still experience histamine-related symptoms.
• Limited availability: DAO testing is not routinely offered in many healthcare settings and may not be covered by insurance.
• Transient values: DAO activity can fluctuate due to recent meals, medications, and stress, potentially skewing results.

These limitations underscore the importance of a holistic, clinically guided approach rather than over-reliance on laboratory values.

Emerging Directions and Future Perspectives

As research into histamine intolerance and DAO continues to evolve, several developments may enhance the accuracy and relevance of DAO testing:

• Standardized assay protocols with uniform reference ranges across laboratories
• Point-of-care DAO activity testing for more immediate clinical decision-making
• Integration with gut microbiome profiling, recognizing the role of bacterial histamine production
• Advanced genetic testing to better understand individual variations in DAO expression and activity

Such advancements will likely improve the reliability and clinical applicability of DAO testing in the coming years.

In summary, testing for diamine oxidase levels represents a valuable, though not definitive, tool in the diagnostic arsenal for histamine intolerance. While serum DAO activity assays can offer insight into a patient's ability to metabolize dietary histamine, results must always be interpreted in context—considering clinical presentation, dietary habits, and potential confounding factors.

For patients suffering from unexplained gastrointestinal, neurological, or dermatological symptoms that worsen with certain foods, DAO testing can help guide dietary strategies and therapeutic decisions, including enzyme supplementation. As with any emerging area in medicine, ongoing research and clinical validation are essential to refining the role of DAO testing in both diagnosis and management.

Creative Enzymes offers high-quality diamine oxidase (DAO) enzymes, including food-grade native pea sprout DAO powder and native porcine-derived DAO powder, ideal for various nutritional and research applications. Contact us for any questions or requests!