Standards
Anatomical Therapeutic Chemical (ATC) classification
A hierarchical classification for substances used in drug-utilization research.
- Kind
- Vocabularies
- Steward
- WHO Collaborating Centre for Drug Statistics Methodology
- Official source
- https://www.who.int/tools/atc-ddd-toolkit/methodology
What it describes
The ATC system groups active substances according to the organ or system on which they act and their therapeutic, pharmacological, and chemical properties. The WHO Collaborating Centre for Drug Statistics Methodology maintains the classification. ATC supports comparable drug-utilization statistics when names, brands, and package sizes differ. Its hierarchy is a classification of use, not an identifier for a particular marketed product.
An active substance can receive different ATC codes when used for distinct therapeutic purposes or routes under the classification rules. Codes and defined daily doses may also be revised. Keep the ATC version and the context of use when comparing datasets. An ATC code does not establish a patient's dose, a product's approval status, or a treatment recommendation.
Structure
Five hierarchical levels, from anatomical main group to chemical substance.
Useful for
- Group medicines for utilization analysis.
- Compare datasets across product names.
- Track classification changes by version.
Limits
- Not a product identifier.
- Does not prescribe an individual dose.
- Codes and assignments can change.
How it is used in practice
ATC places each active substance in a tree. The first level is an anatomical main group, the second a therapeutic subgroup, the third a pharmacological subgroup, the fourth a chemical subgroup, and the fifth the individual chemical substance. A seven-character code therefore carries its own ancestry, and an analyst can aggregate consumption of, say, all substances in a pharmacological subgroup by truncating the code at the right level. The WHO Collaborating Centre publishes the classification together with defined daily doses, which are assumed average maintenance doses for the main indication in adults and serve as a unit for comparing use across products.
Health agencies, researchers and regulators use ATC to compare drug use across countries and years, and many national drug registers record it. Because the classification tracks the substance and not the product, it is useful when brand names, strengths and package sizes vary, and unhelpful when a question depends on a particular formulation.
A worked example
A fictional analyst compares prescription volumes for two fictional compounds, both marketed for related conditions. Their ATC codes share the first four characters, so they sit in one chemical subgroup. The analyst reports volume in defined daily doses, not in packages, because one product comes in packs of 28 and the other in packs of 90.
A reviewer asks which version of the classification and which DDD values were used. The analyst's record states the version date, the source of the DDD values and the rule used when a substance had more than one ATC code. A substance with two codes appears under each in the aggregate, so the totals across groups exceed the number of substances; the record says so explicitly. When a later version changes a DDD, the earlier analysis is not rewritten. It remains a statement about the version on which it was based.
Evaluating a claim that rests on an ATC code
A claim that two products are in the same class rests on a code at a stated level. Matching at the fourth level and matching at the fifth are different claims, and a pharmacological subgroup can contain substances with quite different clinical profiles. If the claim is about therapeutic equivalence, safety or interchangeability, an ATC code is not the right evidence.
Check the version and the assignment. Codes are assigned following the classification's guidelines and revised over time; substances move, and a substance used for different purposes can be assigned more than one code. A dataset that stores ATC codes copied from a vendor without a version cannot be compared reliably with another that used a different edition. Keep the edition, and when comparing across editions, use the published change lists in preference to guesses.
Confusions to avoid
ATC is not a product identifier and not a regulatory approval. A medicine's NDC, marketing authorization number or label identifies the product, and the ATC code is attached to the active substance or combination. Joining a product to an ATC code requires the ingredient and sometimes the route or indication.
It is also not the same kind of thing as a UNII. The UNII identifies a substance in a registry. The ATC code places a substance in a classification of use. Two substances with different UNIIs can share an ATC code, and one UNII can be linked to more than one ATC code. A record that needs both should keep them as separate attributes with separate sources.
Questions a reviewer can ask
Which edition of the classification and which defined daily doses were used? At which level was the comparison made? Does the substance have more than one code, and how were such cases counted? Is the question about use at population level, for which ATC suits, or about a specific product, for which it does not? Which source maps the product to its substance? Are the totals reproducible from a retained copy of the lists used?
Related terms
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Enquiry
Which of your sources carry Anatomical Therapeutic Chemical (ATC) classification?
Tell us which sources carry it, which key the same entity some other way and who may read which. We reply with whether and how a first record would use this vocabulary.
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