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Arden Syntax for Medical Logic Systems

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The Arden Syntax for Medical Logic Systems encodes medical knowledge in knowledge base form as Medical Logic Modules (MLMs). An MLM is a hybrid between a production rule (i.e. an "if-then" rule) and a procedural formalism. Each MLM is invoked as if it were a single-step "if-then" rule, but then it executes serially as a sequence of instructions, including queries, calculations, logic statements and write statements.

Arden was developed for embedding MLMs into proprietary clinical information systems. It was designed to support clinical decision making in particular: an individual MLM should contain sufficient logic to make a single medical decision. Sequencing tasks can be modelled by chaining a sequence of MLMs. MLMs have been used to generate clinical alerts and reminders, interpretations, diagnoses, screening for clinical research studies, quality assurance functions, and administrative support.

With an appropriate computer program (known as an event monitor), MLMs run automatically, generating advice where and when it is needed, e.g. to warn when a patient develops new or worsening kidney failure.

The initial version of the Arden Syntax was based largely on the encoding scheme for generalised decision support used in the HELP (Health Evaluation through Logical Processing) system for providing alerts and reminders, developed at the LDS hospital in Salt Lake City.


  • The target user of the Arden Syntax is the clinician. The Arden Syntax is not a full-feature programming language; for example, it does not include complex structures. MLMs are meant to be written and used by clinicians with little or no programming training.
  • Arden provides explicit links to data, trigger events and messages to the target user. It clearly defines the hooks to clinical databases, and defines how an MLM can be called (evoked) from a trigger event.
  • The Arden Syntax brings particular support for time functions. Almost all medical knowledge involves the time that something happened. Arden ensures that every data element and every event has a data/time stamp that is clinically significant. Many time functions are provided to help users specify the date and time in MLMs. With any other language, these definitions would be more dependent on the person implementing the MLM; the Arden Syntax allows them to be defined explicitly.

    The basic format of Arden Syntax, the MLM, means that it is not the most appropriate format for developing complete electronic guideline applications.

    A problem that occurs with any form of clinical knowledge representation is the need to interact with a clinical database in order to provide alerts and reminders. Database schemata, clinical vocabulary and data access methods vary widely so any encoding of clinical knowledge (such as a MLM) must be adapted to the local institution in order to use the local clinical repository. This hinders knowledge sharing. Arden is the only standard for procedurally representating declarative clinical knowledge (contrast GLIF or PROforma, for example, which are more declarative formats), so this problem is associated with Arden, but it is not unique to it.

    Arden explicitly isolates references to the local data environment in curly braces ["{}"] in a MLM, often referred to as the "curly braces problem". Efforts are underway in HL7 to help solve this problem, but it is not something that the Arden workgroup can do alone; it requires industry-wide standardization.

    Another potential limitation of Arden is that it does not explicitly define notification mechanisms for alerts and reminders. Instead, this is left to local implementation and is, like database queries, contained in curly braces in a MLM. Explicit notification mechanisms in the Syntax itself may be a part of a future edition.

    The Arden Syntax makes knowledge portable, but MLMs developed for one environment are are not easily embeddable within another. Most commercial applications incorporating MLMs are developed by individual vendors primarily for use within their own environments. Vendors who have developed Arden-compliant decision support applications include:
    • Eclipsys Corporation
    • McKesson Information Solutions
    • Siemens Medical Solutions Health Services Corporation
    • MICROMEDEX (MICROMEDEX® Medical Logic Modules).
    Healthcare organisations with Arden-compliant commercial systems include:
    • Alamance Regional Medical Center, Burlington, NC (Eclipsys)
    • Sarasota Memorial Hospital, Sarasota FL (Eclipsys)
    • Columbia-Presbyterian Medical Center, New York, NY (developed with IBM)
    • JFK Medical Center, Edison, NJ (McKesson)
    • Covenant Health, Knoxville, TN (McKesson)
    • St. Mary's Hospital, Waterbury, CT (McKesson)
    • Mississippi Baptist Health Systems, Jackson, MS (McKesson)
    • St. Vincent's Hospital, Birmingham, AL (McKesson)
    • St. Mary's Medical Center, Knoxville, TN (McKesson)
    • Meridian Health Systems / Jersey Shore Medical Center, Neptune NJ (Siemens)
    • Ohio State University, Columbus OH (Siemens)

    For further information about the Arden Syntax, please visit their website.


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