Modeling the Arden Syntax for medical decisions in XML
Introduction
The Arden Syntax is a language designed to represent the context of, support for and logic behind medical decisions and to do so in a syntax that renders these decision executable by computers. It is used to construct executable medical logic modules (MLMs) that represent individual clinical decisions. Arden Syntax version 1.0 was adopted as a standard by the American Society for Testing and Materials as document E 1460 in 1992. Later, version 2.0 was proposed and adopted as a standard for representing MLMs by HL7 in 1999 [1].
Arden Syntax is composed of maintenance, library and knowledge categories with appropriate slots. The maintenance category contains the slots that specify information unrelated to the health knowledge in the MLM. These slots are used for MLM knowledge base maintenance and change control. And they also contain information about the version of the Arden Syntax that is being used. The library category contains the slots pertinent to knowledge base maintenance that are related to the MLM's knowledge. These slots provide predefined explanatory information and links to facilitate searching through a knowledge base of MLMs. The knowledge category contains the slots that actually specify what the MLM does. These slots define the terms used in the MLM (data slot), the context in which the MLM should be evoked (evoke slot), the condition to be tested (logic slot), and the action to take should the condition be true (action slot).
Although Arden Syntax is the HL7 standard, there are problems related to the adoption of Arden Syntax as the standard tool for describing medical logic. Two prominent problems are: (1) the curly braces problem and (2) the compiler problem. The term “curly braces problem” comes from the Arden approach to mapping MLM variables to clinical data sources. The structure and terminology of stored clinical data often vary from institution to institution. The approach taken in Arden is to provide a location, separated from the rest of the MLM by curly braces, where queries, expressed in the query language of the local Electronic Health Record (EHR) can be expressed. Data collected in local hospital environments can then be mapped to MLM identifiers. In the future, XML-based techniques like the Virtual Medical Record (vMR) [1] as well as other techniques based on the HL7 Reference Information Model (RIM) [2] may provide solutions to this problem. In addition to the challenges associated with differing models for the storage and retrieval of stored data, different clinical systems rely on different software development environments. As a result, different Arden Syntax compilers have been necessary to fit MLMs for the specific architectures and information systems of each hospital [3].
Building compilers specific to the Arden Syntax, however, is difficult; several institutions have tried to compile Arden Syntax to C++ [4], [5], to Mumps source code [6], to JAVA [3] or to a pseudo-code compiler-interpreter [7], and then re-compile (or interpret) the source codes to create a functioning execution environment (Fig. 1). The key problem remains: specific compilers are required for hospitals using different computer languages.
XML is commonly recognized as a component of the worldwide web used to format content that is easily transformed to HTML pages. In addition to its characteristics as a markup language, XML is now an industry standard for data communication among different breeds of computer systems. During the past few years, XML has been introduced into the healthcare industry and is now being widely used. Applications of XML in health care have a wide range including academic studies that contain descriptions of the contents of heart sound components [8], representations of models of biochemical reaction networks [9], ADE reports [10], and pharmaceutical inquiries [11]. XML is also entering the medical domain through standards activities that include inter-system messaging (HL7 3.0), the clinical document architecture (CDA), as well as knowledge representations, including clinical guidelines [12], [13], [14], [15], [16], [17], [18], [19], [20], [21] and the Arden Syntax.
XML can be used to convert clinical guidelines to electronic form [8], [15], [16], [19], [21], [22], [23], [24]. This conversion makes it easy to transform the guidelines to human-friendly form and to machine-friendly form, and makes it possible to share guidelines among medical computing groups. Easy transformation of any document to another form is one of the greatest benefits of XML. There has been considerable progress in the representation of clinical guidelines in XML, and several studies in this area are still being conducted by researchers. Other researchers, however, have mainly focused on the representation of expert knowledge [15], [16], [17], [20], [21], [23], [25], [26] in electronic form, or on the extraction of a medical logic module [21], [27] from those electronic forms.
Arden Syntax version 3.0 is currently under development by HL7. This version is expected to be based on XML as is the case with much of the next generation of HL7 standards. In this paper an Arden Syntax in XML, and MLMs written in this XML-based format, are called “ArdenML” and “MLMs in ArdenML” respectively. Sailors has described one goal of ArdenML as “translating MLMs into other program constructs’ knowledge representations without having to build a traditional compiler.” [24] According to his conceptualization, the challenge of creating an ArdenML can be divided into 5 levels, numbered 0–4 (Table 1). Development of a level-4 schema, including operands and operators, is necessary to meet the goal. Recent examples of ArdenML, however, remain at level 3 or have reached only the early stage of level 4. Continued development of xPath and XLST techniques are encouraging the development of ArdenML at the operators and operands level.
The objective of this study is to increase the portability of the Arden Syntax. To address this issue, we will build a XML schema representing this syntax and develop an XSL style sheet that can reverse-parse MLMs in “ArdenML (Arden Markup Language)” [24] to readable HTML, allowing easy comparison to the original text and thus verify MLMs written in this XML model. In this study, XML will be used to present MLM in a form that may substitute for the parse tree that is produced by compiler. A transformation of this XML was used to produce the original text version of MLM for validation purposes. This paper proposes newly developed Arden MLMs with XML format as a first step to address this compiler issue. In this form, the MLMs are, at least partially, precompiled.
Section snippets
Materials and tools
The Arden Syntax for Medical Logic Systems, version 2.1 [28] including a context-free grammar represented by the Backus-Naur Form (BNF) defines the current standard. A context model used in MathML from http://www.w3c.org/math was used in designing the expression language for ArdenML. The goal was to build a schema capable of representing a large group of existing MLMs and to demonstrate the success of this effort by converting these MLMs back to their ASCII form (HTML) using an XSL style sheet.
Results
All the schema, style sheet and examples in both text and ArdenML version are available in the Internet web site (http://61.78.109.24/ArdenML).
Discussion
In this work, we demonstrated one approach to the goal of expressing Arden-based medical logic as XML. Notably, a key challenge was finding a good model for the expression language embedded in Arden Syntax. We were able to find a stable, workable schema by introducing context model from MathML. RuleML [29], a general knowledge representations tool in the web, has also expression language of its own. We, however, took the advantage of MathML over RuleML because of several redundant tags in
Acknowledgement
This work was supported by the Post-doctoral Fellowship Program of Korea Science & Engineering Foundation (KOSEF).
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