OPS, the German coding system for therapeutic and diagnostic procedures, is a large and complex classification system. Its main purpose is to provide codes for billing. Like other systems of this type (e.g. ICD-10) it follows the principle of class disjointness and exhaustiveness. SNOMED CT, on the other hand, aims at providing standardised terms, together with logic-based descriptions, and pursues the goal to make the electronic health record (EHR) computable and interoperable across languages and jurisdictions. We investigated the feasibility of aligning OPS with SNOMED CT, based on the 1000 most frequently used OPS codes. A team of three terminologists performed the mapping (partially overlapping), using the first hundred codes to determine guidelines. From the work, which is currently being extended, we can draw the following conclusions: (i) for less than half of the OPS codes, a semantically equivalent SNOMED CT code can be found; (ii) many maps require SNOMED CT post-coordination but remain approximate; (iii) the mapping work is impaired by imprecise descriptions in either terminology system.
Most artefacts that provide a semantic reference – generically referred to as terminology
systems [
There is one remarkable exception to this trend, viz. terminologies for medical procedures. Procedure terminologies encompass operations, drug and other therapies, as well as imaging and other diagnostic procedures. Here, a multitude of national terminologies coexists. Most of them are better described as catalogues than as terminologies or ontologies, i.e., flat lists of medical interventions, often completed by weight values from which the price of each procedure can be computed. Rather than terminologies or ontologies (which systematise terms and/or their referents), they resemble classification systems (like ICD) with mutually disjoint classes, supported by rules that clarify under which conditions a given procedure belongs to a given class.
Billing is therefore a major driver for procedure coding systems, and the
heterogeneity of national health systems (enhanced by different systems co-existing
within jurisdictions) explains the special status of these artefacts. While there are good
examples that semantic harmonisation of data across national borders is possible when
the data is captured by standards that enjoy global adoption, semantic harmonization of
medical procedures remains a problem [
Medical procedures also constitute an important portion of SNOMED CT (58,213
concepts) [
Therefore, SNOMED CT is often seen as a strong candidate to represent medical
procedures, but capturing procedures with SNOMED CT has never been current practice.
An important step towards having procedures coded in SNOMED CT, could consist in
the construction of mappings to SNOMED CT from the local procedure coding systems,
as there does not seem to be any other suitable standard. ICD-10-PCS, developed for use
in the U.S. and adopted in few additional countries, is typically considered to be very
difficult to map, ICHI (International Classification of Health Interventions [
There are ongoing efforts in some countries to harmonize procedure standards. In the
US, the National Library of Medicine is working on a way to map ICD-10-PCS to
SNOMED. Instead of a map [
Compared to other biomedical terminology systems, SNOMED CT is unique not
only due to its scope and size, but also regarding its ontological foundation, based on the
description logics [
has-part some ((method some Excision - action) and
(‘procedure site direct’ some ‘Tonsillar structure (palatine)’))
This is the case of a so-called pre-coordinated concept (identified by the code 173422009 and the label ‘Tonsillectomy (procedure)’). The syntax also allows for constructing more detailed expressions for which no code is available, e.g. tonsillectomy with an ultrasonic scalpel.
((method some Excision - action) and (‘procedure site direct’ some ‘Tonsillar structure (palatine)’) and (‘using device’ some ‘Ultrasonic scalpel’))
This mechanism, called post-coordination, allows for an increased level of coverage and detail, however, at the price of increased complexity, which means that documentation systems have to deal with logical expressions instead of just codes.
A typical classification system also provides a code for tonsillectomy, but often
comes with additional instructions, such as using different codes for tonsillectomy with
or without adenoidectomy, or demanding additional codes for, e.g. haemostasis after
tonsillectomy, or requiring different codes for different age groups. Table 1 provides a
juxtaposition of a procedure ontology with a procedure classification (see also [
Several scenarios of use justify an alignment between classification-type coding systems and SNOMED CT: 1. Primary effort put into manual administrative encoding using a procedure classification or catalogue. Then, the alignment resource can be used to infer SNOMED codes for adding semantic annotations to the EHR, supporting a broad range of primary or secondary use scenarios (decision support, prediction, cohort building, health statistics), capitalizing on the ontological structure of SNOMED CT. 2. Primary effort put into manually annotating EHR content with SNOMED CT codes. However, inferring the full meaning of a procedure classification code would then require representing also the disjointness conditions and exclusions. For several reasons this exceeds the power of SNOMED CT post-coordination. As an alternative, each procedure code could be expressed as a query on a SNOMED-CT annotated record. 3. The same as 2, but using natural language processing (NLP) for annotating clinical narratives with SNOMED CT codes. This scenario, as well as the previous one, would realistically require additional human encoding efforts, given the high quality required for codes that are used for billing.
Our work described in this paper pursues the first of these three goals, i.e. the
direction from the classification to the ontology. Out of a ranked list of approximately
24,000 codes from the German procedure classification system OPS [
OPS, used in Germany for encoding therapeutic and diagnostic procedures, is a finegrained classification system with 35,641 codes distributed across seven hierarchical levels. Its purpose is to provide codes for billing. Like other systems of this type (e.g. ICD-10) it follows the principle of class disjointness and exhaustiveness. The “Systematic Version” PDF file (2019 release) was used as a reference. In this version, formatted like a book, certain naming principles had to be considered. E.g., the code 5790.26: “Geschlossene Reposition einer Fraktur oder Epiphysenlösung des distalen Radius mit Osteosynthese unter Verwendung eines intramedullären Drahts” [Closed reduction of a fracture or slipped epiphysis of the distal radius by internal fixation using an intramedullary wire]. This label is not pre-synthesised; it has to be constructed by 5-790 **5-790.2 6 ↔
The sixth digit is taken from a list with anatomical sites that can be combined with several 5-character codes. However, completely pre-synthesised texts are also available. Nevertheless is the inspection of the Systematic Version indispensable in order to get access to exclusions, inclusions, and scope notes at all hierarchical levels. E.g., the subchapter 5-79 Reduction of Fracture and Dislocation is preceded by nearly one page of such additional information (e.g. excludes therapy of pseudarthrosis, requires separate encoding of nerve sutures). In addition, under the heading the subsubchapter 5-790, more additional information is given, e.g. that child fractures are included, closed reductions dislocations of joints are excluded, or that arthroscopic assistance requires an additional code. The level of detail is often only fully understandable by specialist surgeons. Expertise and a certain degree of subjective interpretation are also needed in order to understand the meaning of certain terms, which lack precise definition, such as “Epiphyseolyse”.
The SNOMED CT Procedures hierarchy provides formal concept definitions, which can be interpreted as description logics axioms (cf. Background). In contrast, scope notes or text definitions are completely missing, which is particularly challenging where formal definitions refer to undefined primitives, e.g. from the SNOMED CT Qualifier Value hierarchy, e.g. Preperitoneal approach without connection to any anatomy reference. Another peculiarity is what SNOMED CT calls role grouping. This can most straightforwardly be interpreted as asserting a mereological order between procedures and their processual parts. However, these role groups also occur solely in, e.g. in the concept Sigmoidoscopy, which is therefore classified as a taxonomic parent of, e.g.
then be “Procedure with sigmoidoscopy”. The background of this is to optimise term retrieval: searching for “sigmoidoscopy” would then retrieve also data annotated with the concept Sigmoidoscopy with biopsy. Negation cannot be expressed by the SNOMED CT syntax. However, we find “reified” negations in several concepts, such as Computed tomography of head without contrast.
These examples demonstrate, in addition to the basic distinctions exposed in Tab. 1, the wide discrepancy of SNOMED CT and OPS in particular (which can also be extended to classification-like coding systems in general). Whatsoever alignment between OPS and SNOMED CT has to be aware of this. Simple lexical mapping is not sufficient. It would lead to numerous wrong equivalence statements: an OPS code, with its meaning restricted by numerous exclusion rules and with the underlying close-value assumption is rarely fully semantically equivalent to any SNOMED CT concept.
The dataset was provided by TriNetX. They harvested it from two German hospitals,
where OPS is established as the official coding system (together with ICD-10) for the
German DRG (Diagnosis-related groups) payment system [
Two coders were recruited (2nd and 3rd authors), both of them final year medical students, one of which already held a degree in nursing. They were trained supervised by the first author, MD and experienced terminologist / ontologist. Each coder was hired for 26 hours a month over three months. The supervisor had the same time budget. This period covered the whole cycle from guideline creation, training, mapping, validation to the delivery of the map and the final report (this paper).
The 1000-code set OPS1000 was ordered by random. The first author, together with the coders analysed the first 100 codes regarding their alignment with SNOMED CT, i.e. the mapping of one OPS code to one or more SNOMED CT codes. The main purpose of this initial step was to reach a consensus regarding a meaningful, simple and reusable mapping scheme. First, the underlying assumption of the mapping process was formulated: According to assumption 3 in the Background chapter, we defined the mapping task as follows: Given a patient record annotated with the code OPSi , which SNOMED CT expression(s) SCT1...j can be reliably assumed to be instantiated. The range of the mapping should contain one or more SNOMED CT codes or SNOMED CT postcoordinated expressions belonging to one of the semantic types "Procedure", "Regime/Therapy" or "Situation". In addition, a scoring system is used to distinguish either the quality of the mapping or “no mapping”.
For each map, the OPS code is analysed in its hierarchical context, taken into
consideration scope notes, inclusion and exclusion statements. Elements of the OPS label
are translated into English, eliminating doubts regarding the appropriate translation in
online sources whenever necessary. In case of doubt, a search with more general terms
is done. Words or word stems are entered into the SNOMED browser [
As a rule of thumb, maps are preferred that were as close as possible to the original wording. In case no map is achieved, a compositional approach is pursued to approximate the meaning. Full post-coordination using the SNOMED CT compositional grammar is not aimed at, due to its complexity, its experimental status (especially regarding its closeness to description logics) and its irrelevance for current implementations. Postcoordination is therefore restricted to logical conjunction (AND), disjunction (OR) and addition (ADD). The latter is preferred in case the OPS code stands for clearly distinct entities, of which a conjunction (even given the large tolerance how SNOMED CT handles logical conjunctions) is considered inappropriate. E.g., if there is no SNOMED CT concept for an OPS code ‘Procedure P on body site B’, then the post-coordination P AND ‘Procedure on B’ would be an appropriate representation, because it is still one procedure. In contrast, if an OPS code stands for ‘Procedure P followed by Procedure Q’, the preferred SNOMED map would then be P ADD Q, which means that it is represented by actually two separate procedures.
OPS codes are often defined by numerical values or value ranges such as number of therapy units, dosage, frequency, or implicitly by age groups (e.g. adults, children). SNOMED CT procedure concepts never include such criteria, i.e. an exact mapping cannot be expected in these cases. Table 2 gives an overview of the mapping scores we elaborated.
There is no code or expression T that allows any of the above judgement.
For practical purposes, there is still the preliminary category “revisit”, which is set in case a coder is not sure about the decision and wants to mark the code for a group discussion. Each OPS code is seen three times, with the following roles: C: coder - the person who does the OPS-SNOMED mapping; R1: first reviser - the person who checks the decision taken by C, R2: second (senior) reviser: the person (mostly the supervisor) who takes the final mapping decision, sometimes as a result of a group discussion. Comment fields are available for each of the three experts; whenever a map is changed, this is documented by an entry. Each expert uses a different colour for his comments. The pilot phase also yielded the following exclusion recommendations: 1. OPS Codes containing the administration of medicines for which no SNOMED
CT procedure codes exists 2. Codes that contain extremely detailed descriptions of a "complex therapy" 3. Planning phase A of a procedure B if only SNOMED codes for B are present 4. Procedures in cases of doubt 5. Supplementary OPS codes (“Zusatzcodes"), unless containing significant information of the type procedure 6. Retired OPS codes The exclusion criteria were reassessed at the end of the mapping phase.
As a collaborative environment, a Google spreadsheet was created and filled by the coders with OPS codes, texts, logical operators, and comments. Table 3 provides the stepwise approach on a randomised list of OPS codes, identified as OPS1 - OPS1000.
Coders were also asked to skip the mapping of a code whenever this takes more than ten minutes. These codes were tagged as “revisit”. The revisiting of these codes is scheduled to take place once all other codes are consolidated.
Inter-coder agreement was measured at two points (Tab. 3): for the codes OPS301 - OPS400, in order to achieve a preliminary estimation and at the end, for the codes OPS801 - OPS1000.
During the whole process, cases of difficult or controversial mappings were picked out and discussed. Priority was given to those mapping problems that can be seen as prototypical issues not only with regard to OPS, but also to classification-like coding systems in general.
For a final quality check, the OPS codes were re-arranged from a random order to the numeric order of OPS. This revealed many inconsistencies regarding the mapping of similar codes. The mapping guidelines were adjusted in the sense that also supplementary codes were mapped (as long as this yielded significant clinical meaning). In addition, retired codes were mapped. As a matter of principle, codes that only consisted of administration of drugs were not mapped, assuming that in EHRs there are other, more complete and reliable sources of medication information.
The complete time spent amounted to 3 months x 3 experts x 26 hours per expert and month, totalling 234 hours. This corresponds to an average effort of approx. 4.3 OPS codes per hour (14 minutes per code). The mapping of the first 100 codes, including guideline development and documentation required approx. one fourth of the total time. A descriptive analysis of the mappings of the 1000 OPS codes is provided by Table 4. Each code was seen which seen by the three experts and revisited in their original order by at least one expert. None
AND
ADD
OR 3 42 48
The results of inter-coder agreement are provided in Table 5. We compute simple percentage agreements, because agreement by chance is negligible. Coders agree on at least one core SNOMED CT concept per OPS 68% [58%; 76%] code Coders agree on the same set of SNOMED CT concepts per OPS 54% [44%; 63%] code Coders agree on the same set of SNOMED CT concepts per OPS 41% [31%; 50%] code and agree regarding the mapping quality
In Table 6, typical mapping phenomena are presented. All examples are instances of frequently recurring phenomena. The OPS labels, which are only available in German, were translated to English for better understanding.
Procedure with finding: finding is not represented in the map (would require complex postcoordination) Procedure with device: device is not represented in the map (would require complex post-coordination) Procedure with body part: body part is not represented in the map (would require complex postcoordination) 1-265.4 Electrophysiological examination of the heart, catheter-assisted: In tachycardia with narrow QRS complex or atrial tachycardia 1-266.1 Electrophysiological examination of the heart, not catheter-assisted: implanted cardioverter defibrillator (ICD) 1-268.3 Cardiac Mapping: Right Ventricle Part of the procedure requires separate coding 1-430.1 Endoscopic biopsy of respiratory organs: bronchus 312849006 175131000 Percutaneous transluminal electrophysiological studies on conducting system of heart 252425004 Cardiac electrophysiology 21032000 Cardiac mapping 312849006 Biopsy of 10847001 bronchus Bronchoscopy ADD
Logical conjunction of specific procedure and anatomy-related procedure 1-490.6 Biopsy without incision on skin and subcutaneous tissue: lower leg 287538006 Nonsurgical skin biopsy 118714000 Procedure on lower leg Different granularity in SNOMED requires post-coordination in one case but not in another 3-825 Magnetic resonance imaging of the abdomen with contrast 3-826 Magnetic resonance 58713006 Magnetic 51619007 Magnetic resonance imaging of the musculoskeletal resonance imaging of imaging with contrast system with contrast agent musculoskeletal
structures Coordination needed to add the feature that a procedure is a diagnostic one: 1-631 Diagnostic Esophagogastroscopy 392153002 103693007 Esophagogastroscopy Diagnostic procedure Missing of aggregations at the level “vessel” (regardless of whether artery or vein) 3-611.x Phlebography of cervical and thoracic vessels: Other 4008007 60006002 Phlebography of neck Intrathoracic phlebography Exclusion statements for OPS codes (cannot be expressed by SNOMED CT semantics) 432369004 Magnetic resonance imaging of abdomen with contrast 252738008 Video electroencephalogram 40403005 Catheterization of right heart 1-207.2 Video-EEG (10/20 Electrodes). Excl.: Video-EEG during pre and intraoperative epilepsy assessment 1-273.x Right heart catheterization: Other 1-275.0 Transarterial Left Heart Catheter Examination: Coronary angiography without further action 1-650.2 Diagnostic Colonoscopy: Total, with Ileoscopy AND AND AND OR AND 33367005 Coronary angiography 174184006 Diagnostic endoscopic examination on colon 67629009 Catheterization of left heart 235150006 Total colonoscopy 265387003 Diagnostic endoscopic examination of ileum (X AND Y) ADD Z 12 Distinction between logical conjunction “AND” and addition (more than one instance in the target representation (“ADD”))
Given the size of the two terminologies and the fact that the mapping was done only with the most frequent 1000 codes (i.e. the most frequent medical procedures, covering 80.9% of the procedure coding results used in German university hospitals), the relatively low amount of exact mappings and the frequent need of post-coordination of two or more SNOMED CT concepts may be surprising. However, knowing the large structural differences between these two coding systems and their distinct scenarios of use explains 10 Residual class “other”, i.e. logical complement (cannot be expressed by SNOMED CT semantics) 11 Explicit definition “without” (cannot be expressed by SNOMED CT semantics) the differences. For instance, many frequent OPS codes contain numeric criteria (number of treatment sessions, duration of interventions, number of sites where a complex intervention takes place, dosage of drugs). This is mainly because complexity and the treatment costs grow with these numeric values. By including them in the definition of codes, the use of OPS as a tool for billing becomes more convenient, since multiple assignments of the same code, e.g. for each single application of a drug, are not necessary. A similar case is the use of single codes for complex treatments, e.g. stroke. Here the assignment of one single code depends on fine-grained rules (for stroke, comprising 592 words), including the frequency of monitoring, the required diagnostic measures and the specialty of the clinicians involved. In rheumatology, complex therapies with integrated function-oriented and pain-therapeutic treatment sections, often lasting a week or more, are prerequisite for an efficient acute care of chronically ill patients. The combination of a multitude of “small” clinical procedures requires appropriate codes that represent the overall effort without coding each single procedure [14]. There are also cases, in which the exact OPS code depends on the computation of a score that estimates the overall effort spent in complex treatments (32 among 1000 OPS codes).
On the other hand, many of the simple procedures, which are mentioned as constituents of complex therapies, like blood pressure measurement or blood sampling, as well as most lab procedures are missing in OPS. The reason is simple: the effort needed for these actions, in isolation, is just too insignificant. As much a single procedure may be relevant for clinical documentation, if it is cheap, there is no OPS code.
For SNOMED CT, such an overloading of procedure concepts would contradict its main destination as a standard for fine-grained clinical documentation, where reimbursement is not the focus. Although concepts for combined procedures exist, the focus is on encoding every single procedure.
This explains why our map required so many SNOMED CT co-ordinations, even for apparently simple concepts, and why the mapping could often not be considered exact, given the exclusion rules that assure the non-overlapping of OPS classes.
The ontological structure of SNOMED CT allows for complex logical expressions for concept refinement by its so-called post-coordination mechanism. Because such expression cannot be processed by any routine implementation, we decided to restrict ourselves to simple post-coordination patterns. This explains inexact mappings, e.g. when an indication, a body part of a device was missing for a perfect semantic match between an OPS code and a SNOMED CT expression (examples 1 – 3 in Table 6). In other cases, the meaning of an OPS code could be represented by the logical combination of two or three SNOMED CT classes (example 4 - 7). The same meaning could also be achieved by using the SNOMED compositional syntax. The semantic equivalence between such different syntactic forms could be ascertained by a description logics reasoning engine like SNOROCKET [15]. Example 6 shows variation in the degrees of pre-coordination: “Magnetic resonance imaging of the abdomen with contrast” maps to one SNOMED CT concept, whereas “Magnetic resonance imaging of the musculoskeletal system with contrast” requires post-coordination.
Pre-coordinated SNOMED CT concepts often skip certain anatomical hierarchy levels, e.g. they require a distinction between arteries and veins, whether OPS often just refers to “vessels” (Example 8). In other cases, the anatomic delineation of an OPS code (e.g. knee + thigh for certain procedures on skin) has no correlate in SNOMED CT. Postcoordination by simple conjunction is here not possible; the way out would be a disjunctive expression (‘procedure X on skin of knee’ or ‘procedure X on skin of thigh’), which is, however, not supported by SNOMED CT logics.
A tricky issue is the distinction between one single procedure and a set of procedures. This was the reason for the “ADD” operation, which in contrast to “AND” just lumps codes together. Given the special semantics of SNOMED CT procedures (see comment to “role groups”), one could argue that even complex procedures could be expressed as logical conjunctions. This might be an argument in favour of substituting all “ADD” statements by “AND” statements in order to simplify implementations.
The limits of SNOMED CT’s post-coordination power are also reached when it comes to negation, such as in explicit exclusion rules, residual classes (“others”) and implicit negations in labels including “without” (examples 9 - 11). This is explained by the OWL EL profile used for SNOMED CT, which lacks negation.
Finally, a complicating factor was the language gap (German vs. English) and the lack of clear term definitions. The coders, who were not specialists in any surgical or diagnostic discipline, depended on medical textbooks and online references, in order to make clear whether a German term meant the same as its supposed English translation. In many cases, this was difficult.
In the light of all these factors, the relatively low inter-coder agreement rates were not surprising. Especially the distinction between “exact” mapping and “broader” was not easy. That the inter-coder agreement in the last phase was even lower than in the early phase (see Tables 3, 5) is explainable by the fact that in the early phase many complex mappings had been left out and encoded as “revisit”.
Our results reinforce the large difference between medical coding systems even between those that cover the same domain. The dependency of coded information on the specifics of the vocabulary used and the purpose of the codes cannot be emphasized enough. The problem of re-use of administrative codes for other scenarios has been repeatedly addressed [16]. So far, idiosyncratic procedure codes are normally the only source of procedure information, beyond EHR narratives. Mappings to an international standard as SNOMED CT is, in theory, a partial solution to this problem; but to be of high quality it will require leveraging its complete post-coordination mechanism, supported by description logics reasoning, in order to obtain a high coverage and convincing retrieval results. Another way to achieve interoperable EHR information would be the application of natural language processing technology to clinical narratives in which medical procedures are referenced. The quality and comprehensiveness of such data depend, however, on natural language resources like lexicons and annotated corpora for training. Both, however, still constitute a major bottleneck, given the dynamics of medical language on the hand and the difficulties to share clinical real-world data due to privacy issues, on the other hand.
Once the content of EHRs is comprehensively represented by standardised information models and terminologies, a new mapping challenge will arise, viz. inferring administrative codes like OPS from SNOMED CT codes in context. This could put a new task on the agenda, viz. the construction of mappings in the inverse direction. This has already been discussed in the context of ICD-11; a suggested formalism was to express classification codes as queries on SNOMED CT coded EHR data [17]. Thus, a new generation of medical classifications could be rooted in and maintained by using international EHR standards like SNOMED CT.
The exchange of clinical real-world data is a major desideratum; however, no universal coding standards are currently used for medical procedures. We report on mapping codes from OPS, the German coding system for therapeutic and diagnostic procedures, to SNOMED CT concepts, under the hypothesis that this might be a route towards worldwide interoperability of clinical data. A team of three terminologists has mapped the 1000 most frequently used OPS codes to SNOMED CT. After analysing a pilot set of 100 codes, mapping guidelines were derived. An intermediate analysis of the mappings showed that about one third of OPS codes could precisely mapped to one SNOMED CT code or a conjunction of up to three codes. A higher degree of precise maps would require sophisticated post-coordination but even in these cases, maps are still approximate. About 5% of the codes could not be mapped mostly due to complex rules for codes that aggregate many elements of complex therapies or codes for medication administration. The mapping result is affected by several factors such as lack of precise definitions in either terminology system, translation problems, or the need of fine-grained specialist knowledge in some areas. This is also one reason behind the rather low inter-coder agreement.
The team is currently extending the mapping, covering the most frequent 2,125 OPS codes, which correspond to 90% of the encodings in the underlying clinical dataset. This map will be published in early 2020 by TriNetX. [14] H.J. Lakomek et al. Die multimodale rheumatologische Komplexbehandlung (OPS 8-983). [The multimodal rheumatologic complex treatment (OPS 8-983)--challenges, solutions and perspectives]. Z Rheumatol. 64 (2005), 557–563. [15] M.J. Lawley, C. Bousquet. Fast classification in Protégé: Snorocket as an OWL 2 EL reasoner. Proc. 6th Australasian Ontology Workshop (IAOA’10). Conferences in Research and Practice in Information Technology 122 (2010). [16] W.R. Hersh et al. Caveats for the use of operational electronic health record data in comparative effectiveness research. Medical care 51(8 Suppl 3) (2013), S30-7. [17] S. Schulz. J.M. Rodrigues, A. Rector, C.G. Chute. Interface Terminologies, Reference Terminologies and Aggregation Terminologies: A Strategy for Better Integration. Stud Health Technol Inform. 245 (2017), 940–944.