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Revising the UMLS Semantic Network

Steffen Schulze-Kremer
, Barry Smith
and Anand Kumar

IFOMIS, University of Leipzig, Germany
Department of Philosophy, University at Buffalo, NY, USA

The integration of standardized biomedical terminologies into
a single, unified knowledge representation system has formed a
key area of applied informatics research in recent years. The
Unified Medical Language System (UMLS) is the most ad-
vanced and most prominent effort in this direction, bringing
together within its Metathesaurus a large number of distinct
source-terminologies. The UMLS Semantic Network, which is
designed to support the integration of these source-termino-
logies, has proved to be a highly successful combination of
formal coherence and broad scope. We argue here, however,
that its organization manifests certain structural problems, and
we describe revisions which we believe are needed if the
network is to be maximally successful in realizing its goals of
supporting terminology integration.
Terminology, Ontology, UMLS Semantic Network
The J anuary 2003AB version of the UMLS Metathesaurus,
which is the total UMLS concept repository, includes some
900,551 concepts and 2.5 million concept names in its source
vocabularies. [1] The associated Semantic Network (hereafter
SN), consists of 134 Semantic Types together with 54 possible
links between these types, and represents a high-level abstrac-
tion from the UMLS Metathesaurus. SN is a graph containing
more than 6,000 edges organized into a double tree structure.
An additional tree hierarchically classifies all available link
types. We focus here on SNs role as a classificatory (isa) hier-
archy, leaving for another place the scrutiny of SNs non-isa
links. We aim to stay as close as possible to the intentions of
SNs creators by focusing on potential problems which need to
be addressed if the network is to serve its purpose of suppor-
ting integration of diverse biomedical information sources.
Ontology needs some most general term to designate every-
thing which exists (all items, objects, beings, existents), and
the term entity has established itself in this role. Since SNs
top-level division into entities and events corresponds in the
ontological literature to that between continuants and occur-
rents, we here depart from SN usage and talk not of entities
and events but rather of continuant and occurrent entities.
Continuants are entities which endure, or continue to exist:
they preserve their identity from one moment to the next even
while undergoing changes. Examples are physical objects
(organs, cells, genes, molecules, ) but also: shapes, quali-
ties, dispositions, states, roles, spatial regions, body sites, and
functions. Entities in the latter categories, too, endure self-
identically through time. The function of a thermometer to
measure temperature exists self-identically from one moment
to the next and it exists even at those times when it is not
being exercised.
The exercise of a function, in contrast like the performance
of a role, the execution of a plan, the application of a therapy,
the realization of a disposition is an occurrent entity, an enti-
ty which occurs in a given interval of time. Occurrent entities
(processes, events, activities, changes, histories) unfold them-
selves in time; they never exist in full in any single instant.
Ontologists distinguish also between independent and depen-
dent entities, corresponding in first approximation to SNs dis-
tinction between physical objects on the one hand and concep-
tual entities and events on the other. To say that an entity is
dependent is to assert that it requires a support from other
entities in order to be sustained in existence. There is no mass
or shape without some body, no cellular motion without some
cell which moves. Independent entities require no such sup-
port, for they are themselves the substrates for qualities, dis-
positions, motions, functions and other dependent entities.
At those levels of granularity which are of concern in biomedi-
cine, occurrents are always changes of or in someenduring en-
tity or entities; thus they are always dependent entities. Of the
four possible combinations yielded by our two divisions, there-
fore, only three are instantiated: dependent and independent
continuant and occurrent. This system of three categories pro-
vides the top-level architecture inter alia for the DOLCE onto-
logy developed within the framework of the Semantic Web
Initiative as the first module of the Wonderweb Foundational
Ontologies Library [2]. It underlies also a number of other
ontological systems in current use, including LinKBase, the
large terminology-based medical ontology developed by the
company L&C in Belgium [3].
We formulated a series of consensus ontological principles and
used these as the basis for a thorough audit of the SN. We pre-
sent here samples of the results of applying the following prin-
ciples to SNs classificatory hierarchy:
P1. entities in different highest-level categories (independ-
ent continuant, dependent continuant, occurrent) should not
be combined within a single class;
P2. objects should not be combined within a single class
with the roles they play or with the functions they exercise;
P3. entities in reality should not be combined within a sin-
gle class with our knowledge about or with our concepts of
such entities;
P4. what is concrete (what exists in space and time and en-
ters into causal relations) should not be combined within a
single class with what is abstract (for example with abstract
spatial regions, measures, and the like);
P5. classifications should respect the factor of time; for ex-
ample classes should be assigned in a way that is consistent
with the fact that continuant entities endure through time.
We show that failure to respect these principles implies subop-
timal reasoning capabilities: valid inferences will be blocked
and invalid inferences will be admitted.
We focus on the graph generated by SNs class-subclass sub-
sumption links. Working up from the lowest nodes of this
graph, we consider the question of compliance with the five
basic principles mentioned above, and draw conclusions as to
possible revisions of the network. At the same time we add
commentary pointing to other potential problems in SNs or-
ganization, sometimes drawing on examples of assignments of
SN-types within the UMLS. We mark only problematic cases,
using the designation Pn to mark potential problems connected
with non-compliance with the corresponding principle.
Plant (Alga): Plant, like algae, is an independent continuant.
By allowing plant roots isa plant, plant leaves isa plant, etc.,
UMLS runs together isa with part-of relations. Pollen isa plant
may reflect a failure to do justice to the factor of time (it holds
only in a specific stage of the reproduction cycle). P5
Vertebrate (Amphibian; Bird; Fish; Reptile; Mammal) /
Animal (Invertebrate; Vertebrate) / Organism (Plant;
Fungus; Virus; Rickettsia or Chlamydia; Bacterium; Ani-
mal; Archaeon): classified under independent continuants.
Fully Formed Anatomical Structure (Body Part, Organ,
or Organ Component; Tissue; Cell; Cell Component; Gene
or Genome): These again are independent continuants. Fully
Formed Anatomical Structure is defined by SN as: An ana-
tomical structure in a fully formed organism; in mammals, for
example, a structure in the body after the birth of the organ-
ism. One potential problem with this definition pertains to
time. How would it allow one to code terms in literature about
e.g. pressures on the heart during birth? From the definition, it
is clear that structure is intended to denote a concrete physi-
cal object (an independent continuant). Note, however that the
term is used elsewhere in the SN (for example under Chemi-
cal, below) to denote something abstract, namely the way an
object or thing is organized, its Bauplan or the arrangement of
its parts. A minor point: the term carries the erroneous sugges-
tion that it is the entity itself (rather than the organism within
which it is housed) that is fully formed.
There are some odd features of the use of Body Part, Organ, or
Organ Component in the Metathesaurus; thus while hand is a
Body Part, Organ, or Organ Component, fingers is classified
under Body Location or Region. Such problems sometimes
arise due to the fact that the UMLS merges source vocabularies
whose classification hierarchies have different rationales. This
cannot be a reason here, however, for while source vocabular-
ies like MeSH and different flavors of SNOMED classify both
hand and fingers as Body Region or Body Region Structure,
Digital Anatomist classifies both as Body Part Subdivision. In
any case both hand and finger(s) should be classified in the
same way.
Gene or Genome is defined as: A specific sequence of nu-
cleotides along a molecule of DNA or RNA. A genome is,
however, much more than a sequence of genes, and a eukary-
otic genome contains only gene fragments. P4
Anatomical Abnormality (Congenital Abnormality; Ac-
quired Abnormality): Anatomical abnormality is defined as:
An abnormal structure, or one that is abnormal in size or loca-
tion. Thus understood, the term embraces both dependent con-
tinuants (such as abnormalities in shape or position of the
uterus) and independent continuants (such as an acquired fis-
tula). This categorial ambiguity should be eliminated. P1
Anatomical Structure (Embryonic Structure; Fully
Formed Anatomical Structure; Anatomical Abnormality):
Anatomical Structure is defined as: A normal or pathological
part of the anatomy or structural organization of an organism.
Note that in the phrase structural organization, the term or-
ganization is not used in conformity with SNs own definition
(see below) as meaning social organization. Rather it is used
to mean an entitys Bauplan. The latter, however, would be,
not a concrete three-dimensionally extended independent thing
but rather some dependent abstract feature which gives shape
and functionality to an entity of this sort. Then, however, it
should not subsume liver or leukocyte.
Embryonic Structure is: An anatomical structure that exists
only before the organism is fully formed. This definition is
problematic for reasons outlined already under Fully Formed
Anatomical Structure and Anatomical Abnormality. P1, P4, P5
Medical Device (Drug Delivery Device) / Manufactured
Object (Medical Device; Research Device; Clinical Drug):
When we refer to medical devices we can refer either to a
physical object or to the role this object plays in some context.
Under the former heading we are referring to an independent
continuant, under the latter to a dependent continuant. We pro-
pose introducing into SN a new higher-level category of role.
This would allow also a more adequate treatment of terms such
as doctor, nurse, patient, etc., as also of manufactured object
and chemical (see below and [4]). P1, P2
Lipid (Steroid; Eicosanoid) / Organic Chemical (Nucleic
Acid, Nucleoside, or Nucleotide; Organophosphorus Com-
pound; Amino Acid, Peptide, or Protein; Carbohydrate;
Lipid) / Chemical Viewed Structurally (Organic Chemi-
cal; Element, Ion or Isotope; Inorganic Chemical): All of
the above are independent continuants. SNs dichotomy Chem-
ical Viewed Structurally and Chemical Viewed Functionally
reflects, we believe, not a genuine classificatory subdivision,
but rather a distinction between types of classification. Struc-
ture yields one classification, functions a second, and to put
these two together leads to problems of the same sort which we
would face if we were to divide the class of people into: tall
people, people who play tennis, etc. Chemicals viewed func-
tionally should be treated not as special types of chemicals, but
rather in terms of special types of roles or functions. P2
Pharmacologic Substance (Antibiotic): Pharmacologic
Substance is of course properly to be classified under Sub-
stance. Here again, however, there is a dimension of function
that has to be addressed. A given pharmacologic substance will
be unable to perform its function for example when its expiry
date has elapsed. We propose distinguishing Pharmacologic
Substance as an independent continuant, Pharmacologic Func-
tion as a dependent continuant, and Pharmacologic Action as
an occurrent. P2, P5
Biologically Active Substance (Neuroreactive Substance
or Biogenic Amine; Hormone; Enzyme; Vitamin; Immuno-
logic Factor; Receptor) / Chemical Viewed Functionally
(Pharmacologic Substance; Biomedical or Dental Material;
Biologically Active Substance; Indicator, Reagent, or Diag-
nostic Aid; Hazardous or Poisonous Substance) / Chemical
(Chemical Viewed Structurally, Chemical Viewed Func-
tionally): Chemical Viewed Functionally is defined as: A
chemical viewed from the perspective of its functional charac-
teristics or pharmacological activities. For reasons given
above, we suggest that the information captured under this
node should be represented in the SN via the addition of new
upper-level categories of role or function. A feature like haz-
ardous is more properly classified as a role than as a type of
substance, since the same substance may be hazardous in some
contexts but not in others, and at low concentrations it may
even be beneficial. P2, P5
Substance (Body Substance; Chemical; Food): Substance
is defined as: A material with definite or fairly definite chemi-
cal composition. Here again food should more properly be
classified in terms of a substances special role in a certain con-
text. P2
Organism Attribute (Clinical Attribute): The types in
question refer to dependent continuants. See discussion under
conceptual entity below.
Finding (Laboratory or Test Result; Sign or Symptom):
A Finding is: That which is discovered by direct observation
or measurement. A Sign or Symptom is: An observable mani-
festation of a disease or condition based on clinical judgment,
or a manifestation of a disease or condition which is experi-
enced by the patient and reported as a subjective observation.
These terms reflect a running together of ontology and episte-
mology. [5] Classifying Symptom as a subclass of Finding
amounts to the assertion that observable manifestations of a
disease exist only when they are discovered by direct obser-
vation or measurement. This, however, means that your symp-
toms do not exist until they are cognized as such. Symptom
should be reclassified under the heading Organism Attribute.
P1, P3
Molecular Sequence (Nucleotide Sequence; Amino Acid
Sequence; Carbohydrate Sequence): These are independent
Spatial Concept (Body Location or Region; Body Space
or Junction; Geographic Area; Molecular Sequence): Spa-
tial Concept is defined in SN as: A location, region, or space,
generally having definite boundaries, and it thus falls under
the category of independent continuants. However, the use of
the term concept is then inappropriate. For concept are de-
pendent continuants; concepts exist in peoples minds; they are
abstract entities which depend on concept-users. There have
been concepts only during the period in which there have been
concept-using organisms.
Independent continuants like geographic areas and molecular
sequences, in contrast, do not exist in peoples minds. We sur-
mise that the mentioned subclasses are grouped together under
Spatial Concept because they are held to share the following
characteristics: a) they are extended in space and b) their
boundaries are determined not by any underlying physical dis-
continuities but rather by demarcations introduced by human
subjects. The referent of Bethesda, MD satisfies these condi-
tions, and so also does knee-joint but then so also does hand,
which is not classified by UMLS as a conceptual entity. Mo-
lecular sequences, too, can be regarded as the products of de-
marcation or segmentation. We believe that it is appropriate to
include in a top-level ontology the distinction between entities
which have bona fide boundaries and fiat entities (entities
which exist because of demarcations which we draw). The hu-
man body and the liver have bona fide boundaries, while the
boundaries of your hand or finger are fiat in nature. [6] But this
does not mean that the entities in question are special sorts of
concepts. P1, P3, P4, P5
Functional Concept (Body System): Functional concept is
defined in SN as: A concept which is of interest because it
pertains to the carrying out of a process or activity. This defi-
nition runs together functions, which are dependent contin-
uants (for example the function of your heart, which is to pump
blood), and occurrents, which are the realizations or executions
of such functions.
Body system is defined as: A complex of anatomical structures
that performs a common function. Functional concept is sub-
sumed by concept. A body system is however not a concept.
(Concepts do not perform functions or have physical parts.)
Rather, a body system is a certain physical part of an organism,
whose (fiat) demarcation is functionally determined. Body sys-
tems and the associated functions existed for many millen-
nia before there were concept-using organisms. P1, P3, P4, P5
Idea or Concept (Temporal Concept; Qualitative Con-
cept; Quantitative Concept; Spatial Concept, Functional
Concept): Idea or Concept is defined as: An abstract concept,
such as a social, religious or philosophical concept. Note that
this definition is circular, since the term concept appears also
in the definiens.
Given our arguments above, the entities currently coded under
Spatial concept and functional concept should not be included
among the subclasses of idea or concept. Temporal concept,
too, seems to us to be misclassified. As continuants are located
in spatial regions, so occurrents are located in temporal inter-
vals. This analogy is lost if both spatial and temporal terms are
treated under the UMLS-ST equivalent of the category of con-
tinuants. P1, P3, P4
Occupation or Discipline (Biomedical Occupation or Dis-
cipline) / Organization (Health Care Related Organiza-
tion; Professional Society; Self-help or Relief Organization)
Group (Professional or Occupational Group; Population
Group; Family Group; Age Group; Patient or Disabled
Group): Group is defined as: A conceptual entity referring to
the classification of individuals according to certain shared
characteristics. Groups on the one hand endure in time (and
survive changes in their members), but on the other hand they
are dependent upon demarcations or classifications. Thus they
belong to the category of dependent continuants; but again:
they are not concepts. Professional or Occupational Group is
defined as: An individual or individuals classified according
to their vocation. This makes it clear that Group subsumes
also individual members, which yields consequences like: a
member of a physicians organization isa physicians organiza-
tion, an individual is an age-group. A better approach, again,
would be to classify types such as doctor, nurse, patient, etc.,
not in terms of groups but rather in terms of roles or functions
performed by corresponding individuals. P1, P2, P3, P4
Intellectual Product (Regulation or Law; Classification):
Note that the use of the term conceptual entity here does not
cause the problems it causes elsewhere; intellectual products
are indeed such as to depend for their existence on the exis-
tence of concept-using organisms.
Conceptual Entity (Organism Attribute; Finding; Idea or
Concept; Occupation or Discipline; Organization; Group;
Group Attribute; Intellectual Product; Language): Con-
ceptual entity is defined as: A broad type for grouping ab-
stract entities or concepts. We have seen above that many of
the subclasses of this node are neither abstract entities nor con-
cepts and that they belong to no single higher-level ontological
category. Organism Attribute, Occupation or Discipline, Or-
ganization, Group and Group Attribute are entities belonging
not to the abstract realm of concepts but rather to the real world
of space, time and causality, and they should be reclassified
accordingly. P1, P4
Behavior (Social Behavior; Individual Behavior): SN
draws no clear boundary between Social and Individual Behav-
ior. Racism is classified under both headings, smoking and
singing as Individual Behavior, walking as Social Behavior.
Here again we postulate that SNs problematic treatment of in-
dividual human beings will be improved via the addition of a
category of role. P2
Health Care Activity (Laboratory Procedure; Diagnostic
Procedure; Therapeutic or Preventive Procedure): An ac-
tivity is an occurrent, and the same holds for the carrying out
of a laboratory procedure. Procedures themselves, however,
are dependent continuants. A procedure is an intellectual prod-
uct that endures identically through time and is-realised-in its
successive applications, perhaps undergoing changes as meth-
ods and equipment are refined. P1, P5
Research Activity (Molecular Biology Research Tech-
nique): Here again one needs to distinguish between a tech-
nique (which is a dependent continuant) and the application of
a technique, which is an activity (an occurrent). The former
stands to the latter not in an isa relation but rather in an is-
realised-in relation. P1, P5
Occupational Activity (Health Care Activity; Research
Activity; Governmental or Regulatory Activity; Educa-
tional Activity) / Activity (Behavior; Daily or Recrea-
tional Activity; Occupational Activity; Machine Activity):
All of the above are occurrents.
Human-Caused Phenomenon or Process (Environmental
Effect of Humans): Phenomena such as aviation and cultural
evolution (both examples given by UMLS) are dependent con-
tinuants. Processes are occurrents. When water pollution and
deforestation are classified by UMLS as Environmental Ef-
fect of Humans, this leaves open whether we are dealing with
the phenomenon (continuant) or the act (occurrent) of water
pollution or deforestation. To resolve such problems, Phe-
nomenon and Process and their respective subclasses should be
classified separately. P1, P5
Organism Function (Mental Process): Organism Function
is a dependent continuant. Mental Process is defined as: A
physiologic function involving the mind or cognitive proces-
sing. This definition, too, mixes function and process. P1
Molecular Function (Genetic Function) / Physiologic
Function (Organism Function; Organ or Tissue Func-
tion; Cell Function; Molecular Function): Physiologic Func-
tion is defined as: A normal process, activity, or state of the
body. This mixes process (occurrent) and state (continuant).
Disease or Syndrome (Mental or Behavioral Dysfunction;
Neoplastic Process): We believe that neoplastic process is
misclassified as a disease or syndrome. As SN elsewhere rec-
ognizes, a disease is to be distinguished from the processes
which constitute a disease history. But only a continuant can
have a history. An occurrent does not have a history because it
is a history. To this degree, therefore, SN recognizes diseases
as continuants: they are conditions which endure through time
while undergoing changes (for example by becoming chronic,
fibrous, malignant). On the other hand, however, diseases are
listed by SN as a subclass of event. Complex of symptoms,
too, in being included under Disease or Syndrome, is classified
by SN as a subclass of event; symptoms themselves however
are classified under finding, and thus under conceptual entity.
When disease history is classified under Health Care Activity
this is a case of running together the history or course of a dis-
ease on the side of the patient (ontology) with the act of elicit-
ing that history (epistemology). Similar problems pertain to the
UMLS classifications of natural history of disease under
Finding and of cancer patient under Mental or Behavioral
Dysfunction (the latter a reflection of the odd umbrella term
cancer patients and suicide and depression). P1, P3, P4, P5
Pathologic Function (Disease or Syndrome; Cell or Mole-
cular Dysfunction; Experimental Model of Disease): Occur-
rent processes, activities and responses are here combined to-
gether with continuant states and conditions. Pathologic func-
tions of organisms are classified in the same way as pathologic
functions of body systems. Yet the former are classed by SN as
physical objects, the latter as conceptual entities. P1
Biologic Function (Physiologic Function; Pathologic
Function) / Natural Phenomenon or Process (Biologic
Function): Here again the definition of Biologic Function
mixes process (occurrent) and state (continuant). P1
Phenomenon or Process (Injury or Poisoning; Human-
caused Phenomenon or Process; Natural Phenomenon or
Process): Injury or Poisoning is defined as: A traumatic
wound, injury, or poisoning caused by an external agent or
force. A wound (continuant) should be distinguished from a
wounding (occurrent), a state of being poisoned (continuant)
from an act of poisoning (occurrent). P1
Event (Activity; Phenomenon or Process): Phenomena and
processes should not be classified together. P1, P5
The inclusion of the opposition Chemical Viewed Structurally
and Chemical Viewed Functionally suggests that SN might be
better interpreted as classifying not entities but rather the con-
cepts we have of such entities. The concepts we use when re-
ferring to chemicals can after all be divided quite naturally un-
der these two headings. Then, however, the root nodes of SN
should be not: Entity and Event, but rather: Entity Concept and
Event Concept, and the latter should themselves be re-assigned
to the position of daughters of a new root Concept. A restruc-
turing along these lines would however in other ways conflict
radically with SNs current architecture. Above all, it would
contradict the fact that Idea or Concept is already itself a sub-
node of Conceptual Entity. It would also contradict explicit
statements to the effect that SN is an upper-level ontology
in which all concepts are given a consistent and semantically
coherent representation. [7]
A number of proposals have been advanced to increase SNs
effectiveness as a terminology integration platform that can
support enhanced reasoning and information retrieval. Thus [8]
argues that UMLS lacks the requisite granularity, semantic
types and relationships for comprehensively and consistently
representing anatomical concepts in machine readable form.
[9] and [10] propose enhancing the efficiency of UMLS-based
reasoning systems via a clustering of SN nodes to yield more
coarse-grained partitions of the network.
Our proposal is that SNs power to support terminology-based
reasoning can be enhanced through a reclassification along the
lines sketched in the above. As an example of how such a re-
classification would support inferences currently blocked, con-
sider the way in which SN currently views tissues and cells as
physical parts of organs, but views these organs themselves as
mere conceptual parts of body systems, which are in turn con-
ceptual parts of fully-formed anatomical structures, which are
in turn physical parts of organisms. When we reclassify Body
System as a Physical Entity, there is no longer a need for the
distinction between conceptual and physical part-of relations.
Reasoning systems can thus exploit the full power of mereol-
ogy, including the rules governing transitivity of part-of.
The proposed reclassification would lead also to a system of
semantic types for which definitions can be formulated which
are at one and the same time both more intuitive and also more
rigorously formalizable than existing definitions, thereby mak-
ing it easier to train and monitor those with the task of assign-
ing semantic types to new and existing source-terminologies.
Work on this paper was supported by the Wolfgang Paul Pro-
gram of the Alexander von Humboldt Foundation. Our thanks
go also to Olivier Bodenreider.

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Address for correspondence
Steffen Schulze-Kremer, Institute for Formal Ontology and Medical
Information Science (IFOMIS), University of Leipzig, Hrtelstr. 16,
04107 Leipzig, Germany. URL: