Coping with Problems Caused by Duplicate DICOM Unique Identifiers

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					              Solving the Problem of Duplicate Medical Device Unique Identifiers

1. Introduction
Unique identifiers are generated by medical device systems to individually label different
instances of clinical information objects, for example different patients, different studies,
different reports, and different images. In order to protect patient safety, it is absolutely essential
that no two different entities ever be labeled with the same unique identifier. It has been the
federal government‟s experience, however, that occasionally the same unique identifiers are
generated for different medical information objects. This is caused by poorly designed unique
identifier generation methods that fail due to computer malfunction and/or human error. This is
a very serious problem that can affect patient safety.

2. Background
Digital Imaging and Communications in Medicine (DICOM) was the first medical standard to
use unique identifiers. DICOM uses unique identifiers (UIDs) throughout to identify different
instances of clinical information objects. When a duplicate UID is generated, different studies or
different images are assigned the same UID value, and it is possible to associate the study or
image with the wrong patient. It is estimated that the frequency of occurrence of duplicate UIDs
in DICOM is on the order of magnitude of one in a thousand.

HL7 Version 3.0 uses an analogous unique identifier called an Object ID (OID) to label its
information object instances. As similar factors that are present in DICOM implementations are
also present in HL7 ones, it is expected that OIDs will also have a duplication problem.

3. Root Cause of Problem
The root cause of the DICOM/HL7 duplicate unique identifier problem is that while the
standards define the format and rules for the unique identifiers (that is, they are both ISO 8824
Object Identifiers), the vendors are free to choose their method of implementing them. Many
vendors devise private methods that fail in practice. These cause problems with some current

3.1 Problems Using Natural Primary Keys to create the Unique Identifier
Natural primary keys have business meaning because they contain attributes or a combination of
attributes that are unique by virtue of business semantics. For example, an organization‟s HL7
OID may be designed to contain a facility id, a patient id, and a study id. Natural keys are
almost always a problem because organizations inevitably change their business rules. For
instance, an organization might merge with another and have new business rules requiring it to
use different identifier schemes. As a result, the new OIDs that are created might be identical to
those generated by the old method or to those that are of another facility.

3.2 Problems Using a Synthetic value to create the Unique Identifier
In order to create unique identifiers that have no business meaning, a synthetic value should be
used. While there are some very good methods for creating synthetic values in the computer
industry, a number of unsatisfactory methods are common in medical systems.

3.2.1 One-Up Counter
The simplest synthetic value is based upon the one-up counter. The application uses a
monotonically increasing number to generate unique identifiers like xxx.1, xxx.2, xxx.3, etc.
The counter is maintained in a disk file on the device. Whenever there is a disk glitch, the
counter can be reset. It is reset whenever the disk is replaced. The counter may be reset when a
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new version of the software is loaded. In some cases, a vendor‟s technician resets the counter as
part of normal preventive maintenance.

3.2.2 Resetting Local Computer Time
The unique identifier can incorporate the local date and time values. Duplicate unique identifiers
can be generated if the clock is set back. This may happen manually when an operator observes
an incorrect time setting and resets the clock. It may also occur automatically at the termination
of daylight savings time each fall when clocks are turned back one hour to return to standard
time. Duplicate UID problems can also arise when the system clock malfunctions due to a
hardware/software problem.

3.2.3 Same Identifiers at Multiple Sites
Vendor generated unique identifiers in the same namespace at different local sites. As a result,
duplicate values exist across the enterprise. This problem will be a cross-enterprise
interoperability barrier for organizations such as the VA/DoD that have data consolidation and
sharing requirements. You don‟t want duplicates anywhere really, not just in what you define as
the “enterprise”.

3.3 Resultant Problem – Dealing with existing Duplicate UIDs
There are over a billion DICOM images archived world-wide. Maybe as many as a million of
them have duplicate UIDs. How should end-user systems deal with instances of duplicate
unique identifiers? This currently is a big problem. When a duplicate UID occurs, it must be
detected and properly handled by the Picture Archiving and Communications System (PACS).
However, PACS vary in their handling of objects with duplicate UIDs. Some PACS are
designed with the assumption that all UIDs are unique. These PACS could reject images with
duplicate UIDs, although this may cause a limited disruption of service and a possible loss of
data. Other PACS might not detect the error and associate the images with the wrong patient and
study. Another PACS might mask the problem entirely and use other information to associate
the images with the correct patient and study. The problem may then surface later when these
images are passed to another DICOM system, one that might operate in a different way and
miss-associate the images with the wrong patient or study.

4.0 Challenges in Achieving a Solution
There are a number of diverse federal/public/commercial stakeholders that should be organized
and involved to solve this critical medical device issue. It will be a coordination challenge to
utilize and engage the federal government (for example NSF, NSA, NIST, VA/DOD, FDA,
HHS), standards associations (i.e. NEMA, ISO, HIMSS, RSNA, DICOM, HL7, and The Open
Group), medical device vendors (i.e. PACS, RIS, HIS, speciality systems, modalities) and
academia to address this issue. The HCMDSS workshop and NITRD will be important in
facilitating and guiding this discussion, and resolving issues such as Data Stewardship.

5.0 Important research needs
Finding a „best solution‟ to solve duplicate unique identifiers requires thorough research by a
committee of diverse stakeholders working through the risks and impacts of implementation.
NIST should provide technical guidance.

There currently exist multiple models for universally unique id generation. In particular, the use
of natural vs. synthetic primary keys should be explored. Also the use of a decentralized vs.
centralized model could be analyzed. Research into performance limits may be useful in guiding
implementation. Any identifier schemes that pertain to patient safety should have fail-safe
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structures built into them. These structures might include check digits and random components to
assure the uniqueness and integrity of the identifier.

There may be implementation issues imposed by limits of the current DICOM and HL7
standards (64 characters numeric ISO 8824 Object Identifiers). The committee of stakeholders
will need to determine the best way to generate the unique identifiers. This may impact the
DICOM and HL7 standards.

The flip side of the problem is to determine how systems should deal with the duplicate unique
identifiers in new data and in the legacy data. What is the best design for a system that has to
handle duplicate unique identifiers? How should a system handle objects with duplicate unique
identifiers when it encounters them?

Once the „best‟ solutions have been determined, the committee will need to ensure that
organizations are willing to accept the costs of upgrading their software. This requires sufficient
regulatory compliance, so the FDA should be involved. Marketplace persuasion similar to that
used by HIMSS and RSNA for IHE Compliance will play an important role. The certification of
medical devices generating unique identifiers and handling duplicates will be critical to solving
the issue of non-compliance.

6.0 Roadmap for the next 5 to 10 years
There are a number of milestones on the roadmap that will guide the solution. The first
milestone will be educating the diverse stakeholders on the criticality of the issue. The issue of
data stewardship will be addressed.

The second milestone will be the selection of effective methods for generating universally
unique identifiers and the standardization of approved algorithms and approach. This standards
document may include choosing the “best” from multiple algorithms (such as time-based, name-
based, and randomly generated). The approved solution will need to include metrics on how we
can verify the solution. The second milestone will also include the determination of “how best”
to deal with the problem of the duplicate unique identifiers in the data.

The third milestone will be the demonstration of the validity of the standard and the verification
of test tools.

The fourth milestone will be the VA/DoD and other customers requesting and procuring a
certified solution. Vendors will develop to the requirements and certify their solution with the
committee approved guidelines.

The fifth milestone will be the deployment of the solution and resolving implementation issues.
This includes determining migration strategies and the data cleansing process.

Peter Kuzmak is a Computer Specialist with the VA‟s VistA Imaging Project. He is responsible for DICOM issues.
Andrew Casertano provides technical support to the DOD‟s Military Health System on DICOM, HL7 and IHE.
Daniel Carozza, MD consults to the VA on regulatory compliance and DICOM issues.
Ruth Dayhoff, MD is National Director of the VA‟s VistA Imaging Project.
Keith Campbell consults to the VA on electronic health record issues.

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