Units of Measure (UoM)


The Units of Measure (UoM) project is focused on how the value or outcome of a pathology test is represented electronically, with the goal of improving the quality and computability of pathology test results.

The project has delivered the UK Pathology Units List (UKPUL) of unit codes conforming to the Unified Code for Units of Measure (UCUM) as required by Information Standard Notice DAPB4017: Pathology Test and Results Standard. The UKPUL and associated Implementation Guidance are designed to help both software suppliers and healthcare organisations implement computable representations of pathology results in FHIR, without compromising patient safety or usability for both clinicians and patients.

While numerical results and units of measure are a key focus, the scope also extends to other information types and their representation.


The UoM project has involved analysis of both the current state and future needs for describing and reporting measured results in Pathology, in order to deliver a solution that meets the need for a fully computable pathology message structure, without compromising patient safety or usability for both clinicians and patients.

The solution for UoM, and measured results more broadly, will support the other aspects of the Pathology programme in delivering a pathology reporting capability that is understandable and processable by computational means, allowing for new healthcare tools and processes to be developed and used safely.

How the new standard is different

Prior to the publication of Information Standard Notice DAPB4017 and UKPUL there has not been a mandated standard for the representation of units of measure in NHS data exchanges in England.

For the period from 2008 to 2016 (approx.) there was a list of recommended unit codes (the Laboratory Standardised Representation or LSR) authored under the auspices of the Royal College of Pathology (RCPath) and published by CfH/NHS Digital. Many labs will be aware of LSR and will have aimed to conform to the LSR representations when reporting, however LSR did not have the status of a formal standard. As LSR was no longer being updated and publication ceased in 2016, analysis shows conformance is variable, and probably in decline over time.

The new standard for units of measure is different in that-

  • The UoM standard forms part of a coordinated set of standards that also cover Test naming and coding (UTL), and HL7 FHIR resource profiles. These are being developed together to deliver a new generation of information standards for Pathology
  • These standards have improvements in data quality and computability of results as a primary goal
  • The use of machine readable (computable) unit codes conforming to the UCUM standard wherever possible is now mandated under Information Standard Notice DAPB4017
  • The UKPUL provides standardised UCUM codes for a wide range of UoM that are in use for pathology testing in the UK. The list also provides recommended human-readable representations of these units to improve safety and usability in those cases where UCUM may not be optimum for direct display to clinicians or patients
  • Guidance is provided on how units should be represented in FHIR to provide the best support for both machine interpretation (computability) and human use. The guidance also covers how to deal with a number of specific "edge cases" in order to help implementers improve usability and mitigate potential risks

Benefits of adopting a new approach

  1. The new information standards being developed under Information Standard Notice DAPB4017 are focused on improving the integrity and quality of data, with a specific remit to improve computability in order to support a far greater ability to process, validate and interpret the information than has been possible in the past
  2. Implementing UCUM coding of units of measure supports the delivery of measured results as computable Quantities. These quantities can be processed by UCUM-aware software to allow for capabilities including-
    1. Machine checking and validation("spell checking") of unit codes to improve data quality
    2. Unit-aware comparison and conversion of results to facilitate automated processing such as decision support that requires software to interpret and make decisions based on results
    3. Machine conversion and of results into harmonised units to simplify aggregation of data for large volume data analysis

Current Status

NHS Digital has now published a Design Decisions document for on the subject of representing Semi Quantitative Results in FHIR. This is an early draft intended for early review by senior stakeholders. The document can be expected to change as business requirements and other aspects are elaborated.

A table of Standard UCUM codes and Implementation Guidance in draft form were published for review and feedback. Please use the Review Feedback thread in the forum to access the documents and feed back any questions and review comments.

In August 2021 the Data Alliance Partnership Board (DAPB) published the Information Standard Notice DAPB4017: Pathology Test and Results Standard, which requires units of measure conforming to the Unified Code for Units of Measure (UCUM) standard to be used in pathology reporting whenever possible.

If you or a colleague have an interest in the subject of measurement and Units of Measure in pathology reporting, please look through the topics in the UoM Discussion Forum and UoM Design Issues forum in the first instance to get a feel of the kind of issues we are working on. Please feel free to contribute to the forum discussions, or for more information contact pathologyanddiagnostics@nhs.net  to discuss the goals of the workstream and how you can contribute to our understanding of the requirements in this space.

Workstream Updates

Key Documents


Work in Progress (archive)

Full archive of current and previously published work in progress

Tools and Resources


UCUM Validation and Conversion Tool at the US NLM

Web Service for UCUM (documentation page) at the US NLM

UCUM Essence browser (Excel)



Multiple Free Open-Source Software (FOSS) implementations of UCUM are available, plus other UCUM-related tools. Links to some established UCUM libraries are provided below for convenience, but please note inclusion here does not imply any level of approval or endorsement by NHS Digital.

lhncbc/ucum-lhc  (Javascript). This is the open source library that underpins the US NLM UCUM online service (listed above under Tools). This solution implements some extensions to core UCUM, notably synonyms for some unit codes.

FHIR/Ucum-java (Java). Not a formal product of the HL7 organisation, but written and maintained by a key member of the HL7 FHIR community, and used (I understand) in multiple FHIR projects. There appears to be very little documentation for this implementation.

FirelyTeam/Fhir.Metrics (C#). Not quite a full implementation of UCUM, as currently (Jul 2022) this library does not support UCUM "special" units (that is, units that are not on ratio scales).

Join the Conversation

This forum is for general questions and discussion about the UoM project itself, and on the broader topic of Units of Measure in Pathology, including support in IT systems and related technologies including FHIR, SNOMED, UCUM and others.

Request access to the UoM discussion here

UoM User Reference Group (UoM URG)

The UoM URG is a group of group of stakeholders and subject matter experts that work with us to-

  • Understand clinical, technical and operational requirements
  • Assess available solutions and design options
  • Describe the requirements of stakeholder groups that will be affected by the decisions made
  • Make recommendations on how to address challenges

The Design Issues forum while open to all, is primarily for use by URG members to support the work of the group. If you would like to join the URG please click here

Collaborative Documents in Draft