Gastrointestinal (GI) Physiology services undertake the assessment and diagnosis of patients with a variety of gastrointestinal disorders. It is essential for services carrying out these diagnostic tests to appreciate and understand how Uncertainty of Measurement (UoM) can affect the test performance, interpretation of results, and patient management decisions.
As part of Improving Quality in Physiological Services (IQIPS) accreditation under the revised standard (1) (IQIPS Standard v2: 2020), services need to be aware of UoM in their clinical services and incorporate this into service policies. This document aims to provide a general overview of the concept of UoM and its relevance to GI Physiology.
What is Uncertainty of Measurement?
All measurements (including the measurements taken during GI Physiology investigations) are prone to some level of uncertainty or a margin of doubt. UoM describes a quantitative estimate of the doubt that exists for any measurement result and the level of confidence applicable (i.e. a range of values that the true measurement value is believed to lie within, with a stated probability).
Calculating UoM involves identifying possible sources of uncertainty in your measurement, estimating the size of the uncertainty from each source, and calculating the combined uncertainty from the individual aspects. The National Physical Laboratory have published guidance on UoM explaining these principles in a format aimed at beginners, and is a good place to start (2). UKAS have also produced a guidance document, which explains how to express levels of uncertainty and confidence in measurement (3).
Uncertainties can come from a variety of sources (e.g. the equipment or method used, the patient, the test operator, or the environment in which the measurement is carried out).
Why is Uncertainty of Measurement important?
UoM must be considered when interpreting GI Physiology measurement data and when defining “normality” or “abnormality”.
Recognising UoM is particularly important when the degree of variation may place results into different categories. For example, if a numerical value is clearly within a normal range, the UoM may be less of a factor. However, if the numerical value is close to the lower or upper limit of the normal range, the UoM may impact the interpretation of test results, which in turn may impact on the planned management of patients.
It is important that all diagnostic services are aware of UoM when interpreting and reporting results. The results may influence the diagnosis or exclusion of disease, the categorisation of disease severity, the prescription of medications and other interventions (including decision to proceed to surgery, risk stratification, and the type of surgery performed). UoM should absolutely be known when a measurement is deemed critical (i.e. when a small variation would have an impact on patient outcome).
Uncertainty of Measurement: Contributing Factors
In relation to Physiology investigations, there are 5 main contributing factors that can introduce variability into measurements (i.e. potential sources of uncertainty). These include the staff carrying out the measurement, the patient having the measurement taken, the equipment used to make the measurement, the method used to make the measurement, and the environment in which the measurement is carried out (Figure 1).
Figure 1: Contributing factors to measurement variability
Uncertainty of Measurement in GI Physiology
All measurements will have a degree of variability or a margin of doubt, and we need to appreciate the uncertainties in the measurements and how this may affect the diagnosis or impact the patient’s outcome. In order to ensure diagnostic measurements are of the highest quality, GI Physiology services should identify and address UoM in the scope of their investigations. Individual services should decide how UoM may relate to their investigations and put processes in place to assess, quantify, and work within the UoM.
There should be appropriate quality assurance in place for aspects of your service relating to equipment (e.g. endoanal ultrasound probes, ambulatory pH recording boxes, manometric catheters, breath testing equipment). Services should be able to check the equipment measurement range from technical specifications. For example, a sphygmomanometer may be accurate to +/– 3 mmHg (or 2% of the reading above 200 mmHg). Services can apply this knowledge appropriately to make a decision on whether it will affect interpretation and reporting of results.
How to Reduce Uncertainty of Measurement
Good practice can reduce UoM, and examples of ways to minimise potential contributing factors are outlined in Table 1. However, anything that may affect the final reported result or outcome needs to be considered as a contributing factor.
Processes leading to high quality measurement
Table 1: Minimising uncertainty to produce high quality measurement
Qualitative and Quantitative Measurements
When considering UoM, it is helpful to think about qualitative and quantitative aspects.
Qualitative relates to tests that do not involve numerical measurements and depend on empirical observations (e.g. an endoanal ultrasound scan). We also need to consider qualitative aspects that can affect the performance of a test (and therefore the result). If any of these aspects result in limitations, possible ways of minimising them should be used and the limitations acknowledged.
Quantitative relates to numerical measurements (e.g. the pressure measurements obtained during manometry). All measurements have a level of uncertainty, and this is what we need to establish before using any measurements for diagnostic purposes. We also need to consider physiological factors which can affect the quantitative performance of a test, such as patient anxiety. UoM is particularly important where a measurement is critical, for example where one number informs a decision to recommend surgery for a patient. In assessing UoM, we can employ methods to assess and give confidence in measurements, for example:
- Statistical methods (e.g. coefficients of variability, probabilities)
- Control samples to verify techniques or equipment (e.g. ultrasound and flow phantoms, buffers for pH studies)
- Reference ranges (e.g. evidence-based normal values, equipment technical specifications)
GI Physiology Investigations
As a starting point, Table 2 provides examples of qualitative and quantitative factors for specific diagnostic procedures commonly performed in GI Physiology services (due to the wide range of investigations provided by GI Physiology units nationally, this list is not exhaustive). These investigations include:
- High resolution oesophageal manometry (HRM)
- Oesophageal pH and multichannel intraluminal impedance monitoring (pH-MII)
- Wireless pH monitoring
- High resolution anorectal manometry (HR-ARM)
- Endoanal ultrasound (EAUS)
- Hydrogen and methane breath testing (HMBT)
- Sphincter of oddi manometry
- Small bowel manometry
|Wireless pH monitoring||
|Sphincter of Oddi Manometry||
|Small bowel manometry||
Table 2: Quantitative and qualitative aspects
- IQIPS Standards v2 (2020) https://www.ukas.com/wp-content/uploads/filebase/iqips/IQIPS-Standard-v2-2020.pdf
- Bell, S. “A beginner’s guide to uncertainty of measurement”. Measurement Good Practice guide 11 (2). National Physical Laboratory. https://www.dit.ie/media/physics/documents/GPG11.pdf
- M3003 “The expression of uncertainty and confidence in measurement” 2019 United Kingdom accreditation service. M3003 Expression of Uncertainty and Confidence in Measurement (ukas.com)