Home > Knowledge Hub > Sub-specialties > AGIP > Uncertainty of Measurement in GI Physiology – Association of GI Physiologists (AGIP) Report

Uncertainty of Measurement in GI Physiology – Association of GI Physiologists (AGIP) Report

Updated on: 14 Jan 2022   First published on 14 Jan 2022


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.

Contributing factor  

Processes leading to high quality measurement


  • Staff performing and reporting each procedure should be either appropriately trained and accredited by AGIP, or supervised by a fully trained and accredited practitioner, with evidence of this documented and available
  • Staff should be competent and aware of uncertainties for the measurements that they are undertaking, and evidence to support this should be reviewed and updated annually
  • Staff should be up to date with mandatory training and CPD, and should have access to relevant and current evidence-based standard operating procedures and policies
  • All staff carrying out procedures should be peer assessed by another suitably trained and accredited practitioner a minimum of once a year, to demonstrate ongoing competence performing high quality investigations in line with best practice
  • Any individual patient factors (e.g. anatomy, abilities, compliance with test instructions, relevant surgery or interventions) that may affect the measurements should be documented and taken into consideration when interpreting results and reaching a diagnosis
  • An adequate history and symptom profile should be documented before the test, to ensure that suitable preparations are put in place for a successful investigation
  • Patients should be provided with detailed instructions for test preparation, in order to optimise the investigation
  • Patients should be provided with detailed instructions about what to expect during their procedure, including answers to common questions and contact information for the GI Physiology unit, to optimise the investigation and patient compliance
  • The preparation done by the patient (e.g. cessation of medication) should be taken into consideration when reaching a diagnosis
  • Patient current medication (e.g. opioid medication they are unable to stop) should also be taken into consideration when analysing the results and reaching a diagnosis
  • Where patients are required to follow instructions for measurements to be taken (e.g. carrying out manoeuvres during anorectal physiology testing), a standardised and consistent approach should be taken, documented, and available in patient’s electronic record
  • Equipment and consumables used must be appropriate for the diagnostic procedure and patient
  • All equipment should be regularly serviced in line with the manufacturer’s recommendations, with an up-to-date record of servicing
  • Regular equipment and software checks should be performed by a suitably trained operator, and up-to-date equipment maintenance records and fault logs kept
  • Faulty equipment should be reported and must not be used
  • Regular safety testing must be performed, arranged by the operator of the equipment
  • Due to the variety of equipment available to GI Physiology units, all services must be aware of the accuracy and precision of the specific equipment used (this can be obtained from the manufacturer)
  • All equipment must be calibrated and verified prior to use, in line with manufacturer guidelines
  • There should be appropriate quality assurance (QA) and quality control (QC) processes in place to ensure equipment is reading reliably (where appropriate, external QC should be undertaken)
  • There should be a clear audit trail of the equipment and consumables used for each patient’s investigation (e.g. LOT number, serial number) to allow technical issues to be investigated
  • Prior to and post use, all equipment must be clean and visually inspected for any visible damage
  • Equipment (e.g. drugs, catheters, pH buffer solutions, batteries, lubricating gel, cleaning reagents) must be stored correctly, and used before their expiry date
  • All GI Physiology services should have accessible, up-to-date, evidence-based Standard Operating Procedures (SOPs) or protocols that all staff follow, which are regularly reviewed and revised as necessary, as well as professional body guidance produced by AGIP
  • The SOPs/protocols must include the pre-procedure requirements, preparation and calibration of equipment, performing the investigation, sample collection and storage (if applicable), and analysis and reporting of results
  • Where available, investigations should be performed and reported according to relevant, up-to-date and evidence-based recommendations and normative values
  • The results should be interpreted in conjunction with the patient’s history, symptoms, and clinical examination
  • Artefacts that could affect the measurements should be identified and analysis adjusted accordingly
  • The environment should be conducive to making high quality measurements for each investigation (e.g. appropriate temperature and noise levels, suitable lighting, adequate privacy, suitable facilities) and should be monitored through risk assessments
  • The environment each investigation is conducted in should comply with national and local policies relating to infection control, with special considerations where applicable (e.g. COVID-19 policies)
  • GI Physiology investigations should be carried out in a unit that is either nationally accredited (or working towards national accreditation) with UKAS

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
Test Quantitative aspects  

Qualitative aspects


  • Calibration (including drift)
  • Probe accuracy range
  • Effect of artefacts (e.g. channel failure, vascular impression, movement, gastric pressure due to obesity, a curled catheter) on pressure measurement
  • Normal values used
  • Patient preparation (e.g. discontinuing medication, fasting)
  • Patient tolerance for test (e.g. retching/vomiting)
  • Current patient medication (e.g. opioids)
  • Patient compliance with test instructions (e.g. avoiding multiple swallowing, movement artefacts)
  • Test protocol compliance
  • Patient anxiety
  • Calibration (including drift)
  • Probe accuracy range
  • LOS upper border depth (for probe placement)
  • Misplaced pH sensor
  • Patient preparation (e.g. cessation of medication)
  • Compliance with test instructions (e.g. use of the recording box)
  • Available recording time
  • Functional disorders (e.g. rumination)
Wireless pH monitoring
  • Misplaced pH capsule
  • Capsule accuracy range
  • Accurate location of GOJ
  • Patient preparation (e.g. cessation of medication)
  • Compliance with test instructions (e.g. use of the recording box)
  • Available recording time (e.g. early capsule displacement)
  • Functional disorders (e.g. rumination)


  • Calibration (including drift)
  • Probe accuracy range
  • Effect of artefacts (e.g. vascular impression, ultraslow wave) on pressure measurement
  • Compliance with test instructions (e.g. squeeze effort)
  • Patient anxiety
  • Frequency settings
  • Resolution of calipers
  • Ultrasound probe diameter
  • Clock reference points
  • Quality of probe contact and positioning
  • Patient position
  • Brightness and contrast controls
  • Lighting of environment
  • Calibration
  • Breath sample timings
  • Patient preparation (e.g. cessation of medication, fasting, low residue diet)
  • Compliance with test instructions (e.g. smoking cessation)
  • Transport and storage of test samples
Sphincter of Oddi Manometry
  • Probe accuracy range
  • Patient preparation
Small bowel manometry
  • Probe accuracy range
  • Patient preparation (e.g. discontinuing medication, fasting)
  • Compliance with test instructions (e.g. eating schedule)

Table 2: Quantitative and qualitative aspects


Login to your BSG member account to read and post comments on this page