Abstract
Background
Pulmonary embolism (PE) is a condition with significant morbidity and mortality. Diagnostic imaging aims to identify patients with a PE so that they can receive anticoagulation. There is evidence, predominantly from the USA, that the number of diagnostic scans for PE has increased over time and the diagnostic yield (the percentage of positive scans) has decreased. There is limited evidence on whether this is also true in the UK.
Aim
To describe the pattern of diagnostic imaging for acute PE and investigate whether there has been a change in the diagnostic yield over the last 10 years.
Methods
All adult patients who underwent either computed tomography pulmonary angiogram (CTPA) or ventilation-perfusion (V/Q) scan as an investigation for acute PE at Cambridge University Hospitals NHS Foundation Trust (CUH) between 2015 and 2024 were included. This was a retrospective study using data extracted from the hospital electronic health record (EHR) system.
Results
25,672 scans were included, comprising 23,908 CTPA and 1,764 V/Q scans. The annual number of scans increased slightly over the time period, with an average increase of 1.2% per year (p < 0.05). The overall diagnostic yield was 17.0%, with no clear trend over time. There was also no clear trend in the proportion of subsegmental PE detected over time.
Conclusions
The overall number of scans performed and diagnostic yield for PE have remained relatively stable over the last decade. This is the largest study of its kind in the UK and, uniquely, also covers the entire timeline of the COVID-19 pandemic and its aftermath.
Keywords: Pulmonary embolus, Diagnostic yield, EHR data, CTPA, V/Q scan
Graphical Abstract
Highlights
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Diagnostic imaging for PE increased at a rate of 1.2% per year.
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Number of CTPA scans increased while V/Q scans fell.
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Diagnostic yield remained constant at 17%.
Introduction
Pulmonary embolism (PE) is a thromboembolic phenomenon occurring in the pulmonary arterial system.1 Commonly caused by emboli from systemic veins, several risk factors exist for this clinical presentation of venous thromboembolism (VTE), including deep vein thrombosis (DVT), recent surgery, significant immobility and malignancy.2 PE poses a significant challenge to the NHS accounting for 69,064 hospital attendances and 36,757 in-patient admissions between 2021 and 2022.3 There is also a considerable mortality, with 2,638 deaths attributed to PE in 2021 in England and Wales.3, 4 The main objective of diagnostic testing for PE is to identify those patients who would benefit from anticoagulation therapy, in an effort to mitigate preventable/premature morbidity.5
Published European and UK guidelines alike,6, 7 relating to the diagnosis and management of acute PE, recommend the use of clinical judgement and/or prediction rules to classify patients with suspected PE into different categories of clinical pre-test probability (C-PTP). National Institute for Health and Care Excellence (NICE) guidelines advocate the use of a two-level PE Wells score to estimate the C-PTP when PE is suspected: (i) ‘PE unlikely’ ≤4 – D-dimer is the most appropriate next investigation; and, (ii) ‘PE likely’ >4 – chest imaging should be offered.7 The computed tomography pulmonary angiogram (CTPA) is the imaging investigation of choice to diagnose PE.6, 7, 8 However, its inherent ionising radiation may increase the lifetime risk of cancer especially in female and younger patients,9 and the iodinated contrast material administered may be associated with adverse reactions, including anaphylaxis and nephrotoxicity.10 Where radiation or contrast exposure are contraindicated, a ventilation-perfusion (V/Q) scan can be pursued as an alternative imaging modality, reducing radiation exposure and negating the need to administer iodinated contrast material.6, 7, 11
Easy access to imaging diagnostics and physicians’ poor compliance with current guidelines has led to an overutilisation of CTPAs in patients with suspected PE.12 Konstantinides and colleagues,6 NICE7 and worldwide authors12, 13, 14, 15, 16, 17 have highlighted this problem. CTPA yield (the rate of positive CTPA scans for PE) has been found to be especially low in the United States of America (USA)16, 18 where multiple large, multicentre studies over the last decade have demonstrated a CTPA yield varying from 6.2% to 10.0%. However, it is not clear whether the forces driving increased use of diagnostic imaging in the USA are also active in the NHS.
There are limited UK data on the diagnostic yield of CTPA, with only a few studies focusing on this issue.19, 20, 21 A small single-centre study by David et al.19 compared the number of referrals for CTPA in 2006 and 2009 along with the diagnostic yield, finding that the number of referrals had increased but that this had been accompanied by a decrease in the percentage of positive scans from 21% to 11%. Sharma and Lucas,20 small, single-centre study, found a CTPA-driven diagnostic yield for PE of 28.1% (P = 0.03). Lastly, de Boer and colleagues21 undertook the largest published UK study to date. This was conducted in Sheffield Teaching Hospitals, analysing the result of all CTPA scans from 2016 to 2019, including a total of 10,589 scans with a CTPA diagnostic yield of 15.9%.21 Unlike David et al.,19 de Boer et al.’s21 study revealed that CTPA yield for PE remained stable despite significant increase in the volume of CT imaging performed (59% from 2016 to 2019), falling within the recommended target of 15.4–37.4% by The Royal College of Radiologists.22
The aim of the present study is to describe the changing pattern of CTPA and V/Q scan usage over the last 10 years and to investigate whether there has been a change in the diagnostic yield over this time period. This is the largest study of its kind in the UK and, uniquely, also covers the entire timeline of the coronavirus disease 2019 (COVID-19) pandemic and its aftermath, affording the opportunity to look at changes in imaging practice throughout, as well as whether this has had lasting effects.
Methods
Study population
The study included all patients who underwent either CTPA or V/Q scan as an investigation for acute PE at Cambridge University Hospitals NHS Foundation Trust between 1st January 2015 and 31st December 2024.
Exclusion criteria
Patients undergoing CTPA or V/Q scans for indications other than acute PE (e.g. chronic thromboembolic pulmonary hypertension or in preparation for lung volume reduction surgery) were excluded. Repeat scans for the same patient were also excluded. Scans requested more than 72 h after admission to hospital were excluded. Finally, patients were excluded if they had opted out of data-sharing for research purposes.
Imaging
Patients undergoing CTPA were scanned on either a single-source 64-slice system, or a dual-source system (either 2 × 128 or 2 × 192). The hospital uses V/Q SPECT in the investigation of suspected PE, with the exception of pregnant patients in whom a planar perfusion-only study is used.
Data extraction
This study was performed retrospectively using data extracted from the study hospital’s electronic health record (EHR) system. Data were extracted and anonymised by the clinical informatics team. The data extracted included: basic demographics (age [in years], sex), outcomes (length of stay, hospital admission, intensive care unit (ICU) admission, 30 day readmission, 30 day mortality), observations (National Early Warning Score (NEWS2), temperature, heart rate, blood pressure, respiratory rate, oxygen saturation, fraction inspired oxygen) and imaging reports (CTPA, V/Q).
Classification of CTPA and V/Q reports
Imaging reports were interpreted by the creation of a series of search terms (regular expressions) which were used to classify reports as ʻpositive’, ʻnegative’, or ʻnon-diagnostic’. These were refined iteratively until we were confident that the interpretation was accurate. The regular expressions and the R code used is provided in supplementary information. Any reports that could not be automatically classified were read and classified manually (310 reports). Any that were unclear were reviewed by a second person and classified at this stage (28 reports).
For positive scans, a further series of regular expressions was used to classify the size and location of the PE (provided in supplementary information).
Validation of classification of reports
A random selection of 200 reports were manually reviewed and classified, and the results compared with those from the automated classification. In 199 cases (99.5%) the manual classification agreed with the automated classification.
Data analysis
Data were analysed in R Studio using R version 4.4.2. Statistical comparison between patients undergoing CTPA and V/Q scan was performed using a Chi-squared test for sex distribution, and the Brown-Mood median test for all other characteristics.
Study approval
The study received ethical approval by Cambridge University Hospitals NHS Foundation Trust Research and Development Department and Data Access Committee (Reference A097212).
Results
25,672 patient scans were identified for inclusion, of which 23,908 were CTPA scans and 1,764 were V/Q scans. The overall median age was 65 years old and 56% of patients were female. The group who underwent V/Q scan were significantly younger (median age 32) and more commonly female (81.4%), reflecting its use in pregnancy and the post-partum period in particular. Demographics and baseline observation of patients who underwent a CTPA or V/Q scan are shown in Table 1.
Table 1.
Characteristics of patients who underwent a CTPA or V/Q investigation for suspected acute PE. Values given are median (IQR), unless otherwise specified. For observations the closest in time to the scan is given (up to a maximum of 72 h), with the number of patients with no recorded observation within this time period recorded as missing.
| Overall |
CTPA |
V/Q |
P value | ||||
|---|---|---|---|---|---|---|---|
| Missing | Missing | Missing | |||||
| Demographics | |||||||
| n | 25,672 | 0 | 23,908 | 0 | 1,764 | 0 | |
| Age (years) | 65.0 (48.0–77.0) | 0 | 66.0 (51.0–77.0) | 0 | 32.0 (25.0–39.0) | 0 | <0.001 |
| Female (%) | 56.0% | 0 | 54.1% | 0 | 81.4% | 0 | <0.001 |
| Observations | |||||||
| Systolic blood pressure (mmHg) | 125 (112–140) | 393 | 125 (112–141) | 362 | 119 (110–133) | 31 | <0.001 |
| Heart rate (beats per minute) | 82.0 (72.0–94.0) | 338 | 82.0 (71.0–94.0) | 309 | 83.0 (73.0–94.0) | 29 | 0.060 |
| Respiratory rate (breaths per minute) | 17.0 (16.0–18.0) | 481 | 17.0 (16.0–19.0) | 435 | 16.0 (15.0–18.0) | 46 | <0.001 |
| Oxygen saturations (%) | 96.0 (95.0–98.0) | 346 | 96.0 (95.0–98.0) | 307 | 98.0 (97.0–99.0) | 39 | <0.001 |
| Temperature (°C) | 36.8 (36.5–37.1) | 449 | 36.8 (36.5–37.1) | 405 | 36.8 (36.5–37.1) | 44 | 0.006 |
Number of imaging procedures over time
The number of CTPA scans done each year increased slightly over the 10 year period, but this was accompanied by a decrease in the number of V/Q scans. Fitting a linear model, the total number of scans done increased at an average rate of 31 scans per year, equivalent to a rise of 1.2% per year (p < 0.05). There was a sharp drop in the number of V/Q scans performed from 2020 onwards, with a corresponding increase in the number of CTPAs from 2021. The number of scans performed over time is shown in Fig. 1.
Fig. 1.
a. Total number of scans done for acute PE per year between 2015 and 2024. b. Number of scans done per year according to type of scan (CTPA and V/Q).
Diagnostic yield of CTPA and V/Q scan
Change in diagnostic yield over time is shown in Fig. 2a. The overall diagnostic yield of CTPA was 17.2% over the 10 year period, ranging from 14.9% (2019) to 19.1% (2015) and with no clear trend over time. The diagnostic yield of V/Q scan was 16.0%. This showed more variation (ranging from 11.9% (2021) to 29.2% (2023)), although given the smaller number of scans done this increased variation is not surprising.
Fig. 2.
a. Diagnostic yield of CTPA and V/Q scan over the last 10 years along with the Royal College of Radiologists’ (RCR) recommended upper and lower limits for PE detection. b. Classification of PE according to size and distribution, according to the year the scan was done.
Size of pulmonary embolus
For patients with a confirmed PE we classified the PE according to the size based on the imaging report. The proportion of different sizes of PE did not change significantly over the 10 years of the study. This is shown in Fig. 2b.
Outcomes
In total, 4,375 scans confirmed the presence of a pulmonary embolus, a diagnostic yield of 17.0%. Of patients with a confirmed PE, 33.6% were admitted to hospital, with a median length of stay for admitted patients of 4.2 days. 3.1% were admitted to ICU and the 30 day mortality rate was 3.9%.
The trend in outcomes for patients with confirmed PE is shown in Fig. 3. Analysis of the trend over time showed that the percentage of patients admitted to hospital has fallen over time, averaging 35.8% from 2015 to 2019 and falling to 29.6% in 2021–2024. Notably there was a higher admission rate in 2020 (41.5%) in 2020 during the height of the COVID-19 pandemic, likely reflecting diagnosis of PE in patients who were infected with COVID-19. Mean length of stay for admitted patients increased over the time period, from 6.2 days in 2015–2019 to 8.9 days in 2021–2024. This probably reflects the fact that more of the straightforward cases were not admitted and therefore the remaining patients appear to stay longer. There was no significant trend in the proportion of patients requiring ICU admission or in the 30 day mortality over the course of the study period.
Fig. 3.
Outcomes for patients with a confirmed pulmonary embolus over time, showing mortality, ICU admission and hospital admission (percentage of patients with confirmed PE) along with the mean length of stay for admitted patients.
Discussion
Discussion
This study provides a comprehensive analysis of diagnostic imaging undertaken for investigation of acute PE at a large UK teaching hospital over a ten-year period, encompassing both pre- and post-pandemic eras. The most significant finding is that the overall number of scans performed has remained relatively stable. The small increase in the number of scans performed (1.2% per year) seen over time is less than the growth in the local population, which increased 17.6% according to census data over a similar time period (2011–2021). It is also far less than the growth in the number of ED attendances to CUH which have increased by around 50% in the same period. This suggests that, despite increasing demand on services, clinicians are continuing to apply consistent standards in deciding on the requirement for imaging.
The average diagnostic yield of CTPA remained constant at around 17% over the time period, and the distribution of PE size did not change markedly. These observations contrast with reports from other healthcare systems, particularly in the USA, where increased CTPA utilisation has been associated with declining diagnostic yield for PE and a rise in detection of subsegmental emboli of uncertain clinical significance.16, 18 However, our findings are similar to those from previous smaller UK studies,20, 21 and suggest that there is a difference in practice in the NHS when compared to North America, possibly reflecting differences in guideline adherence and clinical decision-making. The steadiness of diagnostic yield for PE (averaging 17.0% across the study period) aligns with the Royal College of Radiologists22’s recommended range (15.4–37.4%), indicating that chest imaging is being used appropriately. Furthermore, there was no evidence of an increase in subsegmental PE diagnoses, which is reassuring given concerns that overdiagnosis may lead to unnecessary anticoagulation treatment and associated risks.
This study is the first to analyse the use of diagnostic imaging for PE during the COVID-19 pandemic and during the subsequent years. The higher rate of admission during 2020 likely reflects the increase in VTE observed in patients with COVID-19, meaning that patients hospitalised during this period may have had a PE as a complication of COVID-19 and therefore a second reason for hospital admission. It is interesting that, despite this association, we did not observe an increase in either the number of scans performed or the diagnostic yield during the COVID-19 pandemic.
One notable change observed during the pandemic was the sharp decline in V/Q scans, with numbers failing to return to pre-pandemic levels thereafter. This likely reflects infection control measures and operational constraints during the pandemic. There have also been sporadic periods when the ventilation agent was unavailable due to supply issues, which may have influenced the choice of imaging modality. However, the persistent reduction raises questions about ongoing access for patients in whom CTPA is contraindicated, such as those with severe contrast allergy, young female patients or pregnancy. Ensuring equitable access to alternative chest imaging modalities remains an important consideration for future service planning and clinical decision-making.
It is striking that CTPA volumes did not increase substantially over the last decade, despite marked rises in emergency department attendances over the same period. This suggests that either clinical judgement and adherence to local guidelines have improved, preventing unnecessary imaging, or that resource limitations have constrained growth in imaging capacity.
Strengths and limitations
The strengths of this study include its large sample size, minimal missing data, and long observation period spanning major healthcare disruptions such as the COVID-19 pandemic. The use of automated classification algorithms, validated against manual review, adds robustness to the findings. However, limitations include the fact that this was conducted in a single centre and the findings may not be representative of other centres. In addition, due to the retrospective design of the study, we were unable to include information that was not consistently entered in structured form within the electronic health record. Of particular note, we were unable to extract data on the patient’s Wells’ score (or the constituent components of this) and on pregnancy status, which limited further subgroup analysis.
Future directions
Future research should investigate the reasons for persistent low V/Q scan utilisation post-pandemic and assess adherence to local guidelines in imaging decision-making. Evaluating patient outcomes in relation to imaging trends, particularly in vulnerable populations, will be essential to ensure that diagnostic strategies remain both effective and equitable.
Conclusion
In contrast to worldwide studies suggesting an increasing rate of diagnostic imaging for acute PE, we found that both the number of scans and the diagnostic yield remained stable over time in a UK teaching hospital.
CRediT authorship contribution statement
Filipa M. Mendes: Writing – review & editing, Writing – original draft, Validation, Investigation, Data curation, Conceptualization. Robert J.B. Goudie: Writing – review & editing, Visualization, Software, Resources, Project administration, Methodology, Investigation, Formal analysis, Data curation. Sarah L. Cowan: Writing – review & editing, Visualization, Validation, Supervision, Software, Methodology, Investigation, Formal analysis, Data curation, Conceptualization.
Ethics approval and consent to participate
The study received ethical approval by Cambridge University Hospitals NHS Foundation Trust Research and Development Department and Data Access Committee (Reference A097212).
Funding
This research did not receive any specific grant from funding agencies in the public, commercial, or not-for-profit sectors.
Declaration of Competing Interest
The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.
Acknowledgements
RJBG is supported by the National Institute for Health and Care Research (NIHR) Cambridge Biomedical Research Centre (NIHR203312). The views expressed are those of the authors and not necessarily those of the NIHR or the Department of Health and Social Care.
The data that support the findings of this study were provided by the NIHR Cambridge Biomedical Research Centre. This work uses data provided by patients and collected by the NHS as part of their care and support. It would not have been possible without access to this data. The NIHR recognises and values the role of patient data, securely accessed and stored, both in underpinning and leading to improvements in research and care (http://www.nihr.ac.uk/patientdata). The authors wish to thank Habeebat Ibraheem and Jakub Jaworski for their careful work extracting clinical data for this study. We also wish to thank Dr Sumit Karia and Dr Ines Harper in radiology for their help.
For the purpose of open access, the author has applied a Creative Commons Attribution (CC BY) licence to any Author Accepted Manuscript version arising.
Footnotes
Supplementary data associated with this article can be found in the online version at doi:10.1016/j.clinme.2026.100641.
Appendix A. Supplementary material
Supplementary material
Data availability
The data that support the findings of this study were provided by the NIHR Cambridge Biomedical Research Centre in anonymised form under a Data Sharing Agreement. This states that the data cannot be deposited into an open access repository to preserve patient confidentiality and privacy. Data are however available from the authors on reasonable request with permission of Cambridge University Hospitals.
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Associated Data
This section collects any data citations, data availability statements, or supplementary materials included in this article.
Supplementary Materials
Supplementary material
Data Availability Statement
The data that support the findings of this study were provided by the NIHR Cambridge Biomedical Research Centre in anonymised form under a Data Sharing Agreement. This states that the data cannot be deposited into an open access repository to preserve patient confidentiality and privacy. Data are however available from the authors on reasonable request with permission of Cambridge University Hospitals.