Introduction
In accordance with guidance from the Chief Medical Officer's office and the Royal
College of Radiologists, the British Society of Thoracic Imaging (BSTI) recognises
that based on the available evidence computed tomography (CT) currently has no upfront
role in the diagnostic work-up of 2019 novel coronavirus (COVID-19) infection (https://www.rcr.ac.uk/college/coronavirus-covid-19-what-rcr-doing/rcr-position-role-ct-patients-suspected-covid-19).
Nevertheless, a number of reports have been published highlighting CT appearances
in COVID-19, raising the possibility of a role for CT in patient management.1, 2,
3, 4, 5 In response to these reports, the BSTI published a preliminary consensus statement
on 6 March 2020.
6
We discuss below what role, if any, CT would play in the detection and management
of COVID-19 infection in the UK, and the logistics of imaging delivery. This role
is heavily predicated on the clinical context as well as the timing of its intended
use within the diagnostic pathway, especially relative to the current reference standard
diagnostic test, real-time reverse transcriptase polymerase chain reaction (RT-PCR)
of a pharyngeal swab,
7
and other clinical and laboratory investigations. Although it may not be feasible
or desirable for isolation purposes to perform a chest radiograph (CXR), we should
acknowledge that pragmatically patients with a respiratory complaint are likely to
present via any number of routes (primary care, emergency departments [EDs] or outpatient
clinics) having already had a CXR, other than to isolation pods outside a hospital,
and work-up of a respiratory complaint would usually include a CXR in such settings.
Cognizant of this fact, in the following discussion we have considered how a CXR would
also fit into diagnostic algorithms, and in particular, how the use of CT would alter
management in settings where a CXR was or was not available.
As such, we deliberate the following questions:
(1)
would a CT thorax contribute to management of symptomatic cases after a rapidly available
RT-PCR result?
(2)
Would a CT thorax contribute to symptomatic cases if an RT-PCR test was not available
or had to be rationed, and (a) a chest radiograph had been performed and was abnormal?
(b) A chest radiograph had been performed and was normal, or was not/could not be
performed?
(3)
Would a CT thorax contribute to the detection and management of COVID-19 in asymptomatic
high-risk cases?
(4)
How should imaging (CT thorax or CXR) be provided?
(5)
What would a COVID-19 diagnostic algorithm look like?
In the following discussion, a high pre-test probability is assumed for symptomatic
cases, based on one or more of: clinical presentation (Pyrexia of 37.8); acute onset
persistent cough, hoarseness, nasal discharge or congestion, shortness of breath,
sore throat, wheezing, sneezing; and compatible laboratory abnormalities (relative
lymphopenia, elevated C-reactive protein [CRP]).
8
Question 1: Would a CT thorax contribute to patient diagnosis after a rapidly available
RT-PCR result?
If the RT-PCR result is positive
CT findings in this setting would not change diagnosis and may be falsely negative
in about 2–3% of RT-PCR positive symptomatic cases.
2
,
5
As such, we do not envisage a role for CT in this setting.
If the RT-PCR result is negative
In this setting, the role of CT is less certain. RT-PCR has a reported sensitivity
of 60–70%
2
,
5
and thus approximately 30–40% of patients with COVID-19 infection could initially
have a false-negative result. False-negative results may arise for a number of reasons
including inadequate sampling technique or low viral load and thus many patients will
require multiple testing to exclude the diagnosis. Ai et al. found that of 64 patients
with initially negative RT-PCR testing, 15/64 (23.4%) had subsequent positive RT-PCR
(mean time interval 5.1±0.5 days). Ten of these patients (i.e., only 15.6% of those
with initial negative RT-PCR) had typical features on CT at the time of initial negative
RT-PCR.
5
Fang et al. described a 29.4% rate of CT abnormality in patients with initially negative
and subsequently positive RT-PCR.
2
As such, in the minority of patients with high clinical suspicion but negative initial
RT-PCR, the presence of typical CT appearances, such as peripheral ground-glass opacity,
could be used to rapidly diagnose COVID-19 infection, until such time as multiple
negative testing is sufficient to exclude or change the diagnosis.
Conversely, it is important to note that a normal CT cannot be used to exclude a diagnosis
of COVID-19 when duration of symptoms is short,
9
although there is some evidence to suggest that the negative predictive value of CT
is higher when symptom duration is >1 week.
10
,
11
Nevertheless, even in this scenario, multiple PCR testing should be the preferred
method to exclude the diagnosis if no limitation on PCR testing (both availability
and turn around times) exists.
Some CT characteristics, such as pleural effusion, lymphadenopathy, and tree-in-bud
nodularity, have been reported as uncommon in COVID-19 and the presence of these features
should prompt for evaluation of alternative diagnoses.
Therefore, we regard the role of CT in COVID-19 confirmed cases following RT-PCR results
to be the same as in any other viral infection, in that it could be used to: (1) find
co-existing or underlying diagnoses; (2) help diagnose complications, or investigate
a clinically discordant picture (e.g., CRP decline, but increasing hypoxia); and (3)
add value in patients with pre-existing lung diseases.
Even if CT is used to aid rapid diagnosis, it needs to be made clear how this would
translate into a change in management. A practical strategy for patients with initially
negative RT-PCR could involve triage into the following categories: (1) self-isolation
at home for those with no CT abnormality, with serial testing later to definitively
exclude the diagnosis; (2) self-isolation or hospital isolation (depending on clinical
status) for patients with typical CT appearances; and (3) urgent evaluation for alternative
diagnoses, while still maintaining isolation, for patients with non-typical CT abnormalities.
It certainly could be argued, however, that a combination of clinical, laboratory
and CXR findings could also be used to triage patients with suspected COVID-19 requiring
hospital admission in the first instance, reserving CT for challenging situations.
The relative proportions of patients in each triage category would probably differ,
given the increased sensitivity of CT for COVID-19, but we are unaware of any data
comparing the sensitivity of CT to a combination of CXR and laboratory findings to
shed any further light on the relative merits of these different strategies. A study
by Xiong et al. offers a compelling argument for such a strategy: in their analysis
of 42 patients, clinical findings and white blood cell count were not well-correlated
with initial CT findings, but CRP, erythrocyte sedimentation rate (ESR), and serum
lactate dehydrogenase (LDH) were significantly moderately to strongly correlated with
the extent and severity of overall involvement and the size of the largest CT lesion.
10
The use of CT as the main diagnostic tool for COVID-19 over and above RT-PCR in China
has been subsequently challenged.
12
It is noteworthy that the fifth edition of the Diagnosis and Treatment Program of
2019 New Coronavirus Pneumonia proposed by The National Health Commission of China
included chest CT findings as a diagnostic criterion,
13
but CT was removed in the more recent sixth and seventh versions,
14
,
15
reflecting the extremely dynamic nature of consensus opinion on this subject.
Question 2: Would a CT thorax contribute to symptomatic cases if an RT-PCR test was
not available or had to be rationed?
A limited or exhausted supply of RT-PCR testing kits is not inconceivable. Even if
a robust supply was in place, other factors, such as interruption of transport infrastructure
and availability of sufficient virology capacity, or the limited number of centres
that could process results, could impede the utility of this test. We consider the
following two scenarios.
If a chest radiograph had been performed and was abnormal
It is salutary that CXR may be abnormal in the majority of COVID-19 cases, especially
severe cases,
16
,
17
with severity defined according to standard definitions.
18
Huang et al. found bilateral radiographic abnormalities in 40/41 (98%) of cases
16
; Guan et al. found radiographic abnormalities in 162/274 patients (59.1%), but CXR
was more likely to be abnormal in severe disease (46/60 [76.7%] with severe disease,
versus 116/214 [54.2%] of non-severe disease).
17
Conversely, CT thorax has a very low specificity, approximately 25%, for COVID-195.
Furthermore, CRP is abnormally elevated in between 61% to 92% of patients.
17
,
19
As such, we do not consider a CT thorax to provide additive benefit to diagnosis over
and above high clinical suspicion, laboratory findings, and typical radiographic abnormalities
in this setting. Further work is needed to understand the potential role of CT in
providing prognostic information, in particular in patients with severe disease, including
guiding management of patients recovering from severe disease.
If a chest radiograph had been performed and was normal, or was not/could not be performed
In this setting, the lack of initial RT-PCR testing availability could necessitate
an alternative test. A CT thorax could then be viewed as the optimum initial diagnostic
tool for all the reasons discussed in Question 1. Assuming it becomes necessary to
elevate CT thorax to this position in the diagnostic work-up, the role of CT thorax
in guiding management would be to triage patients into the categories discussed earlier
(self-isolation at home with repeat testing as necessary, admission and isolation,
or evaluation for alternative diagnoses and potentially aiding triage of unwell patients).
In Table 1
, we illustrate the possible use of CT as a triage mechanism prior to initial RT-PCR
results being available. For all scenarios, we have further assumed that, regardless
of the presence or absence of initial radiographic abnormalities, initial management
would necessitate isolation and standard care given high clinical suspicion. Furthermore,
if a chest radiograph suggests an alternative diagnosis (such as lobar pneumonia),
the CT may provide no additional pragmatic benefit over and above clinical suspicion
and RT-PCR results, and as such, we do not consider it beneficial to explore the permutations
of that scenario.
Table 1
Possible impact of computed tomography (CT) on the diagnosis of COVID-19, assuming
no RT-PCR result is available initially.
Table 1
Scenario no.
CT findings for COVID-19
CT-based triage categorya
Initial RT-PCR result
Would a rapidly available RT-PCR have voided the CT result?
Management post-initial RT-PCR result
Repeat RT-PCR result (if obtained)(4–10 days)
Management following repeat RT-PCR result
1
Typical
Isolation ± admission
Positive
Yesb
Unchanged
Positive
Unchanged
2
Typical
Isolation ± admission
Positive
Yesb
Unchanged
Negative
Unchanged: in this subgroup, 60% of cases show imaging features prior or parallel
to RT-PCR positivity, and the subsequent negative RT-PCR does not decrease clinical
probability of COVID-19 infection; however, the role of CT in evaluating response
in conjunction with RT-PCR requires further work
3
Typical
Isolation ± admission
Negative
Noc
Unchanged
Positive
Unchanged: 16–29% of patients with initial negative RT-PCR could have positive typical
CT findings.
4
Typical
Isolation ± admission
Negative
No
Downgrade to self-isolation depending on clinical status, await repeat RT-PCR
Negative
Possible downgrade: may be considered COVID-19 negative. Repeat CT in this scenario
may also have a role in guiding management along with clinical evaluation.
5
Clear
Self-isolation, do not admit
Positive
Yesb
Possible upgrade to admission depending on clinical status. Negative CT may indicate
non-severe disease. Whether this independently impacts prognosis requires further
work
Positive
Unchanged
6
Clear
Self-isolation, do not admit
Positive
Yesb
Possible upgrade to admission depending on clinical status. Whether this independently
impacts prognosis requires further work
Negative
Unchanged: RT-PCR results would be guiding management here, and a negative follow-up
RT-PCR result does not exclude COVID-19 infection; however, the role of CT in evaluating
response in conjunction with RT-PCR requires further work
7
Clear
Self-isolation, do not admit
Negative
Noc
Unchanged
Positive
Possible upgrade: depending on symptom duration, initial CT could be negative
8
Clear
Self-isolation, do not admit
Negative
Yesb
Unchanged
Negative
Possible downgrade: may be considered COVID-19 negative. In some clinical scenarios,
CT may have a role in determining the frequency and prioritisation of repeat testing,
and the level of cohort isolation required, all of which requires further consensus.
a
The decision to admit will be based on clinical and laboratory parameters.
b
CT demonstration of underlying lung disease or alternative diagnosis could still be
useful.
c
RT-PCR would be false-negative in this scenario.
Question 3: Would a CT thorax contribute to the detection and management of COVID-19
in asymptomatic high-risk cases?
Data on the prevalence of CT abnormalities, and thus the accuracy of CT, in detecting
COVID-19 in asymptomatic contacts of positive or confirmed cases are scarce. At the
time of writing, we are aware of only two publications on the subject. Hu et al.
20
evaluated the laboratory and CT characteristics of 24 asymptomatic close contacts
who were COVID-19 carriers, as confirmed by RT-PCR. Seventeen of 24 (70.8%) had CT
abnormalities, with 12 (50% of total) showing typical peripheral ground-glass opacity.
The minority had lymphopenia (16.8%) and elevated CRP (17.4%). The 17 patients with
CT abnormalities were older (median age 38 versus 14 years old, p=0.012). A more recent
analysis by Inui et al of 112 RT-PCR confirmed cases of COVID-19 from the cruise ship
“Diamond Princess” demonstrated CT abnormalities in only 44/82 (54%) asymptomatic
cases (reference https://doi.org/10.1148/ryct.2020200110). In combination, this reinforces
the unsuitability of CT in isolation as a screening tool for this subgroup. Even if
CT were to be used to screen close contacts, we cannot envisage the practical value
of such a strategy, compared to serial RT-PCR testing (as long as the latter is available),
given that such contacts would be advised to self-isolate. In addition, the specificity
of CT or RT-PCR for screening asymptomatic contacts in this manner is undefined.
Question 4: HOW should imaging (CT thorax or chest radiography) be provided?
Inevitably, the provision of imaging will depend on local protocols for isolation
and identification of suspected COVID-19 patients when they first present; however,
we consider the following broad options. Regardless of the option, the need to perform
a comprehensive clean of equipment between patients (to protect negative patients
from positive ones), and the implications that has for throughput, should be recognised.
Mobile CXR unit alongside dedicated COVID-19 assessment area located outside the main
hospital building
A mobile unit set up in this way would be able to maintain high throughput and fast
image transfer and interpretation. Such a strategy may, with the agreement of local
respiratory and infectious diseases teams, even obviate the need for CT thorax, but
only if (a) RT-PCR testing and results are available rapidly; and (b) there is still
an intention to isolate and treat patients with negative CXR and RT-PCR but high pre-test
probability.
Mobile CT scanner unit alongside dedicated COVID-19 assessment area located outside
the main hospital building
A CT unit set up in this way would also allow high throughput, as the type of CT required
for diagnosis is easy and rapid to perform (unenhanced thoracic CT). Image transfer
and interpretation would take longer relative to CXR, but rapid reporting turnaround
could be aided by structured reporting templates for relevant abnormalities only.
Examples of such templates are provided on the BSTI website at bsti.org.uk. Such a
CT set-up realistically obviates the need for mobile CXR provision.
Mobile CT scanner in a community location
It may be desirable to locate CT scanners in community locations away from the hospital
or take advantage of existing community-located scanners. Adequate operating procedures
to ensure rapid transmission of images to designated reporting centres would be required
in such cases.
ED CT/CXR for COVID-19 patients, mobile CT/CXR for routine ED imaging
As patients with COVID-19 may bypass the dedicated assessment area and present to
ED itself, this strategy would allow imaging to be performed without potentiating
transmission to other ED attendees via the scanner. Regular ED work could be transferred
to mobile scanners alongside the ED; however, such a strategy would not obviate the
need for local cleaning procedures. In addition, logistically, such a set-up would
be challenging due to default image transfer protocols and worklist interactions between
ED scanners, radiology information systems (RIS) and picture archiving and communications
systems (PACS).
Fixed site non-ED CT scanner
For trusts where CT scanner(s) are solely based within the radiology department contingency
plans for “scanner downtime” during deep cleans and policies to minimise cross-contamination
would need to be instigated. An alternative would be for trusts to consider a mobile
CT scanner unit outside the hospital; however, for inpatients with suspected or confirmed
COVID-19 infection who require further imaging, such as CT pulmonary angiography,
or for very unwell patients, a fixed site ED or non-ED CT scanner would be the only
options. In this circumstance, it may be pragmatic to ring-fence consecutive slots
for the confirmed COVID-19 cases on the nominated scanner to make cleaning more practical.
Question 5: What would a COVID-19 diagnostic algorithm look like
Figure 1
illustrates the diagnostic work-up algorithm now endorsed by the British Society of
Thoracic Imaging, Dr Cliff Mann, National Clinical Director of Urgent and Emergency
Care, Dr Matt Inada-Kim, National Clinical Advisor in Sepsis and Deterioration, and
Professor Erika Denton, National Clinical Director for Diagnostics. The algorithm
consolidates the foregoing discussion on the role of imaging by (1) placing clinical
and laboratory assessment, and the patient's clinical condition, at its centre; (2)
using CXR as the initial diagnostic imaging tool; and (3) suggesting a potential role
for CT in cases with a normal or indeterminate CXR.
Figure 1
BSTI/NHSE Radiology decision tool for suspected COVID-19 (available at https://www.bsti.org.uk/standards-clinical-guidelines/clinical-guidelines/bsti-nhse-covid-19-radiology-decision-support-tool/).
Figure 1
Conclusion
The putative role of CT in the diagnosis, triage, and prognostication of patients
with COVID-19 infection continues to be refined. We explore scenarios integrating
CT into the diagnostic algorithm; however, the clinical value of this integration,
compared to alternative strategies incorporating quick and immediately scalable standard
clinical and laboratory assessment (even in the absence of RT-PCR availability) is
unknown, because published studies on thoracic CT in COVID-19 infection to date have,
frustratingly, described these assessments without analysing their relative merit
as part of an alternative diagnostic strategy. In the absence of such data, we can
only reiterate the need for strategic thinking that explores all available options
to achieve maximum public benefit, optimise throughput in the shortest timeframe,
and minimise detriment.
Declaration of Competing Interest
The authors declare no conflict of interest.