Suspected empyema or complicated parapneumonic effusion

BTS pleural disease guideline 2023; ACR AC Acute Respiratory Illness

The question is whether the pleural fluid is infected and organised, and that is answered by pleural enhancement — so this is a contrast decision above all. The corollary is that an unenhanced CT reported as "effusion, no empyema" is not an answer.

A septic patient with a pleural effusion, or a patient with pleural infection whose drain is not working and in whom the question is loculation and whether surgery is needed.

Referenced decision support — confirm against your local protocol.

Decision support, not a directive. Protocols and timings shown are typical published ones — your local protocol takes precedence, and the vetting radiologist decides.

The request in front of you

Everything is optional. Leave a field alone and the answer assumes nothing — the verdict updates as you go.

Study requested

What the referrer actually asked for. It is evidence of intent, not a constraint on the right answer.

Contrast as written

What the request form says, not what it should say.

Age

Pick a band, or type an exact age if it matters.

years

Pregnancy status

Fever or sepsis

Renal risk factors

The question a vetter can actually answer from the request. An explicit “none known” is a real answer, and it removes checks rather than deferring them.

Previous contrast reaction

The pathway — tap anything already done

Marking a study complete moves the answer on. A patient arrives partway through a pathway far more often than at the start of one.

Accept as requested

CT Chest — Contrast-Enhanced (Venous Phase)

CT Chest

What we'd amend, and why

Infected pleura becomes inflamed, vascular and thickened, and on a venous-phase acquisition the visceral and parietal layers both enhance and separate around the fluid — the split pleura appearance. That enhancement is the discriminator between a simple transudate and an organising empyema, and it exists only with intravenous contrast at a venous delay. The same acquisition shows what a drain will meet: loculation, pleural thickness, gas, and whether the underlying lung will re-expand.

Nothing needs resolving before this goes ahead. Routine checks below are owned downstream.

How was this decided?

pathwayadult — Adults
rulerule-contrast-reaction-premed — Prior contrast reaction and elective premedication; checked by Nurse before the scan
rulerule-metformin — Metformin and iodinated contrast; checked by Radiographer at the scanner
rulerule-paeds-dose — Child-sized technique and contrast dose; checked by Radiographer at the scanner
rulerule-pregnancy-ionising — Pregnancy status before an ionising exposure; checked by Radiographer at the scanner
rulerule-renal-iodinated — Kidney function and intravenous iodinated contrast; checked by Radiographer at the scanner
rulerule-iv-access — Intravenous access adequate for the planned injection; checked by Radiographer at the scanner

Decision support only. Local protocol takes precedence.

Handled at the scanner(4)nothing for you to do

Settled and owned downstream. Each returns to a radiologist only on the stated trigger.

Metformin and iodinated contrast
Confirm whether the patient takes metformin or a metformin-containing combination, and if so whether ACR Category II applies (eGFR below 30, known or suspected AKI, or an arterial catheter study likely to cause renal embolisation). If Category I — that is, no AKI and eGFR at or above 30 — no action of any kind is needed.
Radiographer at the scannerAt the scanner
Flags back if: The patient takes metformin AND meets ACR Category II — eGFR below 30 mL/min/1.73 m2, known or suspected acute kidney injury, or an arterial catheter procedure with likely renal arterial embolisation. Metformin plus a normal or mildly reduced eGFR is explicitly NOT a flag-back: there is no need to stop metformin before or after intravenous iodinated contrast in Category I patients, and no need to re-check creatinine afterwards.
Child-sized technique and contrast dose
Confirm that a size- or weight-based protocol is selected — child-sized kV and mAs against size-based diagnostic reference ranges — and that contrast volume is calculated by weight rather than taken from an adult default. Weight-based iodinated contrast volumes of roughly 1.5–2.0 mL/kg are widely used in paediatric CT.
Radiographer at the scannerAt the scanner
Flags back if: No paediatric or size-based protocol exists on the scanner for the requested examination, or the requested coverage or number of phases exceeds what the clinical question needs — for example a multiphase study where a single phase answers it, or whole-body coverage for a focal question.
Pregnancy status before an ionising exposure
Make the pregnancy enquiry immediately before the exposure and record the answer. In the UK this is a statutory operator duty discharged at the time of exposure under the employer’s written procedures required by IR(ME)R 2017 — it is not something the vetting radiologist can perform or pre-empt, and a request is complete without it.
Radiographer at the scannerAt the scanner
Flags back if: The patient states that she is, or may be, pregnant AND the uterus is in or near the primary beam. The exposure is then paused for re-justification by the IR(ME)R practitioner before it proceeds.
Intravenous access adequate for the planned injection
Site and test a cannula that supports the protocol flow rate, preferring an antecubital or large forearm vein, and observe the injection for extravasation. A 20-gauge or larger cannula is preferred for flow rates of 3 mL/s or more.
Radiographer at the scannerAt the scanner
Flags back if: No cannula can be sited that supports the protocol flow rate — for example only a 22-gauge hand or foot cannula for a CT angiogram needing 4–5 mL/s; or the only available access is a central line or port that is not labelled power-injectable; or an extravasation occurs.

Pathways

Big forks are separate pathways; the first whose conditions match is the one used.

Septic with a pleural collection — sample and drain, then map

Role	Study & protocol	Why this answers the question
First line	Thoracic / pleural ultrasound Thoracic ultrasound — pleural assessment and site marking usually appropriate	Sepsis with an undrained collection is a source-control problem, and neither the diagnosis nor the treatment of pleural infection comes from the CT. The diagnosis is made on the aspirate — frank pus, a pH below roughly 7.2, or organisms — and the treatment is drainage; ultrasound delivers both in one bedside attendance, by grading the fibrinous septation that decides drain size and intrapleural fibrinolytics, and by marking a safe intercostal space with a recorded depth in the position the patient will actually sit in. It needs no scanner slot, no transfer of a septic patient off the ward, and no contrast load on kidneys already under a septic insult. BTS is explicit that no pleural procedure should be performed without image guidance, and a CT report describing a drainable collection does not discharge that requirement — so a septic patient sent to the scanner before anyone has aspirated the fluid has usually got the sequence, rather than the studies, wrong.
First line	CT Chest CT Chest — Contrast-Enhanced (Venous Phase) usually appropriate	The venous-phase CT keeps its place in the same episode and usually on the same day, and a request for it in a septic patient is accepted as written. Pleural enhancement is what separates an organising empyema from a sterile effusion, and the acquisition maps extent, gas, loculation, the state of the underlying lung and any parenchymal source — none of which ultrasound can supply. The only claim this pathway makes is about order: where sepsis is driving the clock, the fluid that proves the diagnosis and the drain that treats it come first, and the map follows.

Timing cuts both ways, and the request should say how long the pleural sepsis has run. CT is least informative in the first day or two, before the process has organised — another reason the early answer in a septic patient comes from the fluid rather than from the scanner.
Where the bedside ultrasound has already been done by the respiratory or critical care team, say so on the request: that step is complete and the card resumes at the CT.

Adults

Matches your inputsDefault

Role	Study & protocol	Why this answers the question
First line	CT Chest CT Chest — Contrast-Enhanced (Venous Phase) usually appropriate	Infected pleura becomes inflamed, vascular and thickened, and on a venous-phase acquisition the visceral and parietal layers both enhance and separate around the fluid — the split pleura appearance. That enhancement is the discriminator between a simple transudate and an organising empyema, and it exists only with intravenous contrast at a venous delay. The same acquisition shows what a drain will meet: loculation, pleural thickness, gas, and whether the underlying lung will re-expand.
First line	Thoracic / pleural ultrasound Thoracic ultrasound — pleural assessment and site marking usually appropriate	Two things ultrasound does that CT does not. It shows septation: fine fibrinous strands within the fluid are conspicuous on a high-frequency probe and frequently invisible on CT, and their presence is what moves management from a simple aspiration to a larger drain with intrapleural fibrinolytics or to a surgical referral — so the modality that grades the septation is the modality that sets the treatment. And it marks a safe site, in the position the patient will actually occupy for the procedure, with a recorded depth to the pleura and to the underlying lung. BTS guidance is explicit that pleural procedures should be image-guided, because the alternative — a mark chosen from a radiograph, or a mark made hours earlier with the patient sitting differently — is the established mechanism of liver, spleen and lung injury. Ultrasound also settles the question a white hemithorax on a film cannot: whether that opacity is fluid, consolidation, collapse or pleural thickening, and whether an echogenic collection that looks solid is in fact drainable.
Problem solving	CT Chest CT Chest — Unenhanced	Where iodinated contrast genuinely cannot be given, an unenhanced study still shows gas in the pleural space, gross loculation, pleural calcification and the volume and position of fluid — enough to plan a drain when the diagnosis is already established by aspiration. What it cannot do is distinguish an infected from a sterile collection, so it should never be used to exclude empyema.

Pitfalls

Accepting an unenhanced request. This is the defining error for this indication, because the diagnostic finding is an enhancement finding.
Treating the CT as the pre-procedure study. It defines extent and infection; it does not mark a safe intercostal space in the position the patient will be drained in, and it under-reads the septation that decides whether a small-bore drain will work.
Reading pleural thickening and enhancement as necessarily infective — malignant pleural disease enhances too, and nodularity, circumferential involvement and mediastinal pleural thickening favour malignancy.
Scanning very early, before the pleural process has organised, and reporting the absence of loculation as evidence against pleural infection.
Confusing a peripheral lung abscess with an empyema. The distinction changes management completely — abscesses are treated medically, empyemas are drained — and rests on the angle the lesion makes with the chest wall and the shape of the wall.
Forgetting that the underlying lung matters: trapped, non-expandable lung changes what drainage can achieve and should be commented on.

Priors — what to pull first

Compare with the radiograph or CT from the start of the illness — an effusion that was already large before antibiotics behaves differently from one that has appeared during treatment.
If a drain has been placed, note where it is: an apparently failed drain is often a correctly positioned drain in the wrong locule.

What makes a good request

Thoracic ultrasound and contrast CT are both first-line here and they answer different halves of the question. Ultrasound decides whether the collection is septated and where a needle can safely go; CT decides whether the pleura is enhancing and how far the process extends. Neither substitutes for the other, and the BTS position is unambiguous that no pleural procedure should be done without ultrasound guidance — a CT report describing a drainable collection does not discharge that requirement.
A good request states whether fluid has been sampled and what it showed, whether a drain is in place, and how long pleural sepsis has been running.
Timing matters: CT is most informative when the pleural process has had time to organise, and least informative in the first day or two of an effusion.
In many departments the ultrasound is performed by the respiratory team at the bedside rather than booked as a radiology examination. That is a service-model question, not a reason to leave it out of the pathway — the request still has to say whether it has happened.

Scoring this once it is done

The classification and risk tools this question ends in.

eGFR (CKD-EPI 2021 Creatinine)CO-RADS

How these studies are acquired

Contrast, phases and timing for every study on the pathways above.

Thoracic ultrasound — pleural assessment and site marking CT Chest — Contrast-Enhanced (Venous Phase)CT Chest — Unenhanced

Confirm locally

CT Chest — Contrast-Enhanced (Venous Phase): timings are typical — confirm against local protocol.
Timings, contrast volumes and rates above are typical published values. Your department's protocol, scanner and patient population decide the actual numbers.
Safety thresholds and premedication policy follow local policy where it differs from the cited guidance.

References

Implemented from the cited published sources. Educational and workflow support only; confirm against current guidelines and local policy before clinical use.