Undiagnosed unilateral pleural effusion

BTS pleural disease guideline 2023; ACR AC Acute Respiratory Illness

Three studies, and the vetting decision is almost never which one — it is the order. Ultrasound characterises the fluid and marks a safe needle site, which no CT can do. CT looks for the cause, and it can only do that properly while the fluid is still there: pleural nodularity is obvious outlined by effusion and nearly invisible once the pleura are drained together. So the sequencing rule that matters is CT before complete drainage, not after it.

A unilateral effusion on a chest radiograph in a patient in whom the cause is not established — new breathlessness, a pleuritic ache, weight loss, an asbestos history, or an effusion found incidentally — with or without a plan to sample or drain it.

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

Known cancer

Back pain in a cancer patient is a different question, and staging phase changes which contrast phases are needed.

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

Thoracic ultrasound — pleural assessment and site marking

Thoracic / pleural ultrasound

What we'd amend, and why

Ultrasound does two things here that nothing else does. It characterises the fluid: anechoic and free-flowing, complex non-septated, complex septated or homogeneously echogenic, and that grading is a better predictor of whether a needle or a small drain will succeed than anything on the CT, because fine fibrinous strands are conspicuous on a high-frequency probe and routinely invisible on cross-section, and it is the degree of septation that moves management from a needle to a large drain with intrapleural agents. And it marks a site — an intercostal space chosen with the patient sat as they will be sat for the procedure, with the depth to the parietal pleura and the depth to the underlying lung both recorded, which is the number that actually prevents a needle going too deep. BTS guidance is explicit that pleural procedures should be image-guided; the practice it exists to replace is a mark taken from a radiograph, or a mark made hours earlier in a different posture, and both are established mechanisms of liver, splenic and lung injury. It also settles the question a plain film often cannot answer at all: whether a dense hemithorax is fluid, consolidation, collapse or tumour.

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

How was this decided?

pathwayadult — Adults — effusion of undetermined cause

Decision support only. Local protocol takes precedence.

Worth asking the referrer (3)

None of these hold the request up. They sharpen the protocol or the plan that follows.

Has the fluid been sampled or drained, and is a drain planned in the next day or two?
It decides when the CT should be booked rather than whether it should be done. A CT after a therapeutic drain is a materially worse study for the question being asked.
Is there known malignancy, asbestos exposure, weight loss, or a significant smoking history?
It moves the contrast-enhanced CT from a second step to something that should be arranged alongside the ultrasound and before the fluid is removed.
Can the patient sit forward, or can they only be scanned and drained supine?
The achievable drainage site depends on posture, and a mark made in a position the patient cannot reproduce is not a usable mark.

Pathways

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

Known malignancy — new effusion in a patient under cancer care

Role	Study & protocol	Why this answers the question
First line	CT Chest CT Chest — Contrast-Enhanced (Venous Phase) usually appropriate	In a patient already known to have cancer, a new unilateral effusion is a staging event until proved otherwise, and the CT is being asked a question that has a short shelf life. The malignant signs are all pleural-surface signs — nodularity, circumferential thickening, parietal thickening greater than about a centimetre, and involvement of the mediastinal pleura — and every one of them is easier to see when the visceral and parietal layers are held apart by fluid. Drain the effusion completely and those same nodules lie against the chest wall with nothing to outline them; drain it and introduce air, and the study is degraded further. That is why this step sits ahead of the drainage plan rather than after it. The acquisition itself is a pleural-phase one: contrast at a venous delay of roughly a minute is what makes the pleural layers enhance and separate from the fluid they contain, and it also re-stages the lung, the nodes and the upper abdomen in the same breath-hold, which is usually the other half of what the oncologist needs.
First line	Thoracic / pleural ultrasound Thoracic ultrasound — pleural assessment and site marking usually appropriate	The CT says what the pleura looks like; the ultrasound says what can be done about it today. It confirms that the opacity is fluid rather than consolidation, collapse or tumour, grades the internal echogenicity and septation that decides whether a small-bore drain will work at all, and returns a marked site with a recorded depth to the pleura and to the underlying lung, taken in the position the patient will actually occupy. In this group it also carries diagnostic weight of its own: pleural nodules and diaphragmatic thickening are visible sonographically, and the yield of a pleural aspiration is highest when the operator can see what they are aiming at. None of that is available from a CT report, and BTS guidance treats image guidance as a requirement of the procedure rather than a refinement of it.
Second line	Chest radiograph Chest radiograph — PA (± lateral)	Once the effusion is known and the cross-sectional plan is made, the radiograph stops being a diagnostic test and becomes a monitoring one: drain position, the volume that came out, whether the lung re-expanded or the hemithorax simply stayed opaque because the lung is trapped. That is a legitimate and cheap use of it, and it is worth stating explicitly on the request so nobody expects it to answer the cause.

This fork is entered from a recorded oncology context. Clinical suspicion of malignancy without a cancer diagnosis on the record — asbestos exposure, weight loss, a heavy smoking history, a rapidly reaccumulating effusion — should be handled the same way, and the request should say so, because the sequencing advice is the whole point and the tool cannot infer it.
No dedicated pleural-phase protocol exists in this vocabulary, so the pathway resolves to the venous-phase chest acquisition, which is the closest available and the one whose timing the pleural signs depend on. A delay towards the longer end of the venous range is the conventional pleural technique; confirm against local protocol.
Where the pleural fluid has already given a malignant cytology, the CT question changes from "is this malignant" to "what is the primary and what is the stage", and the coverage should extend to the abdomen.

Adults — effusion of undetermined cause

Matches your inputsDefault

Role	Study & protocol	Why this answers the question
First line	Thoracic / pleural ultrasound Thoracic ultrasound — pleural assessment and site marking usually appropriate	Ultrasound does two things here that nothing else does. It characterises the fluid: anechoic and free-flowing, complex non-septated, complex septated or homogeneously echogenic, and that grading is a better predictor of whether a needle or a small drain will succeed than anything on the CT, because fine fibrinous strands are conspicuous on a high-frequency probe and routinely invisible on cross-section, and it is the degree of septation that moves management from a needle to a large drain with intrapleural agents. And it marks a site — an intercostal space chosen with the patient sat as they will be sat for the procedure, with the depth to the parietal pleura and the depth to the underlying lung both recorded, which is the number that actually prevents a needle going too deep. BTS guidance is explicit that pleural procedures should be image-guided; the practice it exists to replace is a mark taken from a radiograph, or a mark made hours earlier in a different posture, and both are established mechanisms of liver, splenic and lung injury. It also settles the question a plain film often cannot answer at all: whether a dense hemithorax is fluid, consolidation, collapse or tumour.
First line	Chest radiograph Chest radiograph — PA (± lateral) usually appropriate	The erect film is where most of these effusions are found, and it is co-first-line rather than a formality because one observation on it redirects everything that follows: the direction of mediastinal shift. A large effusion pushes the mediastinum away from itself. A large opaque hemithorax with the mediastinum central, or pulled towards the opacity, is not a simple effusion — it means there is volume loss on that side as well, which points to a central obstructing tumour, a collapsed lung, or a pleural process that has trapped the lung and fixed the mediastinum. That distinction changes the CT from "characterise the pleura" into "find the endobronchial lesion", and it changes what should be said to the referrer about how much fluid it is safe to remove. The film also establishes laterality and volume, dates the effusion against any previous film, and occasionally supplies the diagnosis outright in the form of a mass, rib destruction or pleural plaques.
Second line	CT Chest CT Chest — Contrast-Enhanced (Venous Phase) usually appropriate	The CT is the study that looks for the cause, and it is second only in sequence — in most undiagnosed unilateral effusions it will be needed. It is a contrast study and a pleural-phase one, because every finding it is being asked for is an enhancement finding: nodular pleural thickening, circumferential involvement, parietal thickening beyond about a centimetre and mediastinal pleural disease are the features that separate malignant from benign pleural disease, and at an arterial delay the pleura has barely enhanced and blends with the fluid. So a CT pulmonary angiogram performed for breathlessness, reported as showing an effusion with no pleural abnormality, has not assessed the pleura at all. The same acquisition looks for the parenchymal lesion, the hilar or mediastinal node, the destroyed rib and the sub-diaphragmatic cause — pancreatitis, a subphrenic collection, cirrhosis — that a chest-only view of the problem will miss. Two timing corollaries follow. It should be booked before the effusion is fully drained, for the reason this card exists. And in suspected pleural infection there is a floor as well as a ceiling: a CT done in the first day or two, before the process has organised, under-shows loculation.

The two first-line steps answer different halves of the same question and are not alternatives to each other: the film says how big it is and which way the mediastinum has gone, the ultrasound says what the fluid is like and where a needle can safely go. Requesting either is right; requesting the CT before either has been done is usually premature.
Where the clinical picture points to malignancy, the contrast CT stops being second in sequence and should be arranged alongside the ultrasound and ahead of drainage. That is the substance of the malignancy pathway on this card.
A transudate with an obvious cause does not need a CT at all. The pleural fluid protein and LDH, read against the serum, are what decide whether this pathway continues.

Pitfalls

Booking the CT after a therapeutic drain. This is the defining sequencing error of the indication: pleural nodularity is conspicuous when outlined by fluid and hard to see once the pleural surfaces are apposed, so the study most likely to make the diagnosis is the one done while the effusion is still present. If drainage cannot wait, a diagnostic aspiration rather than a full drain preserves the study.
Accepting an unenhanced CT for an undiagnosed effusion. Every malignant pleural sign the request is asking about depends on enhancement.
Reading a CT pulmonary angiogram as having excluded pleural malignancy. It is arterial-timed, the pleura has barely enhanced, and "effusion, no pleural nodularity" from that acquisition means very little.
Assuming an anechoic, free-flowing, non-septated effusion is benign. Sonographic complexity predicts drainability, not aetiology — a large proportion of malignant effusions are entirely anechoic.
Treating a large opaque hemithorax as a large effusion without looking at the mediastinum. If it has not shifted away, there is volume loss too, and draining aggressively risks re-expansion pulmonary oedema in a lung that was never going to re-expand.
Using a mark made earlier on a ward round, or made with the patient sitting differently from how they will be positioned for the procedure. The mark and the procedure belong to the same episode and the same posture.
Attributing a unilateral effusion to heart failure. Effusions in heart failure are usually bilateral and, when asymmetrical, right-sided — a unilateral left effusion with a normal heart size deserves the full pathway.
Confining the search to the chest. Pancreatitis, a subphrenic collection, cirrhosis with hepatic hydrothorax, ovarian pathology and renal failure all present as a pleural effusion, and a chest-only reconstruction will not show the cause.
Forgetting that a small effusion adjacent to consolidation is a parapneumonic effusion until sampled, and that the vetting question then becomes the empyema question, which is a different card and a different urgency.

Priors — what to pull first

Find the oldest available chest film. When the effusion first appeared, and whether it has recurred after previous drainage, is worth more than any single measurement on the current study.
Look specifically for pleural plaques on old imaging in anyone with an occupational history — they date the exposure and change the threshold for pursuing a pleural biopsy.
If a previous CT exists, check whether it was arterial or venous phase before accepting a report of "no pleural abnormality" from it.

What makes a good request

Laterality is doing real work in the title. A bilateral, symmetrical, small effusion in a patient with a raised jugular venous pressure and cardiomegaly is a heart failure problem and is investigated with an echocardiogram and a diuretic, not with a CT. A genuinely unilateral effusion has a much narrower differential — malignancy, infection, pulmonary embolism, tuberculosis, a sub-diaphragmatic cause — and earns cross-sectional imaging.
BTS guidance is that thoracic ultrasound should be used in the assessment of all patients with a suspected pleural effusion, and that all pleural procedures should be image-guided. A request that plans an aspiration on the basis of a radiograph alone has skipped the step that prevents the injury.
A good request states the side and the approximate size, whether the fluid has already been sampled and what it showed (protein, LDH, cytology, pH), whether a drain is in or planned and when, the smoking and asbestos history, any known malignancy, and whether the patient can sit forward to be scanned or drained.
The single most useful thing a vetter can add to this request is the timing instruction. Where malignancy is a real possibility, the CT should be booked before the effusion is drained — or, if a drain is unavoidable first, immediately after a diagnostic aspiration of a few hundred millilitres rather than after a therapeutic drain has emptied the hemithorax.
Where the effusion is small, the ultrasound may be the only imaging needed: an anechoic free-flowing collection in a patient with a clear clinical explanation does not automatically require a CT, and the pleural fluid result may end the question.

Scoring this once it is done

The classification and risk tools this question ends in.

eGFR (CKD-EPI 2021 Creatinine)CO-RADS Lung Cancer TNM Staging (9th Edition)Lung-RADS v2022 (Screening LDCT)RECIST 1.1 Tumor Response PERCIST 1.0 PET Response

How these studies are acquired

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

CT Chest — Contrast-Enhanced (Venous Phase)Thoracic ultrasound — pleural assessment and site marking Chest radiograph — PA (± lateral)

Confirm locally

Thoracic ultrasound — pleural assessment and site marking: 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.