Suspected pyelonephritis or renal abscess

ACR AC Acute Pyelonephritis (2022 update)

Uncomplicated pyelonephritis is a clinical diagnosis and needs no imaging at all. Imaging is for the complicated patient: not responding, immunocompromised, diabetic, stone disease, or a single or transplanted kidney — and there the question is drainable collection or obstruction, which requires contrast.

Fever, flank pain and pyuria in a patient with complicating factors, or failure to improve after 48-72 hours of appropriate antibiotics.

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

Haemodynamic state

The fact that lets a pathway waive its own requirements. A crashing patient does not wait for a score.

Fever or sepsis

Immunosuppressed

Changes the differential rather than the modality.

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 Abdomen and Pelvis — Portal Venous Phase

CT Abdomen and Pelvis

What we'd amend, and why

Contrast is what converts this from a scan of an inflamed kidney into an answer. Infected parenchyma perfuses poorly and appears as wedge-shaped or striated low enhancement against normally enhancing cortex; an abscess declares itself by an enhancing rim around a non-enhancing centre, which is the finding that changes antibiotics into drainage. An unenhanced study can still show gas and a stone, so it is not worthless, but it cannot separate focal pyelonephritis from an abscess and cannot assess perfusion at all.

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

How was this decided?

pathwaystandard — Adults — complicated presentation
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.

Haemodynamically unstable — suspected obstructed infected system

Role	Study & protocol	Why this answers the question
First line	Ultrasound renal tract Ultrasound renal tract	This card already says ultrasound is the right first test when the specific question is obstruction, when the patient is too unstable to move, or when iodinated contrast must be avoided — and in a shocked patient with urosepsis all three are usually true at once. The finding that has to be excluded now is an obstructed infected system, and that is a decompression emergency rather than an imaging question: pus under pressure does not respond to antibiotics, and the treatment is a nephrostomy or a retrograde stent, not a better scan. Ultrasound answers the obstruction question at the bedside, in minutes, with no contrast and no transfer. Its limitations are the ones the standard arm states and they still apply here — it is insensitive to focal parenchymal inflammation and to small abscesses, so a normal study in a patient who is not improving must escalate rather than reassure — and the study is only useful if the drainage pathway is arranged alongside it rather than after the report.
Second line	CT Abdomen and Pelvis CT Abdomen and Pelvis — Portal Venous Phase	Contrast CT follows decompression and resuscitation rather than preceding them. It is still the study that answers what ultrasound cannot — gas within the renal parenchyma, a non-enhancing centre inside an inflammatory mass, perinephric extension — and once the patient is stable enough to travel it should be done rather than deferred. What it must not become is the reason a septic patient leaves the resuscitation area before the obstruction has been relieved. Acute kidney injury from the sepsis itself is common in this group, so expect the renal flag-back; the trade-off is set out in this card's contrast note rather than resolved by cancelling the study.

This arm is deliberately tested before the pregnancy arm. The pregnancy arm's own problem-solving step already argues that undiagnosed maternal sepsis is the dominant risk to the fetus, and the MRI rung it contains is not available to a patient too unstable to lie still in a magnet for half an hour.
The ultrasound is not the end of the pathway. Where the patient is not improving, or where the question is a drainable collection rather than obstruction, escalate — a negative bedside study in a deteriorating patient is not a negative result.

Pregnant

Role	Study & protocol	Why this answers the question
First line	Ultrasound renal tract Ultrasound renal tract	Pyelonephritis is common in pregnancy and the imaging question is usually obstruction rather than parenchymal characterisation. Ultrasound answers it without radiation, and the physiological dilatation of pregnancy is best interpreted by someone who knows it is expected.
Second line	MRI abdomen and pelvis MRI abdomen/pelvis — non-contrast rapid protocol	Where ultrasound cannot resolve a suspected collection, unenhanced MRI with diffusion-weighted imaging identifies an abscess by restricted diffusion without gadolinium and without ionising radiation.
Problem solving	CT Abdomen and Pelvis CT Abdomen and Pelvis — Portal Venous Phase	Pregnancy is not an absolute bar and the pathway must say so, otherwise it reads as a prohibition and the vetter has nowhere to go. Where ultrasound and MRI have not resolved a septic patient — emphysematous change, a drainable collection, an obstructed infected system — contrast CT is justified, because undiagnosed maternal sepsis is the dominant risk to the fetus. Fetal dose from a single abdominopelvic acquisition sits well below the threshold at which deterministic harm is described, and iodinated contrast has no established fetal effect. Document the justification, and reach for it as a decision rather than a default.

Septic or immunosuppressed — the complicated patient this card is written for

Role	Study & protocol	Why this answers the question
First line	CT Abdomen and Pelvis CT Abdomen and Pelvis — Portal Venous Phase usually appropriate	This is the patient the card was written for, stated as a fact rather than inferred from the request wording: imaging is for the complicated patient — not responding, immunocompromised, diabetic, stone disease, a single or transplanted kidney — so the question here is not whether to image but how quickly, and what the report has to answer. The study and the protocol are the same as the standard arm and for the same reason: contrast is what converts a scan of an inflamed kidney into an answer, because infected parenchyma perfuses poorly and an abscess declares itself by an enhancing rim around a non-enhancing centre. What changes is the urgency and the disposition attached to it. Two findings alter management the moment they are seen and both are actively sought here — an obstructed infected system, which needs drainage tonight, and gas within the renal parenchyma, which is a surgical emergency — so this scan should be booked with the drainage pathway already arranged rather than reported into a vacuum. Immunosuppression carries its own reading instruction: in a transplanted kidney the graft position and the vascular anatomy from the operation note are needed before the study can be interpreted at all.
Reasonable alternative	Ultrasound renal tract Ultrasound renal tract	Where obstruction alone is the question, where the patient cannot travel, or where iodinated contrast must be avoided, ultrasound answers the immediately actionable part quickly and without contrast. It does not answer the rest: it is insensitive to focal parenchymal inflammation and to small abscesses, so in this group a normal ultrasound in a patient who is not improving should escalate rather than reassure.

Fever or immunosuppression recorded on the request does not by itself make an unenhanced study acceptable. Accepting one and then reporting no abscess is the pitfall this card already names: the study could not have shown one.

Adults — complicated presentation

Matches your inputsDefault

Role	Study & protocol	Why this answers the question
First line	CT Abdomen and Pelvis CT Abdomen and Pelvis — Portal Venous Phase usually appropriate	Contrast is what converts this from a scan of an inflamed kidney into an answer. Infected parenchyma perfuses poorly and appears as wedge-shaped or striated low enhancement against normally enhancing cortex; an abscess declares itself by an enhancing rim around a non-enhancing centre, which is the finding that changes antibiotics into drainage. An unenhanced study can still show gas and a stone, so it is not worthless, but it cannot separate focal pyelonephritis from an abscess and cannot assess perfusion at all.
Reasonable alternative	Ultrasound renal tract Ultrasound renal tract	The right first test when the specific question is obstruction, when the patient is too unstable to move, or when iodinated contrast must be avoided. It is insensitive to focal parenchymal inflammation and to small abscesses, so a normal ultrasound in a patient failing to respond should escalate rather than reassure.

Pitfalls

Imaging an uncomplicated first presentation that is responding to antibiotics. It changes nothing and finds incidentalomas.
Accepting an unenhanced study for a patient failing to respond, then reporting no abscess — the study could not have shown one.
Reading a striated nephrogram in isolation as diagnostic; it occurs in obstruction and in renal vein thrombosis too.
Overlooking gas. A small volume of intraparenchymal gas is easy to miss on soft-tissue windows and is the finding with the highest immediate mortality.
Scanning a patient with an obstructed infected system and no arrangement for drainage. The imaging finding needs an intervention pathway attached to it.

Priors — what to pull first

Previous imaging showing stone disease, scarring or a duplex system changes the interpretation and often the treatment.
In a transplanted kidney, the graft position and the vascular anatomy from the operation note are essential to reading the study.

What makes a good request

Imaging is usually not appropriate for a first presentation of uncomplicated pyelonephritis in an otherwise well patient. A request for one is a reasonable point at which to ask what would change.
The two findings that change management immediately are an obstructed infected system, which needs drainage tonight, and gas within the renal parenchyma, which is a surgical emergency.
Focal pyelonephritis and a small abscess look similar early; what separates them is a non-enhancing centre, and that distinction exists only on a contrast-enhanced study.
The striated nephrogram of pyelonephritis is most conspicuous on a nephrographic acquisition at around 90-100 seconds; a standard portal-venous timing shows it adequately in most cases.

Scoring this once it is done

The classification and risk tools this question ends in.

eGFR (CKD-EPI 2021 Creatinine)

How these studies are acquired

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

Ultrasound renal tract CT Abdomen and Pelvis — Portal Venous Phase MRI abdomen/pelvis — non-contrast rapid protocol

Confirm locally

CT Abdomen and Pelvis — Portal 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.