Suspected septic arthritis (including the acutely limping child)

ACR AC Suspected Osteomyelitis/Septic Arthritis (2022 rev); ACR AC Acutely Limping Child

A joint infection is diagnosed by aspirating the joint and treated by washing it out, not by imaging it. Imaging exists to find the effusion, to guide the needle and to answer the next question — is there osteomyelitis. The vetting priority is that nothing in the imaging pathway delays aspiration, antibiotics or surgical lavage: cartilage is destroyed in hours, and an MRI booked in front of the theatre list is the mechanism by which that happens.

An acutely painful, swollen, hot joint with restricted movement, usually with fever and raised inflammatory markers; in a child, refusal to weight-bear or a limp.

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

Ultrasound musculoskeletal / soft tissue

What we'd amend, and why

Ultrasound confirms and localises the effusion in real time and guides the aspiration that makes the diagnosis, including in joints — hip, shoulder, small joints — where a blind attempt often fails. It is fast, portable and does not stand between the patient and the needle, which is its main advantage over every other modality here. Nothing on this pathway may be allowed to delay the treatment: the fluid is sampled, antibiotics follow the cultures, and the orthopaedic team is told now, because a septic joint is washed out and no imaging finding changes that. An absent or equivocal effusion on ultrasound does not overrule a convincing clinical picture — only the fluid does.

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

How was this decided?

pathwayadult — Adults

Decision support only. Local protocol takes precedence.

Worth asking the referrer (1)

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

Which joint, for how long, and is there a prosthesis in it?
It decides the imaging region, the urgency and whether the arthroplasty pathway applies instead.

Pathways

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

Febrile child — effusion and guided aspiration on the same attendance

Role	Study & protocol	Why this answers the question
First line	Limb radiograph Limb radiograph — two orthogonal views usually appropriate	Radiographs keep their place in the febrile child because the differential is wider than infection and several of the alternatives are visible on a film and time-critical in their own right — a slipped upper femoral epiphysis, Perthes disease, a toddler’s fracture and, rarely, a bone tumour. An anteroposterior pelvis with a frog-lateral is the standard pair for the hip. What fever changes is that they are taken alongside the ultrasound on the same attendance rather than as a gate in front of it: bone change in joint infection appears late, so a normal film here neither confirms nor excludes anything.
First line	Ultrasound musculoskeletal / soft tissue Ultrasound musculoskeletal / soft tissue usually appropriate	Equally endorsed once the child is febrile, and the step that carries the urgency: ultrasound is the sensitive test for a hip effusion, which radiographs cannot show, and it guides aspiration in a joint that is deep and difficult to enter blind. In a febrile child with raised inflammatory markers an effusion is an indication to aspirate, and aspiration is both the diagnostic test and part of the treatment — so a request that asks for this study has the sequence right. Ultrasound cannot distinguish a septic effusion from transient synovitis and must never be used to argue against aspirating a convincing clinical picture.
Problem solving	MRI Hip MRI Hip — routine	MRI is for the child who is not improving, or in whom adjacent osteomyelitis, pyomyositis or a subperiosteal collection is suspected — it defines the extent of infection and what needs draining. It frequently requires sedation or general anaesthesia in a small child, which is precisely why it follows aspiration rather than preceding it, and why booking it first in a febrile child adds hours to a condition where hours matter.

Transient synovitis is the commonest cause of an irritable hip, but it is a diagnosis made after septic arthritis has been excluded, not instead of excluding it — and the fever is precisely why that exclusion is done with a needle rather than a probe.
Neither study on this arm is a reason to wait for the other. If the ultrasound list cannot take the child now, that is a conversation with the orthopaedic team, not a delay to antibiotics after cultures.

Children and adolescents

Role	Study & protocol	Why this answers the question
First line	Limb radiograph Limb radiograph — two orthogonal views usually appropriate	Radiographs come first in a limping child because the differential is wider than infection and several of the alternatives are visible on a film and time-critical in their own right: a slipped upper femoral epiphysis, Perthes disease, a toddler’s fracture and, rarely, a bone tumour. An anteroposterior pelvis with a frog-lateral is the standard pair for the hip, and the dose from two views in a child is small compared with the cost of missing a slipped epiphysis.
Second line	Ultrasound musculoskeletal / soft tissue Ultrasound musculoskeletal / soft tissue	Ultrasound is the sensitive test for a hip effusion, which radiographs cannot show, and it guides aspiration in a joint that is deep and difficult to enter blind. An effusion in a febrile child with raised inflammatory markers is an indication to aspirate; ultrasound cannot distinguish a septic effusion from transient synovitis, and should never be used to argue against aspiration when the clinical picture is convincing.
Problem solving	MRI Hip MRI Hip — routine	MRI is for the child who is not improving, or in whom adjacent osteomyelitis, pyomyositis or a subperiosteal collection is suspected — it defines the extent of infection and what needs draining. It frequently requires sedation or general anaesthesia in a small child, which is precisely why it follows aspiration rather than preceding it.

Transient synovitis is the commonest cause of an irritable hip, but it is a diagnosis made after septic arthritis has been excluded, not instead of excluding it.

Adults

Matches your inputsDefault

Role	Study & protocol	Why this answers the question
First line	Ultrasound musculoskeletal / soft tissue Ultrasound musculoskeletal / soft tissue usually appropriate	Ultrasound confirms and localises the effusion in real time and guides the aspiration that makes the diagnosis, including in joints — hip, shoulder, small joints — where a blind attempt often fails. It is fast, portable and does not stand between the patient and the needle, which is its main advantage over every other modality here. Nothing on this pathway may be allowed to delay the treatment: the fluid is sampled, antibiotics follow the cultures, and the orthopaedic team is told now, because a septic joint is washed out and no imaging finding changes that. An absent or equivocal effusion on ultrasound does not overrule a convincing clinical picture — only the fluid does.
Reasonable alternative	Limb radiograph Limb radiograph — two orthogonal views	Radiographs give the baseline against which later bone changes are judged and exclude fracture, chondrocalcinosis and established erosive disease. They are complementary to aspiration and are never the reason to wait.

Pitfalls

Using imaging to decide whether to aspirate. Neither ultrasound nor MRI can distinguish a septic from a reactive effusion; only the fluid can.
Recording "no effusion on ultrasound" as excluding septic arthritis — a small or loculated effusion in a deep joint is easy to miss, and a convincing clinical picture overrides the scan.
Booking an MRI first in a child, adding sedation and hours of delay to a condition where hours matter.
Treating a normal radiograph as reassuring; bony change in septic arthritis appears late and its absence proves nothing.
Forgetting the joint above and below in a limping child — hip pathology presents as knee pain often enough to be a rule rather than an exception.
Letting an MRI slot sit in front of the theatre list. Definitive treatment is aspiration and surgical washout; MRI answers the question of what else is infected, and in a joint that needs lavage it belongs after source control, not before it.
Aspirating after the first dose of antibiotics when the sample could have been taken first. The culture is what selects the antibiotic and its duration, and in a stable patient a few minutes of sequencing buys the whole microbiological answer — but in a septic one, antibiotics are not held for the list.

Priors — what to pull first

Previous radiographs of the joint separate acute change from long-standing arthropathy, which is the commonest source of false alarm.
For a prosthetic joint, retrieve the operative record and any previous aspirate results before planning.

What makes a good request

Aspiration is both the diagnostic test and part of the treatment. If ultrasound is being requested to decide whether to aspirate a clinically septic joint, the sequence has been inverted.
Aspiration precedes antibiotics wherever it can be done without delaying them — a sample taken after the first dose is frequently sterile, and the organism is what determines the course. In a septic patient antibiotics do not wait for the radiology list; the two are arranged in parallel, and the orthopaedic team is told at the same time as the request is made, because definitive treatment is surgical lavage and that is a theatre decision, not a scan result.
A normal radiograph is expected early: bone changes take days to weeks to appear, so a normal film neither confirms nor excludes anything.
In a limping child, the clinical prediction tools in common use — fever, refusal to weight-bear, raised ESR or CRP and a raised white cell count — stratify risk between transient synovitis and a septic hip. They inform how hard the joint is pursued; none of them is a rule for withholding aspiration from a child whose picture is convincing, and none is offered as a validated instrument on this site.
In a prosthetic joint the pathway is different again and involves the arthroplasty team; aspiration is usually performed under image guidance and with antibiotic timing agreed in advance.
The neonate and small infant are the hardest version of this. There may be no fever and no hot joint — only reduced spontaneous movement of a limb, pain on handling or a baby who has stopped feeding — and at this age septic arthritis and adjacent osteomyelitis routinely coexist because the metaphyseal vessels cross the physis. Ultrasound of the joint and of the adjacent metaphysis is the immediate study, the threshold for aspiration is low, and the orthopaedic team is involved from the outset rather than after imaging.

How these studies are acquired

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

Limb radiograph — two orthogonal views Ultrasound musculoskeletal / soft tissue MRI Hip — routine

Confirm locally

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.