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OverviewAetiologyPathophysiologyClinical features and red flagsInvestigationsDifferential diagnosisManagementComplicationsMonitoring and follow-upSpecial situationsCommon pitfallsQuestionsSources
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Educational draft · awaiting clinical reviewThe full textbook explains uncertainty but does not replace live national or local guidance, specialist advice, or current prescribing information.

Acute monoarthritis

Rheumatology · UK scope · Sources checked 27 Aug 2026 · Clinical review pending

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Time-critical presentation

A hot, severely painful or rapidly restricted joint is septic arthritis until urgent assessment proves otherwise, particularly with fever, bacteraemia risk, immunosuppression, a prosthesis or recent surgery. Stabilise sepsis, take blood cultures and aspirate promptly; do not delay necessary antibiotics in an unstable patient.

Overview

Assess one newly painful swollen joint as a time-critical diagnostic problem, obtain synovial fluid safely, treat suspected infection without avoidable delay, and distinguish crystal, traumatic and inflammatory causes.

Acute monoarthritis means new inflammation or dysfunction centred on one joint, usually evolving over hours to days. Establish exact onset, trauma, fever, recent infection, skin breach, injection, surgery, prosthesis, antimicrobial exposure, immunosuppression, anticoagulation and previous attacks. Ask about sexual exposure and disseminated infection when relevant without stereotyping. Examine physiology, skin, heart, other joints and the complete limb neurovascular status.

Septic arthritis and crystal arthritis overlap clinically. Fever may be absent; CRP may be normal early; and crystals may coexist with bacterial infection. The safest decision is therefore based on aspiration, culture, blood cultures and illness severity rather than a single score. Ultrasound can confirm a difficult effusion and guide aspiration, but it does not identify the organism or replace synovial analysis.

Management is simultaneous rather than sequential when infection is plausible: resuscitate, obtain cultures, arrange drainage and start locally recommended intravenous antibiotics after samples if the patient is stable enough. Native and prosthetic joints follow different surgical and microbiological pathways. Once culture results and clinical response are available, narrow treatment, ensure adequate source control and revisit crystal or inflammatory alternatives.

Aetiology

01

Joint infection

Haematogenous bacterial seeding, direct inoculation or spread from adjacent tissue can infect native or prosthetic joints, with risk increased by immunosuppression and damaged joints.

02

Crystal inflammation

Monosodium urate or calcium pyrophosphate crystals activate intense innate inflammation, producing abrupt pain, swelling and erythema that can closely mimic infection.

03

Traumatic and inflammatory causes

Haemarthrosis, fracture, internal derangement, palindromic inflammation and early systemic arthritis may initially affect a single joint before the wider pattern emerges.

Pathophysiology

  1. 1
    Synovial inflammatory surge

    Bacteria, crystals or immune activation recruit neutrophils and inflammatory mediators into the synovial space, rapidly increasing pressure and pain.

  2. 2
    Cartilage vulnerability

    Bacterial enzymes, neutrophil products and raised intra-articular pressure can damage cartilage within a short period, making source control urgent.

  3. 3
    Systemic dissemination

    An infected joint can seed or arise from bloodstream infection, while inflammatory vasodilatation and cytokine release may progress to sepsis and organ dysfunction.

Clinical features and red flags

Septic native joint phenotypeRed flag

Acute severe pain, effusion and markedly painful passive movement with fever, rigors, skin infection or bacteraemia risk requires urgent aspiration and source-control planning.

Prosthetic-joint emergencyRed flag

New pain, swelling, wound change or systemic illness around a prosthesis may represent periprosthetic infection and needs urgent orthopaedic review before casual aspiration or oral antibiotics.

Crystal-arthritis phenotype

An explosive attack peaking within hours, podagra, tophi or previous self-limited episodes supports gout; acute knee or wrist disease in older people suggests calcium pyrophosphate arthritis.

Haemarthrosis or injury

Trauma, anticoagulation, bleeding disorder, rapid tense swelling or radiographic injury suggests blood in the joint and requires structural and haemostatic assessment.

Periarticular inflammation

Preserved passive range with focal superficial tenderness points toward bursitis, tendon disease or cellulitis, although adjacent infection can spread and still warrants close review.

Investigations

  1. 01
    Synovial-fluid aspirationFirst step
    Why
    Obtain the decisive specimen for microbiology and crystal identification before treatment changes the yield.
    Interpretation and limitations
    Record appearance and volume; send Gram stain and culture even when crystals are seen. Cell counts overlap across infection and sterile inflammation, and inadequate volume should prioritise culture.
  2. 02
    Blood cultures and sepsis blood panel
    Why
    Detect bacteraemia, organ dysfunction and a systemic inflammatory response in suspected septic arthritis.
    Interpretation and limitations
    Take at least the locally specified culture sets before antibiotics when feasible. FBC, CRP, renal, liver and lactate results support severity assessment but cannot exclude a local infection.
  3. 03
    Compensated polarised-light microscopy
    Why
    Identify monosodium urate and calcium pyrophosphate crystals within synovial fluid.
    Interpretation and limitations
    Needle-shaped, strongly negatively birefringent urate and rhomboid, weakly positively birefringent CPP crystals support diagnoses; expertise and prompt specimen handling affect sensitivity.
  4. 04
    Ultrasound and plain radiography
    Why
    Locate an effusion, guide difficult aspiration and identify fracture, chondrocalcinosis, prosthesis or established structural disease.
    Interpretation and limitations
    Normal early radiographs do not

Differential diagnosis

01

Septic arthritis

Marked pain on passive movement, systemic illness, bacteraemia risk or a prosthesis raises concern, but mild fever or normal blood tests cannot exclude infection.

02

Gout or CPP crystal arthritis

Rapid onset, previous stereotyped attacks, tophi or chondrocalcinosis supports crystal disease; definitive identification requires correctly examined synovial fluid when uncertainty matters.

03

Trauma or haemarthrosis

Injury, anticoagulation, bleeding disorder or lipohaemarthrosis redirects assessment towards structural damage and bleeding while infection may still coexist.

04

Periarticular mimic

Bursitis, cellulitis and tendinopathy may appear joint-centred, but passive range and precise localisation can show that the synovial cavity is not the primary source.

Management

01Unwell hot jointResuscitate, culture and drainFirst stepAcute monoarthritis with sepsis, systemic toxicity, rapidly worsening pain or high-risk host features.+
  1. 1Perform ABCDE, obtain intravenous access, analgesia, blood cultures and organ-function tests, and contact orthopaedics and microbiology immediately.
  2. 2Aspirate the joint urgently unless this would delay life-saving treatment; start locally approved intravenous antibiotics after samples in a stable patient or immediately if unstable.
  3. 3Give orthopaedics explicit ownership of native-joint drainage and source control. Select repeated aspiration, arthroscopic washout or open drainage according to the joint, macroscopic or radiological damage, organism and response; review cultures and clinical progress with microbiology, then narrow treatment when results permit.
02Stable but uncertainLet fluid answer the central questionA new effusion without shock where infection, crystals and bleeding remain plausible.+

Complications

01

Rapid joint destruction

Untreated septic arthritis can erode cartilage and subchondral bone, leaving permanent pain, instability and severe functional loss despite later microbiological cure.

02

Sepsis and metastatic infection

Bacteraemia may cause shock, endocarditis, vertebral infection or other seeded sites, especially with Staphylococcus aureus or delayed source control.

03

Recurrent crystal disease

Repeated gout or calcium pyrophosphate attacks cause disability, and uncontrolled urate deposition can produce tophi, erosions and urate nephrolithiasis.

Monitoring and follow-up

  • Record temperature, heart rate, blood pressure, consciousness, urine output and lactate trajectory when systemic infection is possible.
  • Trend joint pain, passive range, swelling and ability to bear weight after aspiration or washout; persistent deterioration suggests inadequate source control.
  • Review blood and synovial cultures until final, including after discharge, with a documented person responsible for acting on late growth.
  • Monitor renal, hepatic and haematological parameters according to the antimicrobial or anti-inflammatory regimen and comorbid risk.
  • After a crystal flare, review recurrence, serum urate at an appropriate stable interval and adherence or tolerance if urate-lowering therapy begins.

Special situations

Crystals and bacteria can coexist

Finding urate or CPP crystals explains inflammation but does not sterilise the sample; maintain culture and clinical review when infection remains plausible.

Appearance is not enough

Cloudy or purulent fluid raises concern, yet bloody, inflammatory and partially treated samples overlap, so laboratory analysis and trajectory are indispensable.

Deep joints hide

Hip, shoulder and sacroiliac infection may lack an obvious superficial effusion; inability to move or bear weight can be the strongest clue.

Prostheses need a separate route

Sampling technique, antibiotic timing and surgical strategy affect later diagnosis and implant retention, so involve the responsible orthopaedic team early.

Serum urate is a longitudinal measure

It supports long-term gout assessment but can be normal during an acute flare and never substitutes for aspiration when sepsis is possible.

Common pitfalls

  1. 01

    Starting oral antibiotics for a prosthetic-joint presentation before cultures and orthopaedic discussion in a stable patient.

  2. 02

    Excluding septic arthritis because the patient has no fever, a normal radiograph or only a modest CRP rise.

  3. 03

    Stopping microbiological investigation after crystals are reported in an immunosuppressed or systemically unwell patient.

  4. 04

    Injecting corticosteroid into a hot joint before infection has been adequately excluded.

  5. 05

    Allowing a difficult aspiration to end assessment rather than arranging image guidance or operative sampling.

Practice

Two practice questions

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Question 1 of 2
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RheumatologyOriginal SBA

Crystals in a hot joint

A 71-year-old with fever and an acutely swollen knee has calcium pyrophosphate crystals reported in turbid synovial fluid. Which interpretation is safest while culture is pending?

Choose one answer, then submit
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Sources and review status5 sources · checked 27 Aug 2026 · clinical review pending+
Sources

Sources and review status

National guidance is shown before implementation-dependent detail. Typical adult dose examples remain subject to patient factors, contraindications and the live BNF or specialist protocol. Source check completed 27 Aug 2026; clinical approval remains outstanding.

  • NICE NG219 goutCrystal arthritis diagnosis, aspiration and flare management.
  • NICE NG253 suspected sepsis in people aged 16 or overRecognition, cultures, antibiotics and urgent escalation for systemic infection.
  • UK SMI B26 sterile-site fluidsMicrobiological collection and processing standards for synovial and other sterile-site fluids.
  • British Society for Rheumatology guidelinesSpecialist standards and guideline access for hot swollen joints and inflammatory disease.
  • EBJIS guideline for management of septic arthritis in native joints (SANJO)Current consensus guidance for aspiration and blood-culture timing, empirical-treatment selection, native-joint drainage, microbiology review and de-escalation; local resistance and prescribing protocols still determine the exact UK regimen.

Editorial contact: Doctor’s Passport editorial team. Source-check date is separate from clinical approval. Report a content problem · Content changes

Authoring stateComplete draftClinical stateAwaiting reviewJurisdictionUnited Kingdom

Key points

  • Treat acute monoarthritis as septic arthritis until infection has been actively considered and, where feasible, synovial fluid has been obtained.
  • Examine whether pain is truly intra-articular: severe restriction of both active and passive movement supports joint pathology; focal bursal or tendon pain may preserve passive range.
  • Take blood cultures before antibiotics when systemic infection is suspected, but never postpone antimicrobials for sampling in a haemodynamically unstable patient.
  • Aspirate using aseptic technique and send sufficient fluid for Gram stain, bacterial culture, cell count where locally available and compensated-polarised-light microscopy for crystals.
  • Neither a low synovial white-cell count nor identified urate or CPP crystals reliably excludes concurrent infection; culture and clinical trajectory remain essential.
  • Serum urate may fall during a gout flare and cannot diagnose the cause of an acutely swollen joint by itself.
  • A painful prosthetic joint, joint after recent procedure, or hot joint in an immunosuppressed patient needs early orthopaedic and microbiology involvement.
  • After immediate danger is controlled, determine whether the episode is the first manifestation of gout, inflammatory arthritis, haemarthrosis or structural injury and plan prevention.
exclude infection. Chondrocalcinosis supports CPP deposition but is neither required nor sufficient to explain the acute episode.
  • 05
    Targeted infection testing
    Why
    Identify gonococcal, mycobacterial, fungal or adjacent sources when exposure, host factors or chronicity suggests them.
    Interpretation and limitations
    Discuss sample type and prolonged or molecular testing with microbiology before collection; a routine negative culture after antibiotics may require repeat or operative sampling.
    • 1
    Document risk factors and full examination, withhold unnecessary antibiotics until prompt synovial and blood samples are secured, and avoid injecting corticosteroid into an undiagnosed hot joint.
  • 2EscalationRequest culture and crystal examination together, add imaging for trauma or a difficult deep joint, and provide analgesia while maintaining a clear escalation threshold.
  • 3Review preliminary microscopy and the patient's trajectory the same day; do not discharge solely because crystals are present or inflammatory markers are modest.
    • 03Crystal disease confirmedTreat the flare and prevent recurrenceA stable patient has a compatible phenotype and crystals with no current microbiological or clinical evidence of infection.+
    1. 1Choose an anti-inflammatory option using renal, gastrointestinal, cardiovascular, anticoagulant and interaction risks, and give written advice on expected response and urgent return features.
    2. 2AlternativeRecheck if improvement is not prompt because infection, fracture or an alternative inflammatory diagnosis may have been missed despite the initial result.
    3. 3Assess indications for long-term urate-lowering therapy after the flare and address medicines, kidney disease, alcohol and other modifiable contributors without blame.
    Key medicines and prescribing safety1 treatment · regimens, roles and cautions+
    First-line antimicrobial treatment when septic arthritis remains likely, given after diagnostic sampling where stability allows and always alongside urgent drainage and source-control planning.

    Empirical intravenous antibacterial therapy for suspected native-joint septic arthritis

    There is no single universal UK agent or dose. If the patient is stable, obtain synovial fluid and blood cultures first, then start the current local intravenous regimen; in sepsis or septic shock, take blood cultures promptly but do not delay time-critical intravenous antibiotics for joint aspiration. Microbiology must select and document the agent, dose and interval using Gram stain, likely source, prior cultures, allergy, renal and hepatic function, native versus prosthetic joint, and MRSA, resistant Gram-negative or gonococcal risk.

    Do not use a casual oral regimen before sampling in a stable undiagnosed hot joint. A prosthetic joint follows a separate implant-infection pathway. Review cultures, susceptibilities, renal function, toxicity and clinical response promptly; narrow, change route and set duration only with orthopaedic and microbiology oversight, reassessing source control if improvement is inadequate.