Musculoskeletal & Integumentary · Topic 4 of 11
Acute Monoarthritis: Septic Arthritis & Gout
Musculoskeletal · acute hot joint
Acute Monoarthritis: Septic Arthritis & Gout
A hot, swollen joint is septic until the tap says otherwise: aspirate first, then treat infection or crystals.
Key numbers
Synovial WBC above 50,000 suggests infectionWBC > 50,000/mcL
Gout flare: low-dose colchicine1.2 mg + 0.6 mg at 1 h
Allopurinol first-line; target urate below 6Urate < 6 mg/dL
01 · DoArthrocentesis first in acute monoarthritis
Aspirate any acutely hot, swollen joint before antibiotics: send cell count and differential, Gram stain, culture and crystal analysis. If infection cannot be excluded, treat as septic arthritis pending culture.
02 · Defined bySynovial WBC above 50,000 suggests infection
Septic joint fluid usually has a WBC above 50,000/mcL (sometimes above 100,000) with more than 95% neutrophils; a count under 50,000 does not exclude infection, especially early or partially treated.
03 · GiveGout flare: low-dose colchicine
Treat a gout flare early with low-dose colchicine — 1.2 mg, then 0.6 mg one hour later, best within 12–24 hours of onset — or an NSAID or glucocorticoid; all are first-line (ACR 2020).
01
Causes & risk factors
Why it happensRisk: damaged or prosthetic joint, IVDU, immunosuppressionSeptic arthritis risk rises with rheumatoid arthritis or other joint damage, a prosthetic joint, prior joint infection, injection drug use, diabetes, hemodialysis and immunosuppression; S. aureus is the most frequent adult pathogen.
02
Presentation
What you'll seeGonococcal: tenosynovitis, pustules, migratory arthralgiaSuspect disseminated gonococcal infection in sexually active young adults with migratory polyarthralgia, tenosynovitis and pustular or petechial skin lesions; test urethra, cervix, rectum and pharynx by NAAT, as joint cultures are often negative.
03
Diagnosis & monitoring
Work-upNeedle-shaped, negatively birefringent crystals mean gout must knowGout is confirmed by needle-shaped, strongly negatively birefringent monosodium urate crystals in synovial fluid; crystals do not exclude a coexisting infection, so still send Gram stain and culture.
Gout fluid WBC overlaps the septic rangeSynovial fluid in a gout flare is inflammatory, usually 2,000–100,000 WBC/mcL, so the count alone cannot separate crystal from septic arthritis.
Serum urate may be normal during a flareAt least 30% of patients have a normal serum urate during an acute flare, so a normal level does not exclude gout; recheck it after the flare to guide therapy.
04
Do right now
In this orderFirst moves
1
→Arthrocentesis first in acute monoarthritis
2
→Septic arthritis: IV antibiotics plus joint drainage
3
Gout flare: low-dose colchicine1.2 mg + 0.6 mg at 1 h
Why each step
Arthrocentesis first in acute monoarthritisAspirate any acutely hot, swollen joint before antibiotics: send cell count and differential, Gram stain, culture and crystal analysis. If infection cannot be excluded, treat as septic arthritis pending culture.
Septic arthritis: IV antibiotics plus joint drainageTreat nongonococcal septic arthritis with IV antibiotics and drainage — daily large-bore needle aspiration, arthroscopic washout or arthrotomy; arthroscopy has fewer complications. Splint briefly, then start range-of-motion exercises.
Gout flare: low-dose colchicineTreat a gout flare early with low-dose colchicine — 1.2 mg, then 0.6 mg one hour later, best within 12–24 hours of onset — or an NSAID or glucocorticoid; all are first-line (ACR 2020).
05
Management
OrdersEmpiric vancomycin; add gram-negative cover if at riskFor gram-positive cocci or a negative Gram stain, start vancomycin (MRSA is common); add an antipseudomonal agent such as ceftazidime, cefepime or piperacillin-tazobactam with injection drug use, immunosuppression or a vascular catheter.
Gonococcal arthritis: ceftriaxone 1 g IV dailyTreat gonococcal arthritis with ceftriaxone 1 g IV daily until 24 hours after improvement, then oral therapy guided by susceptibility for 7–14 days; add doxycycline if chlamydia is not excluded.
Start urate-lowering therapy for tophi or frequent flaresStart urate-lowering therapy for tophaceous gout, radiographic gout damage or frequent flares (2 or more a year) — strong ACR 2020 recommendations; asymptomatic hyperuricemia is not treated.
Allopurinol first-line; target urate below 6Allopurinol is first-line, even in CKD: start at 100 mg/day or less (lower in CKD) and titrate to a serum urate below 6 mg/dL; give colchicine or NSAID prophylaxis for at least 3–6 months.
06
Red flags & don'ts
Avoid harmTest HLA-B*5801 before allopurinol in high-risk groups avoidTest for HLA-B*5801 before allopurinol in patients of Southeast Asian descent (Han Chinese, Korean, Thai) and African American patients — carriers risk severe hypersensitivity. Allopurinol and febuxostat are contraindicated with azathioprine or mercaptopurine.
Reduce colchicine with CKD or CYP3A4 inhibitors interactionReduce or avoid colchicine in renal insufficiency and with CYP3A4 inhibitors (clarithromycin, azole antifungals, grapefruit juice); with statins or macrolides it can cause neuromyopathy. Diarrhea is the common adverse effect.
07
The rules
Know theseNo intra-articular steroid until infection is excludedAvoid intra-articular corticosteroids and potent anti-inflammatory therapy until a bacterial cause is definitively excluded, because initial bacteriologic tests can be falsely negative.
Continue urate-lowering therapy through a flareDo not stop urate-lowering therapy during a flare; it can even be started during a flare if anti-inflammatory treatment is given.
08
Septic arthritis or acute gout?
Commonly confused| Synovial WBC | Usually > 50,000/mcL, > 95% neutrophils |
|---|---|
| Crystals | None — but crystals can coexist with infection |
| Gram stain and culture | Gram stain positive in only 50–75%; culture definitive |
| Typical joint | Any joint; damaged or prosthetic joints at highest risk |
| Treatment | IV antibiotics plus joint drainage |
09
More detail
ReferenceShow 7 reference facts
Staphylococcal and most other bacterial joint infections need at least 3–4 weeks of antibiotics (IV then oral); streptococcal and Haemophilus infections usually need about 2 weeks of oral therapy after IV.
If colchicine, NSAIDs and glucocorticoids are contraindicated or fail, the IL-1 inhibitor anakinra 100 mg SC daily can treat a flare without affecting glucose, kidney function or fluid balance.
Glucocorticoid options for a flare: intra-articular injection when infection is not suspected, or oral prednisone about 0.5 mg/kg daily with a taper, continued a few days past resolution.
A first gout flare is usually monoarticular, often nocturnal, and most often hits the first metatarsophalangeal joint (podagra); the instep, ankle, knee, wrist and elbow are also common.
Gout flares are triggered by sudden urate shifts — starting urate-lowering therapy, thiazide or loop diuretics, alcohol, purine-rich food, surgery or acute illness; decreased renal excretion is the main cause of hyperuricemia.
For hard-to-reach joints such as the hip, sacroiliac or sternoclavicular joint, use ultrasound or MRI to find effusion or abscess and guide aspiration; imaging cannot distinguish infection from crystal arthritis.
Gram stain shows organisms in only 50–75% of acute bacterial joint infections, most often staphylococci; culture is definitive but less sensitive after prior antibiotics.
Crystals in the fluid do not clear the joint of infection — culture every hot joint.
Sources · Merck Manual Professional — Acute Infectious Arthritis (reviewed Dec 2024) · Merck Manual Professional — Gout · FitzGerald JD et al. 2020 American College of Rheumatology Guideline for the Management of Gout. Arthritis Care Res 2020;72:744–760 (abstract via Europe PMC) · Starter sheet built from the verified AGACNP fact map (facts reviewed 2026-10-15).