Gout: an animated, cited guide

This page needs JavaScript for its animations. The outline and key answers are below, and every claim in the full version carries a numbered citation.

Not medical advice. Gout Companion is an educational synthesis of published literature. It does not replace a clinician.

What is in it

What gout is Who gets it, the numbers, and the four stages from high urate to tophi.
Where urate comes from Purines, xanthine oxidase, and how the kidney and gut handle urate.
Crystals vs serum urate Why crystals form above about 6.8 mg/dL and dissolve below the target.
A flare, hour by hour From a shed crystal to the NLRP3 inflammasome, interleukin-1β and neutrophils, then resolution.
Colchicine inside the cell From the pill to the neutrophil to the microtubule.
Treating a flare Colchicine, NSAIDs, glucocorticoids, IL-1 blockers and ice.
Colchicine dosing and safety Published doses, kidney function and drug interactions.
How urate-lowering drugs work Allopurinol, febuxostat, uricosurics and pegloticase.
Treat to target Titrating to below 6 mg/dL, and why flares rise in the first months.
Safety of urate-lowering drugs HLA-B*58:01, the CARES and FAST trials, interactions.
Diagnosis and the ACR/EULAR score Crystals under polarised light, imaging, and the 2015 classification calculator.
The differential Calcium pyrophosphate crystal arthritis, septic arthritis, cellulitis and others.
Gout, kidneys and heart Chronic kidney disease, stones, blood pressure and cardiovascular risk.
Living with gout Diet, alcohol, weight, other medicines and a flare plan.
Nursing care Assessment, giving the medicines, teaching and documentation.
Toward a cure Dissolving crystals for good, and the research pipeline.
History and people From podagra to polarised light and allopurinol.
Flashcards, quiz and glossary Revision for students.
Sources Every reference behind the app.

Part 4 · Diagnosis & complications

The differential

A hot, swollen joint is not always gout: septic arthritis, CPP crystal arthritis and cellulitis can look the same, and infection can sit alongside crystals. Pick a clue to light up the conditions it points to, then sweep the joint-fluid white count to see how the odds of infection change.

Red flagsThink infection, and aspirate the joint

Fever, sweats or rigorsBut their absence does not exclude infection: fever had a sensitivity of only 57%. ²⁴⁸
Prosthetic joint, diabetes, RA, skin infection or HIVEach is a risk factor for septic arthritis. ²⁴⁸
A shoulderSeptic shoulders in crystal arthritis were bacteraemic in 90%. ³⁰⁵
A ruptured or discharging tophusRuptured, infected tophi were the commonest route in 30 people with septic gout. ⁴⁴²
An atypical pictureEULAR 2018 says atypical presentations should be investigated further. ³³²

Crystals do not exclude infection. Infection was found in 1.5% of 265 crystal-positive aspirates, and in 5% of crystal-positive joints in another series. ^305,363 Coexistence is very unlikely only when the Gram stain is negative, CRP is under 100 mg/L and the joint white count is under 10,000/µL. ³⁰⁵

Side by side

Pick a clue

Compare one feature

A dash means the sources used here give no distinguishing feature for that cell. A trauma or fracture row is left out because no source was checked for it.

Joint fluid: how likely is infection?

White count

Likelihood ratios are pooled from 14 studies (6,242 patients, 653 with septic arthritis). Each LR belongs to its own cut-off (above or below it), not to the band between two cut-offs. ²⁴⁸

Exam hookGout and septic arthritis can coexist: crystals in the fluid do not exclude infection (1.5–5%). Synovial white count: under 25,000/µL LR 0.32; over 50,000 LR 7.7; over 100,000 LR 28. Fever is absent in many: sensitivity 57%. ^248,305,363

In plain wordsA hot, swollen joint with fever or chills needs a doctor the same day. A joint infection can look just like gout, and both can happen at once. The surest way to tell them apart is to take fluid from the joint and test it for crystals and for germs.

CPP crystal arthritis: its own criteria

The 2023 ACR/EULAR CPPD criteria have an entry step (joint pain, swelling or tenderness), exclude cases fully explained by another disease, and classify at a score above 56. Sensitivity and specificity were 92.2%/87.9% in derivation and 99.2%/92.5% in validation. Crowned dens syndrome is a sufficient criterion. ^2,3

When gout turns septic

In 30 people with septic gout, 30% had a normal blood count and 10% a joint count under 6,000/mm³. S. aureus caused 16 cases, and a third had no fever. ⁴⁴² A 50,000/µL cut-off missed 39% of 49 culture-proven septic joints, and 55% had a negative Gram stain. ²⁶⁰

A gout score is not an infection test

Comparing 82 gout and 54 septic joints, the Janssens score averaged 8.6 vs 3.6 (AUC 0.933), but 3 of the 54 septic joints still scored "high". ²²⁶

Cellulitis and its mimics

Cellulitis has no effective diagnostic test, and an organism is identified in about 15%. ³²¹ Of 259 people admitted with "lower-limb cellulitis", 30.5% had another diagnosis. ⁴²⁹

Rheumatoid and psoriatic arthritis

RA 2010: synovitis in at least 1 joint and a score of 6 or more of 10 from joint count, serology, acute-phase reactants and duration. ⁶ CASPAR: inflammatory joint disease plus 3 or more points (specificity 0.987, sensitivity 0.914). ³⁹⁹

Reactive arthritis

Acute oligoarthritis of larger joints, 2–4 weeks after a gut or urogenital infection, with an incidence of 0.6–27 per 100,000 a year. HLA-B27 is not a diagnostic test. ¹⁵⁵