Part 4 · Diagnosis & complications
Diagnosis & the ACR/EULAR score
Seeing urate crystals in joint fluid settles the diagnosis; without them, the 2015 ACR/EULAR criteria add up points, and 8 or more classifies as gout. Turn the compensator to see the crystals change colour, score a case in the calculator, then step through the diagnostic pathway.
Under the polarised microscope
Schematic field, not to scale. The colour rule follows published descriptions of compensated polarised microscopy. 186,322,451
2015 ACR/EULAR classification calculator
These are classification criteria, built so that research studies enrol comparable patients. Their authors say they are not meant for making a diagnosis in clinic, and that joint or tophus aspiration remains essential to establish one. 290 There are no exclusion criteria, because gout can coexist with other diseases and microscopy can miss crystals. 290
Examples are worked from the published weights, not patients from a study. 290
Performance in the independent SUGAR validation set (n = 330), with microscopy as the reference. The box that matches the items you have used is outlined. At the chosen cut-off, 13.9% were false negatives and 10.5% false positives. 290
A primary-care rule without joint fluid
The Janssens (Nijmegen) rule was built in 328 Dutch primary-care patients with one hot joint, with crystals as the reference. Its 7 items had an AUC of 0.85 (0.81–0.90). 180 It is a probability aid, not a substitute for aspiration when infection is possible. 226
Weights and bands from Janssens 2010 as reproduced by Lee 2015. Band prevalences are from the derivation study, as reported by Lee 2015. Secondary-care validation: a score of 8 or more had a PPV of 0.87 and a score of 4 or less an NPV of 0.95 (AUC 0.86). 180,204,226
Imaging that sees urate
Ultrasound: the double-contour sign
A bright, irregular line over the surface of the cartilage, running parallel to the bone line. A real one stays when the probe angle changes; an artefact disappears. 290 It was seen in 92% of 37 crystal-proven gouty joints and in none of 33 controls. 410 In SUGAR, any of double contour, tophus or snowstorm had sensitivity 76.9% and specificity 84.3%. 301
It fades: it vanished in every joint where serum urate stayed under 6 mg/dL (360 µmol/L) for 7–18 months. 411 EULAR names ultrasound as the first imaging choice when crystals cannot be sought. 332
Dual-energy CT: colour-coded urate
Scans at 80 and 140 kV separate urate from calcium. Nail-bed, skin, motion, beam-hardening and vascular artefacts do not count. 290 In one accuracy study sensitivity was 0.90 and specificity 0.83; all 4 false negatives were first flares with symptoms under 6 weeks, and all 7 false positives had advanced knee osteoarthritis. 25
Pooled joint-level sensitivity was 0.83, but only 0.55 in disease of 6 weeks or less. 144 Some artefact appeared in 90% of 50 scans. 247 Established A negative DECT does not exclude early gout. 25,144
X-ray: the overhanging edge
The criteria erosion is a cortical break with a sclerotic margin and an overhanging edge. DIP joints and the "gull wing" pattern are excluded because they occur in osteoarthritis. 290 X-rays add little early; in late or severe gout, asymmetrical swelling (LR 4.13) and subcortical cysts without erosion (LR 6.39) help. 445
Erosions are driven by tophus: all 56 CT erosions larger than 7.5 mm contained tophus. 71
Early evidence One meta-analysis found DECT more accurate overall (0.89/0.91), but ultrasound more sensitive in disease of 2 years or less (0.93 vs 0.75). 365
The diagnostic pathway
A simplified teaching sequence drawn from the guidelines and studies cited in the notes. Real work-ups overlap and loop back.
Trained eyes agree
Trained observers identified MSU crystals with sensitivity 95.3%, specificity 97.2% and kappa 0.93; for CPP, 92.7%, 92.1% and 0.79. 239 In an online test of 110 participants, only 39% met the primary outcome. 22
Crystals stay between flares
In quiet, previously inflamed knees or first MTPs, crystals were found in all 43 people not on urate-lowering drugs and in 34 of 48 who were; 90% of positive samples showed them in the first 5 microscope fields. 307 In untreated gout, 36 of 37 previously inflamed knees held crystals. 306
Handle the sample well
MSU crystal concentration falls over time at room temperature but not in the fridge, and dried cytospin preparations allowed delayed analysis for at least 12 months. 149 A lab report should state birefringence, shape, inside or outside cells, and number per high-power field. 186
A normal urate in a flare
In 41 patients, serum urate was normal during the flare in 20 (49%): mean 7.5 mg/dL (446 µmol/L) in flares vs 8.5 mg/dL (506 µmol/L) between them. 416 In 339 people, 14% had a urate of 6 mg/dL (357 µmol/L) or less at the flare. 354
Not on urate alone
EULAR 2018: gout should not be diagnosed on hyperuricaemia alone, and every person with suspected gout should have a search for crystals. 332 In people with asymptomatic hyperuricaemia, a first-MTP double contour was found in 0.31 (pooled). 384 G-CAN calls this "asymptomatic MSU crystal deposition", not gout. 32
How the older criteria did
In the same validation set, ACR 1977 had sensitivity 1.00 but specificity 0.51; Rome 0.97/0.78; New York 1.00/0.78; Mexico 1.00/0.44; Netherlands 0.95/0.59. 290 Across older sets, sensitivity was better in established disease and specificity in early disease. 404