Diagnosis of gout by ultrasound

OBJECTIVES

To establish the usefulness of ultrasonography (US) for diagnosing gout and to determine whether there are sonographic features that are characteristic for gout but not for other arthropathies.

METHODS

We retrospectively compared joint images of gout patients with matching images from patients with other rheumatic conditions. Images of 37 joints of 23 patients with monosodium urate (MSU) crystal-proven gout were reviewed. MSU crystals were identified in at least one joint in each patient. Our control group had 23 randomly selected patients with 33 examined joints with rheumatic conditions other than gout.

RESULTS

Specific diagnostic features included a hyperechoic, irregular band over the superficial margin of the articular cartilage described as a double contour sign in 92% of gouty joints and in none of the controls (P < 0.001); hypoechoic to hyperechoic, inhomogeneous material surrounded by a small anechoic rim, representing tophaceous material, was seen in all gouty metatarsophalangeal (MTP) joints, in all metacarpophalangeal (MCP) joints and in none of the controls (P < 0.001); erosions adjacent to tophaceous material were seen in 65% of MTP joints and in 25% of MCP joints. One erosion was seen in a MTP joint in a control patient with psoriatic arthritis.

CONCLUSIONS

US can detect deposition of MSU crystals on cartilaginous surfaces (P < 0.001) as well as tophaceous material and typical erosions. US may serve as a non-invasive means to diagnose gout.

Dual energy computed tomography in tophaceous gout

OBJECTIVE

To evaluate the potential utility of a dual energy CT (DECT) scan in assessing urate deposits among patients with tophaceous gout, and obtain computerised quantification of tophus volume in peripheral joints.

METHODS

20 consecutive patients with tophaceous gout and 10 control patients with other arthritic conditions were included. DECT scans were performed using a renal stone colour-coding protocol that specifically assessed the chemical composition of the material (ie, urate coloured in red, calcium coloured in blue). An automated volumetric assessment of DECT was used to measure the volume of urate deposits in all peripheral joint areas.

RESULTS

All 20 patients with gout showed red colour-coded urate deposits on their DECT scans, whereas none of 10 controls showed urate deposits. DECT scans revealed a total of 440 areas of urate deposition in 20 patients, whereas physical examination showed 111 areas of urate deposition (mean 22 vs 6 per patient, respectively, p<0.001). Total urate volume in a given patient ranged from 0.63 cm(3) to 249.13 cm(3), with a mean of 40.20 cm(3).

CONCLUSIONS

DECT scans can produce obvious colour displays for urate deposits and help to identify subclinical tophus deposits. Furthermore, tophus volume can be measured by DECT scans through an automated volume estimation procedure.

Dual-energy CT for the diagnosis of gout: an accuracy and diagnostic yield study

OBJECTIVES

To assess the accuracy of dual-energy CT (DECT) for diagnosing gout, and to explore whether it can have any impact on clinical decision making beyond the established diagnostic approach using polarising microscopy of synovial fluid (diagnostic yield).

METHODS

Diagnostic single-centre study of 40 patients with active gout, and 41 individuals with other types of joint disease. Sensitivity and specificity of DECT for diagnosing gout was calculated against a combined reference standard (polarising and electron microscopy of synovial fluid). To explore the diagnostic yield of DECT scanning, a third cohort was assembled consisting of patients with inflammatory arthritis and risk factors for gout who had negative synovial fluid polarising microscopy results. Among these patients, the proportion of subjects with DECT findings indicating a diagnosis of gout was assessed.

RESULTS

The sensitivity and specificity of DECT for diagnosing gout was 0.90 (95% CI 0.76 to 0.97) and 0.83 (95% CI 0.68 to 0.93), respectively. All false negative patients were observed among patients with acute, recent-onset gout. All false positive patients had advanced knee osteoarthritis. DECT in the diagnostic yield cohort revealed evidence of uric acid deposition in 14 out of 30 patients (46.7%).

CONCLUSIONS

DECT provides good diagnostic accuracy for detection of monosodium urate (MSU) deposits in patients with gout. However, sensitivity is lower in patients with recent-onset disease. DECT has a significant impact on clinical decision making when gout is suspected, but polarising microscopy of synovial fluid fails to demonstrate the presence of MSU crystals.

“Crystal Clear”—Sonographic Assessment of Gout and Calcium Pyrophosphate Deposition Disease

OBJECTIVES

To date, high-resolution ultrasound (US) has not been fully exploited in the field of crystalline arthropathy. Both gout and calcium pyrophosphate deposition (CPPD) disease are significant diseases within the purview of the rheumatologist. The aim of this pictorial review was to present the principal findings in patients with crystal deposition in gout and CPPD.

METHODS

US pictures were obtained from 60 consecutive patients, 34 with CPPD disease and 26 with gout, whose diagnosis was confirmed by synovial fluid analysis. The US examinations were performed using the following US systems: Diasus (Dynamic Imaging, Livingstone, UK) and Logiq 9 (General Electric Medical Systems, Milwaukee, WI).

RESULTS

Pictorial evidence of the principal US findings in gout includes monosodium urate (MSU) deposition on the surface of articular cartilage, various patterns within synovial fluid ranging from completely anechoic fluid to collections filled with aggregates of variable shape and echogenicity, microdeposition within tendons, and tophus formation. In CPPD, the hallmark US features include crystal deposition within articular cartilage, calcification of fibrocartilage, together with focal crystal deposition within tendons.

CONCLUSION

US is an impressive imaging modality in crystalline arthropathy. The anatomical location of the crystal deposits, clearly depictable by US, allows differentiation between MSU and CPPD aggregates.

Urate crystal deposition in asymptomatic hyperuricaemia and symptomatic gout: a dual energy CT study

BACKGROUND

The aim of this study was to compare the frequency and volume of dual energy CT (DECT) urate deposits in people with asymptomatic hyperuricaemia and symptomatic gout.

METHODS

We analysed DECT scans of the feet from asymptomatic individuals with serum urate ≥540 µmol/L (n=25) and those with crystal proven gout without clinically apparent tophi (n=33).

RESULTS

DECT urate deposits were observed in 6/25 (24%) participants with asymptomatic hyperuricaemia, 11/14 (79%) with early gout (predefined as disease duration ≤3 years) and 16/19 (84%) with late gout (p<0.001). DECT urate deposition was observed in both joints and tendons in the asymptomatic hyperuricaemia group, but significantly less frequently than in those with gout (p≤0.001 for both joint and tendon sites). The volume of urate deposition was also significantly lower in those with asymptomatic hyperuricaemia, compared with the early and the late gout groups (p<0.01 for both comparisons). Similar urate volumes were observed in the early and late gout groups.

CONCLUSIONS

Although subclinical urate deposition can occur in people with asymptomatic hyperuricaemia, these deposits occur more frequently and at higher volumes in those with symptomatic gout. These data suggest that a threshold of urate crystal volume may be required before symptomatic disease occurs.

Dual energy CT in gout: a prospective validation study

OBJECTIVE

The authors prospectively determined: (1) the specificity and sensitivity of dual energy CT (DECT) for gout; and (2) the interobserver and intraobserver reproducibility for DECT urate volume measurements.

METHODS

Forty crystal-proven gout patients (17 tophaceous) and 40 controls with other arthritic conditions prospectively underwent DECT scans of all peripheral joints using a gout protocol that color-codes the composition of tissues. A blinded radiologist identified urate deposition to calculate specificity and sensitivity of DECT for gout. Inter-rater volumetric reproducibility was determined by two independent radiologists on 40 index tophi from the 17 tophaceous gout patients using automated software.

RESULTS

The mean age of the 40 gout patients was 62 years, the mean gout duration was 13 years and 87% had a history of urate-lowering therapy (ULT). The specificity and sensitivity of DECT for gout were 0.93 (95% CI, 0.80 to 0.98) and 0.78 (0.62 to 0.89), respectively. When the authors excluded three gout cases with unreadable or incomplete scans, the sensitivity was 0.84 (95% CI, 0.68 to 0.94). The urate volumes of 40 index tophi ranged from 0.06 cm(3) to 18.74 cm(3) with a mean of 2.45 cm(3). Interobserver and intraobserver intraclass correlation coefficients for DECT volume measurements were 1.00 (95% CI, 1.00 to 1.00) and 1.00 (95% CI, 1.00 to 1.00) with corresponding bias estimates (SD) of 0.01 (0.00) cm(3) and 0.01 (0.03) cm(3).

CONCLUSIONS

These prospective data indicate high reproducibility of DECT urate volume measures. The specificity was high, but sensitivity was more moderate, potentially due to frequent ULT use in our patients.

Mechanisms of bone erosion in gout: a quantitative analysis using plain radiography and computed tomography

OBJECTIVE

The underlying basis of bone erosion in gout remains speculative. The aim of this study was to understand the mechanisms of bone erosion in gout using non-invasive imaging techniques.

METHODS

Paired plain radiographs and computed tomography (CT) scans of 798 individual hand and wrist joints from 20 patients with gout were analysed. Radiographs were scored for erosion (0-5) using the Sharp/van der Heijde method. CT scans were scored for the presence and diameter of bone erosions and tophi. The presence of intraosseous tophus (tophus visualised within bone) was recorded. The relationships between radiographic erosion, CT erosion and tophus scores were analysed.

RESULTS

With increasing radiographic erosion scores, the percentage of joints with intraosseous tophus increased (p<0.001). For those joints with a radiographic erosion score of 4 or 5, 96/98 (98%) had CT evidence of intraosseous tophus. There was a significant relationship between the radiographic erosion scores and intraosseous tophus size (p<0.001). For those joints with CT erosion, 194/237 (81.8%) had visible intraosseous tophus. Of the joints with CT erosions greater than 5 mm, 106/112 (94.6%) had visible intraosseous tophus and all (56/56) erosions greater than 7.5 mm had intraosseous tophus. There was a strong correlation between CT erosion diameter and intraosseous tophus diameter (r = 0.93, p<0.001). Intraosseous tophi were larger than non-intraosseous tophi, but had similar density and calcification.

CONCLUSION

There is a strong relationship between bone erosion and the presence of intraosseous tophus. These results strongly implicate tophus infiltration into bone as the dominant mechanism for the development of bone erosion and joint damage in gout.

2018 updated European League Against Rheumatism evidence-based recommendations for the diagnosis of gout

Although gout is the most common inflammatory arthritis, it is still frequently misdiagnosed. New data on imaging and clinical diagnosis have become available since the first EULAR recommendations for the diagnosis of gout in 2006. This prompted a systematic review and update of the 2006 recommendations. A systematic review of the literature concerning all aspects of gout diagnosis was performed. Recommendations were formulated using a Delphi consensus approach. Eight key recommendations were generated. A search for crystals in synovial fluid or tophus aspirates is recommended in every person with suspected gout, because demonstration of monosodium urate (MSU) crystals allows a definite diagnosis of gout. There was consensus that a number of suggestive clinical features support a clinical diagnosis of gout. These are monoarticular involvement of a foot or ankle joint (especially the first metatarsophalangeal joint); previous episodes of similar acute arthritis; rapid onset of severe pain and swelling; erythema; male gender and associated cardiovascular diseases and hyperuricaemia. When crystal identification is not possible, it is recommended that any atypical presentation should be investigated by imaging, in particular with ultrasound to seek features suggestive of MSU crystal deposition (double contour sign and tophi). There was consensus that a diagnosis of gout should not be based on the presence of hyperuricaemia alone. There was also a strong recommendation that all people with gout should be systematically assessed for presence of associated comorbidities and risk factors for cardiovascular disease, as well as for risk factors for chronic hyperuricaemia. Eight updated, evidence-based, expert consensus recommendations for the diagnosis of gout are proposed.

High-resolution ultrasonography of the first metatarsal phalangeal joint in gout: a controlled study

OBJECTIVE

To compare high-resolution ultrasound (HRUS) with conventional radiography in the detection of erosions in the first metatarsophalangeal joints (1st MTPJs) of patients with gout and to identify the characteristic sonographic features of gout.

METHODS

HRUS examination of the 1st MTPJs of both feet was performed by two independent sonographers. The presence of joint and soft-tissue pathology was recorded. x Ray examination of the feet was performed on the same day and reported by the same radiologist.

RESULTS

39 male patients with gout and 22 age-matched control subjects (14 with an inflammatory arthropathy and 8 disease free) were studied. The agreement on erosion between HRUS and x ray was poor, kappa = 0.229 (non-weighted), with McNemar's test being significant (p<0.001) indicating a large number of false negative x rays. 22 MTPJs in patients with gout had never been subjected to a clinical attack of acute gout. In these MTPJs, there were 10 erosions detected by HRUS and 3 erosions on x ray. HRUS features significantly more prevalent in the patients with gout were hard and soft tophus-like lesions (p<0.01) and the double contour sign (p<0.01).

CONCLUSIONS

These data show that HRUS may assist in the management of gout in two ways: first, by aiding in the diagnosis by identifying the sonographic features that may be representative of the disease, and, second, by allowing the early detection of erosive joint damage and/or tophaceous deposits even in clinically silent joints.

Tophaceous gout and high level of hyperuricaemia are both associated with increased risk of mortality in patients with gout

BACKGROUND

While several studies have reported a link between the presence of gout and adverse cardiovascular (CV) events in the general population, none has addressed the question of whether the mortality risk of patients with gout is influenced by disease severity.

METHODS

We applied survival analysis methodology to prospectively collected data on clinical and radiographic measures of disease severity and mortality in a specialty clinic based cohort of 706 patients with gout (1992-2008). Standardised mortality ratios (SMR) were calculated to assess the magnitude of excess mortality among patients with gout compared with the underlying general population.

RESULTS

Mean follow-up was 47 months. Tophaceous deposition was present in 30.5% of patients; >4 joints were involved in 34.6% of cases. Mean annual flare rate was 3.4. Arterial hypertension (41.2%), hyperlipidaemia (42.2%), diabetes mellitus (20.1%), renal function impairment (26.6%) and a previous CV event (25.3%) were recorded. 64 (9.1%) patients died, death being attributed to vascular causes in 38 (59%) patients. SMR for gout patients was 2.37 (95% CI 1.82 to 3.03), 1.57 (1.18 to 2.05) and 4.50 (2.06 to 8.54) overall, and in men and women, respectively. The presence of tophi and the highest baseline serum urate (SU) levels were independently associated with a higher risk of mortality, in addition to age, loop diuretic use and a history of a previous vascular event. In the multivariable survival regression models, with time varying covariates, the presence of tophi remained a significant mortality risk after adjustment for baseline SU levels (1.98; 1.24 to 3.20).

CONCLUSIONS

High baseline SU level and the presence of subcutaneous tophi were both associated with an increased risk of mortality in patients with gout, in most cases attributed to a CV cause. This suggests a plausible pathophysiological link between greater total body urate load and CV disease.

Ultrasound-detected musculoskeletal urate crystal deposition: which joints and what findings should be assessed for diagnosing gout?

OBJECTIVE

The primary objective of this prospective case-control study was to assess the diagnostic value of several intra-articular and periarticular ultrasound (US)-detected abnormalities in the upper and lower limbs in gout. The secondary objective was to test the concurrent validity of US abnormalities using as gold standard the microscopic demonstration of monosodium urate (MSU) crystals.

METHODS

Ninety-one men with gout and 42 age-matched controls were prospectively recruited. All patients with gout and controls underwent US assessment of several US abnormalities in 26 joints, six bursae, eight tendons, 20 tendon compartments, four ligaments, and 18 articular cartilages by experts in US blinded to the patients' group. Patients with gout and controls with US abnormalities were asked to undergo US-guided aspiration for microscopic identification of MSU crystals. Interobserver and intraobserver reliability of the US assessment was evaluated in a web-based exercise.

RESULTS

The assessment of one joint (ie, radiocarpal joint) for hyperechoic aggregates (HAGs), two tendons (ie, patellar tendon and triceps tendon) for HAGs and three articular cartilages (ie, first metatarsal, talar and second metacarpal/femoral) for double contour sign showed the best balance between sensitivity and specificity (84.6% and 83.3%, respectively). Intraobserver reliability was good (mean κ 0.75) and interobserver reliability was moderate (κ 0.52). The aspirated material from HAGs was positive for MSU crystals in 77.6% of patients with gout and negative in all controls.

CONCLUSIONS

Our results suggest that US bilateral assessment of one joint, three articular cartilages and two tendons may be valid for diagnosing gout with acceptable sensitivity and specificity.

Imaging modalities for the classification of gout: systematic literature review and meta-analysis

BACKGROUND

Although there has been major progress in gout imaging, no gout classification criteria currently include advanced imaging techniques.

OBJECTIVE

To examine the usefulness of imaging modalities in the classification of gout when compared to monosodium urate (MSU) crystal confirmation as the gold standard, in order to inform development of new gout classification criteria.

METHODS

We systematically reviewed the published literature concerning the diagnostic performance of plain film radiography, MRI, ultrasound (US), conventional CT and dual energy CT (DECT). Only studies with MSU crystal confirmation as the gold standard were included. When more than one study examined the same imaging feature, the data were pooled and summary test characteristics were calculated.

RESULTS

11 studies (9 manuscripts and 2 meeting abstracts) satisfied the inclusion criteria. All were set in secondary care, with mean gout disease duration of at least 7 years. Three features were examined in more than one study: the double contour sign (DCS) on US, tophus on US, and MSU crystal deposition on DECT. The pooled (95% CI) sensitivity and specificity of US DCS were 0.83 (0.72 to 0.91) and 0.76 (0.68 to 0.83), respectively; of US tophus, were 0.65 (0.34 to 0.87) and 0.80 (0.38 to 0.96), respectively; and of DECT, were 0.87 (0.79 to 0.93) and 0.84 (0.75 to 0.90), respectively.

CONCLUSIONS

US and DECT show promise for gout classification but the few studies to date have mostly been in patients with longstanding, established disease. The contribution of imaging over clinical features for gout classification criteria requires further examination.

Gout: a clinical and radiologic review

Gout is a group of diseases characterized by arthritis and results from a disturbance of urate metabolism with the deposition of monosodium urate crystals in the joints and soft tissues. Often, but not invariably, the serum urate levels are elevated as a result of overproduction or underexcretion of uric acid. Clinical manifestations include acute and chronic arthritis, tophaceous deposits, interstitial renal disease, and uric acid nephrolithiasis. The diagnosis is based on the identification of uric acid crystals in joints, tissues, or body fluids. Acute episodes are treated with colchicine, NSAIDs, or steroids. Long-term management includes treatment with uricosuric agents or xanthine oxidase inhibitors.

Systemic staging for urate crystal deposits with dual-energy CT and ultrasound in patients with suspected gout

Objective of the study is to compare the diagnostic accuracy for detecting monosodium urate crystal deposits between dual-energy CT (DECT) and ultrasound (US). Sixty consecutive patients (49 men, mean age 62 years) with clinically suspected gout were included in this case-control study. DECT and US of feet, knees, hands and elbows were performed in all patients. Polarisation microscopy of synovial fluid or a score incorporating serum uric acid level, first MTP joint involvement, gender, previous patient-reported arthritis attack, cardiovascular diseases, joint redness and onset within 1 day was used as standard of reference. Standard of reference classified 39 patients as gout positive. Sixteen patients had gout and a concomitant rheumatic disease. Sensitivities for diagnosis of gout disease were 84.6 % (33/39) for DECT and 100 % (39/39) for US. Specificities were 85.7 % (18/21) for DECT and 76.2 % (16/21) for US. Positive and negative predictive values were 91.7 % (33/36) and 75.0 % (18/24) for DECT, 88.6 % (39/44) and 100 % (16/16) for US, respectively. Urate crystals were detected most frequently in MTP1 joints (DECT 20/78, US 58/78), any other toe joints (DECT 25/78, US 62/78) and knees (DECT 41/78, US 31/78). The volumetry of DECT computed a mean urate crystal deposit load of 2.1 cm(3) (SD 9.6 cm(3)). A mean effective dose of ≤0.5 mSv was estimated. DECT is more specific for the diagnosis of gout than US. However, it fails to detect small urate crystal deposits. It might be particularly useful for patients with ambivalent findings, concomitant rheumatic diseases and with non-conclusive joint aspiration.

A radiologic reevaluation of gout: a study of 2,000 patients

Radiographs were available for study for 466 of 2,000 patients with primary gout who were followed clinically for as long as 30 years. The radiographic manifestations at various stages of the disease and the effect of therapy were studied in 80 patients. New radiographic signs, such as lace pattern, intracortical lesions, and subchondral compressions, are emphasized as well as the frequency of tophaceous lesions affecting the dorsum of the feet.

A systematic review of ultrasonography in gout and asymptomatic hyperuricaemia

Gout is one of the most common inflammatory arthritides. The literature reveals that management of this condition is often suboptimal. Imaging techniques, such as ultrasound (US), may assist in the diagnosis and management of gout and asymptomatic hyperuricaemia (AH). To undertake a systematic review evaluating US as an outcome tool in gout and asymptomatic hyperuricaemia, articles published in Medline and PubMed (1975-February 2012) were identified. Data was extracted and categorised into four different groups namely tophi, articular cartilage, soft tissue pathologies and bony changes, with a focus on validity, responsiveness, reproducibility and feasibility. Lesions reported in the literature include tophi, cartilage abnormalities, soft tissue lesions and erosions. US is able to detect tophi, using MRI as the gold standard, and is sensitive to change. The double contour sign seen overlying cartilage is specific to gout and sensitive to change. Synovial pathology is identified in gout, with some reporting intrasynovial hyperechogeneicity is suggestive of gout. US was less sensitive than MRI to cortical erosions in gout, but better than conventional radiography. Interobserver reliability when assessed ranged from fair to substantial agreement for soft tissue changes and was very good for assessing tophi, double contour and erosions. US is a promising tool which could be used in the diagnosis and management of gout. More studies are needed to assess responsiveness, reliability and feasibility.

Validation of a radiographic damage index in chronic gout

OBJECTIVE

To identify a valid method to measure radiographic damage in patients with chronic gout.

METHODS

The scoring method that best represented radiographic damage in individual joints was analyzed by comparing a gold standard rheumatologist consensus global score with recognized scoring methods, including the Sharp/van der Heijde erosion and narrowing scores, Ratingen destruction score, and Steinbrocker score. Ninety-five proximal interphalangeal joints from 12 patients with gout were included in this analysis. Scoring of hand and feet radiographs from an additional 35 patients with gout was used to analyze the sites to be included in a scoring system and the additional features to be recorded.

RESULTS

For individual joints, the combination of the Sharp/van der Heijde erosion and narrowing scores correlated best with the consensus global score. In addition, the limits of agreement were narrowest for the combined Sharp/van der Heijde erosion and narrowing score. All joint areas in the Sharp/van der Heijde rheumatoid arthritis score and the distal interphalangeal joints were affected by chronic gout and contributed to the total score. Additional features (extraarticular erosions, joint space widening, and ankylosis) occurred infrequently, and scoring of these features did not increase the reliability of the total score. The reliability of the total score was high: intraclass correlation coefficient for intraobserver reproducibility was 0.993-0.998 and for interobserver reproducibility was 0.963-0.966. The modified Sharp/van der Heijde score was able to discriminate between early and advanced disease.

CONCLUSION

A modified Sharp/van der Heijde system accurately and reliably represents radiographic joint damage in chronic gout.

Dual-energy CT in gout - A review of current concepts and applications

Dual‐energy computed tomography (DECT) is a relatively recent development in the imaging of gouty arthritis. Its availability and usage have become increasingly widespread in recent years. DECT is a non‐invasive method for the visualisation, characterisation and quantification of monosodium urate crystal deposits which aids the clinician in the early diagnosis, treatment and follow‐up of this condition. This article aims to give an up to date review and summary of existing literature on the role and accuracy of DECT in the imaging of gout. Techniques in image acquisition, processing and interpretation will be discussed along with pitfalls, artefacts and clinical applications.

Gouty arthritis: a prospective radiographic evaluation of sixty patients

A prospective analysis of 60 patients with gout was undertaken to evaluate the radiographic spectrum of gouty arthritis in patients treated in the era of hypouricemic therapy. Twenty-two of these patients were clinically tophaceous; 36 were considered to have radiographic findings diagnostic of gouty arthritis by strict radiographic criteria. Up to 24% of the patients denied symptoms in joints with radiographic changes of gout; 42% with no evidence of tophi on clinical examination had radiographic changes characteristic of gout. Radiographic assessment can be extremely helpful in the management of gout by documenting the degree and extent of bony involvement, particularly in patients with limited symptoms or without clinical tophi.

The pathogenesis of podagra

Precipitation of crystals from solution is a relatively simple phenomenon that should be explained by changes in concentration, temperature, pH, or other local factors. Which of these factors determines the preferential precipitation of urate crystals at the base of the big toe? Podagra correlates with degenerative joint disease of the toe, often follows abuse of the feet, and classically occurs in the middle of the night. These characteristics suggest that crystals may form in resolving synovial effusions. The urate concentration must transiently rise in such an effusion because water will leave the joint space twice as fast as urate. If the local concentration surpasses the solubility of urate, crystals will form that may than induce acute podagra.