From the Guidelines
For diagnosing gout, serum uric acid testing is not the primary diagnostic tool, instead, the diagnosis of gout should be based on the identification of monosodium urate (MSU) crystals in synovial fluid or tophus aspirates, and when this is not feasible, a clinical diagnosis can be made based on the presence of hyperuricaemia and associated clinical features of gout, with C-Reactive Protein (CRP) testing being useful as a secondary measure to assess inflammation. The most recent and highest quality study, published in 2020 in the Annals of the Rheumatic Diseases 1, recommends a three-step approach for the diagnosis of gout, which includes the identification of MSU crystals, clinical diagnosis, and imaging. Some key points to consider when diagnosing gout include:
- The specificity of hyperuricaemia for the diagnosis of gout is low, ranging from 53% to 61% 1
- The absence of hyperuricaemia has a markedly low negative likelihood ratio, indicating that after an episode of acute arthritis has settled, the absence of hyperuricaemia does not completely exclude a diagnosis of gout but makes the diagnosis very unlikely 1
- CRP is a marker of inflammation and can help assess the severity of a gout flare or monitor the effectiveness of treatment, but it is not specific to gout and can be elevated in many inflammatory conditions 1
- The reference standard for diagnosing acute gout is joint aspiration with synovial fluid analysis for MSU, but this is not always feasible in primary care or emergency medicine settings 1. Overall, while both serum uric acid and CRP tests can be useful in the diagnosis and management of gout, the identification of MSU crystals and clinical diagnosis are the primary diagnostic tools.
From the Research
Diagnostic Testing for Gout
- C-Re�active Protein (CRP) and serum uric acid testing are two methods used to diagnose gout, but their diagnostic value can vary.
- According to a study from 2014 2, normal serum uric acid levels were found in 63.3% of patients with acute gout attack, suggesting that serum uric acid testing may not always be diagnostic.
- In contrast, a study from 2011 3 found that the mean serum uric acid level was significantly higher in patients with acute gout compared to those with hyperuricemia, indicating that serum uric acid testing may be useful in diagnosing gout.
- elevations in CRP were found in 30% of patients with acute gout attack, suggesting that CRP may be a useful marker for inflammation in gout, but not necessarily diagnostic of the condition itself 2.
Comparison of Diagnostic Tests
- A study from 2024 4 found that general practitioners often diagnose gout without testing serum uric acid, which can lead to diagnostic uncertainty.
- The study suggested that serum uric acid testing is important for confirming a diagnosis of gout, and that a level >0.35mmol/l is largely determinative for the clinical diagnosis of gout.
- Another study from 2017 5 found that febuxostat adherence and lower baseline serum uric acid level were the strongest predictors of attaining a serum uric acid goal in patients with gout, highlighting the importance of serum uric acid testing in managing the condition.
- A study from 2017 6 found that patients with gout differ from healthy subjects in renal response to changes in serum uric acid, suggesting that serum uric acid testing may be useful in understanding the underlying mechanisms of gout.
Limitations of Diagnostic Tests
- Serum uric acid testing may not always be diagnostic, as normal levels can be found in patients with acute gout attack 2.
- CRP testing may not be specific for gout, as elevations can be found in other inflammatory conditions.
- The diagnostic value of serum uric acid and CRP testing may depend on the specific clinical context and patient population.