“Synovial fluid analysis is the gold standard for the diagnosis of gout, but often fluid is not readily obtainable or able to be viewed under an appropriate microscope. This diagnostic tool is easy to use and will provide help in the rapid and correct diagnosis of gout,” Howard Smith, MD, a rheumatologist at the Cleveland Clinic in Ohio, toldMedscape Medical News. Dr Smith was not involved in the study.

In 2010, a team of rheumatologists in the Netherlands first described the diagnostic rule and reported its validity in primary care patients with an area under the receiver operator characteristic curve of 0.85 (95% confidence interval, 0.81 – 0.90).

Validation in Secondary Care Population

Laura B.E. Kienhorst, MD, from the Department of Rheumatology at Rijnstate Hospital in the Netherlands, and colleagues describe the ability of the rule to diagnose gout in a secondary care population of patients in an article published online September 16 in Rheumatology.

In the current study, patients with monoarthritis (n = 390) were first diagnosed according to the diagnostic rule, and then their joint fluid was aspirated and analyzed for the presence of monosodium urate crystals. Patients were followed for up to 17 months.

Patients who were found to have these crystals (n = 219) were classified as having gout, even if the crystals were detected months after the baseline diagnosis. When arthrocentesis yielded inadequate joint fluid (n = 12), patients were classified as nongout.

“A strength of our diagnostic study is the prospective design with a long follow-up period in which patients with unspecified arthritis were re-evaluated in the case of any new arthritis. Not excluding patients with unspecified arthritis is consistent with clinical practice in which unspecified monoarthritis is prevalent,” the authors write. They also note, however, that the study was not fully blinded.

The investigators report that a score of 8 points or higher had a positive predictive value of 0.87, and a score 4 or fewer points had a negative predictive value of 0.95. They calculated the area under the receiver operating characteristic curve for the diagnostic rule to be 0.86 (95% confidence interval, 0.82 – 0.89).

Rule Helps Diagnose Gout

A score of 4 or lower from the diagnostic rule indicates a low probability of gout and directs the practitioner to consider a diagnosis other than gout.

“When it [the rule] indicates a high probability, the physician can treat the patient as having gout. But when there is uncertainty (an intermediate probability) about the diagnosis, the patient would require a consultation with a rheumatologist for joint fluid analysis or extensive follow-up. Physician reexamination of every patient is advisable in the case of a recurrent arthritis because there always remains a risk of missing other important rheumatic diagnoses if patients are false-positively classified as gout,” elaborated Dr Kienhorst.

Previous studies have compared the gold standard of monosodium urate crystals with the American College of Rheumatology criteria for classifying patients with gout and revealed limited validity, making the high positive predictive value and negative predictive value for the diagnostic rule especially noteworthy.

The authors and Dr Smith have disclosed no relevant financial relationships.

Rheumatology. Published online September 16, 2014. Abstract