Rare diseases

Classification is not diagnosis


Clinicians need to distinguish carefully between the criteria used to classify a disease and the criteria used to diagnose it, senior rheumatologists have warned.

Professors Robert Landewé and Désirée van der Heijde, from Amsterdam and Leiden in The Netherlands, were commenting on a recently published set of ‘diagnostic’ criteria for cryopyrin-associated periodic syndrome (CAPS).

“CAPS encompasses a group of very rare disorders with incompletely understood pathogenesis and diverse clinical presentations,” Landewé and van der Heijde said.

“The authors justify their efforts by claiming that there is a significant delay in diagnosis, which, in their opinion, is due to the lack of recognition and clear diagnostic criteria.”

While they should be commended for their efforts, the CAPS authors had in fact developed classification criteria rather than diagnostic criteria.

“The most important shortcoming of the criteria in terms of their diagnostic potential is the absence of a ‘true’ gold standard for CAPS,” Landewé and van der Heijde said.

The criteria were claimed to have a sensitivity of 81% and a specificity of 94%. This might seem quite good, but not when applied to a disease with a prevalence of just three in a

million.

“Every true criteria-positive case of CAPS will be counterbalanced by 25,000 false criteria-positive cases,” they said.

Fundamental concerns about diagnostic criteria in rheumatology have led to both the American College of Rheumatology and the European League Against Rheumatism to cease endorsing them.

“Diagnostic criteria will often be used in common clinical practice as a checkbox to be ticked by the diagnostician,” they said.

“[This] policy may lead to tremendous and potentially dangerous over-diagnosis as well as missing other relevant diagnoses and to over-treatment (eg, with biological drugs).”

“To us it seems that patients with CAPS or conditions mimicking CAPS are not served by using the criteria in a diagnostic context.

“The authors should…rename them as classification criteria, implying that these can only be applied in patients with a diagnosis of CAPS.”

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