FDG PET is almost always positive in relapsing polychondritis — which is not the same as diagnostic

Design

Systematic review and meta-analysis of three comparative or descriptive studies, Joanna Briggs Institute risk-of-bias appraisal

Population

97 patients with suspected relapsing polychondritis who underwent 18F-FDG PET/CT

Primary outcome

Pooled PET positivity rate, with pooled baseline SUVmax

Effect

Positivity 94% (95% CI 73 to 99, I² 0%); pooled SUVmax 4.0 (3.5 to 4.6); asymptomatic cartilage involvement in more than 25%

Three studies with 97 patients between them were pooled for 18F-FDG PET/CT in suspected relapsing polychondritis, from 962 screened articles. Pooled PET positivity was 94% (95% CI 73 to 99, I² 0%) and pooled baseline SUVmax was 4.0 (3.5 to 4.6, I² 32%). PET identified asymptomatic cartilage involvement in more than a quarter of patients, had a higher positivity rate at sites that cannot be examined or biopsied — the peripheral airways in particular — and PET parameters tracked inflammatory markers.

A positivity rate is not a diagnostic accuracy. These studies report how often PET was positive in patients who had or were suspected of having the disease, without a comparison group of patients with something else that inflames cartilage or airways. So the 94% is closer to a sensitivity than to anything else, and specificity is simply unmeasured. With three studies and 97 patients in a rare disease, that is the ceiling of what the literature currently supports.

Where it is genuinely useful is the part clinical examination cannot reach. Relapsing polychondritis kills through airway involvement, and tracheobronchial cartilage is neither visible nor safely biopsied; a whole-body study that shows uptake there, or finds asymptomatic costal or laryngeal involvement, changes the assessment of how extensive the disease is. Use it for mapping extent and for monitoring response, where SUV change against a baseline is interpretable, rather than as the test that makes or excludes the diagnosis.

• Use FDG PET/CT to map disease extent and to monitor treatment, not to confirm or exclude the diagnosis.
• Look specifically at tracheobronchial and costal cartilage — the sites examination misses.
• Record baseline SUVmax so later studies are comparable; pooled baseline here was 4.0.
• Interpret a positive study alongside inflammatory markers, which correlated with PET parameters.
• Specificity is unmeasured in this evidence; other causes of cartilage and airway inflammation were not compared.

The statistics, in plain English

A confidence interval of 73 to 99% around a pooled positivity rate of 94% is what 97 patients across three studies buys — the point estimate looks precise and the interval says otherwise. Heterogeneity of 0% means the three studies agreed, but three studies agreeing is weak evidence of consistency. The deeper limitation is the design rather than the numbers: without a control group of patients with alternative diagnoses, a high positivity rate cannot distinguish a test that detects this disease from one that lights up in any cartilage inflammation. That is why the authors call for prospective work on diagnostic accuracy rather than more positivity rates.

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