FDG PET in relapsing polychondritis: a high positivity rate on thin evidence

Design

Systematic review and meta-analysis of pooled proportions, JBI risk-of-bias appraisal

Population

3 studies, 97 patients with suspected or confirmed relapsing polychondritis undergoing 18F-FDG PET/CT

Primary outcome

PET positivity rate

Effect

94% (95% CI 73 to 99, I² = 0%); pooled baseline SUVmax 4.0 (3.5 to 4.6)

Three studies and 97 patients is the entire pooled literature on FDG PET/CT in relapsing polychondritis. Within that, the PET-positivity rate was 94 per cent, with no heterogeneity between the three, and pooled baseline SUVmax was 4.0.

Two observations are more useful than the headline rate. PET found asymptomatic cartilage involvement in more than a quarter of patients, and it performed better than clinical assessment at sites that cannot be examined — the peripheral airways above all, where disease is what kills in this condition. PET parameters also tracked inflammatory markers, which is what makes it attractive for monitoring treatment response.

A positivity rate is not diagnostic accuracy. These were patients with suspected or established disease, so the studies cannot say how often PET is positive in the conditions relapsing polychondritis is mistaken for — infection, vasculitis, IgG4-related disease. Order it to map extent and follow treatment, not to make the diagnosis.

• Use PET to map airway and asymptomatic cartilage involvement once the diagnosis is suspected clinically.
• Record baseline SUVmax if PET will be used for monitoring; the comparison is the point.
• Do not use a positive PET to establish the diagnosis — specificity is unstudied.
• Correlate with CRP and ESR; the PET parameters moved with them.
• Flag airway findings explicitly to the referring team; that is where the mortality lies.

Why it matters

It supports PET for the part of this disease the clinic cannot see, and withholds it from the part clinicians most want to use it for.

Don't overread it

Positivity rate without a comparator group says nothing about specificity or diagnostic accuracy.

The statistics, in plain English

A pooled proportion of 94 per cent with an interval from 73 to 99 tells you how uncertain a three-study, 97-patient estimate is: the lower bound is much further from the point estimate than the upper, because proportions near 100 per cent compress at the top. I squared of 0 per cent means the three studies agreed — with three studies, that is weak reassurance rather than strong. No study reported specificity, so no predictive value can be calculated from these data.

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