Background: Differentiation of gout and calcium pyrophosphate deposition disease (CPPD) is sometimes difficult as patients often present with a similar clinical picture. Arthrocentesis and subsequent polarization microscopy (PM) remains the gold standard but novel diagnostic approaches such as non-invasive dual energy computed tomography (DECT) have recently been validated for gout. Currently, limited data is available on DECT in patients with CPPD.

Objectives: To analyse the diagnostic impact of DECT in gout and CPPD when compared to the gold standard of PM. We further compared the results of PM to ultrasound (US), conventional radiographs (CR), and suspected clinical diagnosis (SCD). Additionally, 15 laboratory parameters were analysed.

Methods: Twenty-six patients diagnosed with gout (n = 18) or CPPD (n = 8) who received a DECT and underwent arthrocentesis were included. Two independent readers assessed colour coded, as well as 80 and 120 kV DECT images for signs of monosodium urate (MSU) crystals or CPP deposition. US and CR from the patient’s initial visit along with the SCD were also compared to PM. US examinations were performed by certified musculoskeletal ultrasound specialists. The association of up to 15 laboratory parameters such as uric acid, thyroid stimulating hormone, and C-reactive protein (CRP) with the PM results was analysed.

Results: Sensitivity of DECT for gout was 67% (95% CI 0.41-0.87) with a specificity of 88% (95% CI 0.47-1.0). Concerning CPPD, the sensitivity and specificity of DECT was 63% (95% CI 0.25-0.91) and 83% (95% CI 0.59-0.96) respectively. US had the highest sensitivity of 89% (95% CI 0.65-0.99) with a specificity of 75% (95% CI 0.35-0.97) for gout, while the sensitivity and specificity for CPPD were 88% (95% CI 0.47-1.0) and 89% (95% CI 0.65-0.99) respectively. The SCD had the second highest sensitivity for gout at 78% (95% CI 0.52-0.94) with a comparable sensitivity of 63% (95% CI 0.25-0.92) for CPPD. Uric acid levels were elevated in 33% of gout patients and 25% of CPPD patients. While elevated CRP levels were observed in 59% of gout patients and in 88% of CPPD patients, none of the 15 analysed laboratory parameters were found to be significantly linked.

Conclusion: DECT provides a non-invasive diagnostic tool for gout but might have a lower sensitivity than suggested by previous studies (67% vs 90%¹). DECT sensitivity for CPPD was 63% (95% CI 0.25-0.91) in a sample group of eight patients. Both US and the SCD had higher sensitivities than DECT for gout and CPPD. Further studies with larger patient cohorts are needed in order to determine the diagnostic utility of DECT in CPPD.

References: [1]Bongartz, Tim; Glazebrook, Katrina N.; Kavros, Steven J.; Murthy, Naveen S.; Merry, Stephen P.; Franz, Walter B. et al. (2015): Dual-energy CT for the diagnosis of gout: an accuracy and diagnostic yield study. In Annals of the rheumatic diseases 74 (6), pp. 1072–1077. DOI: 10.1136/annrheumdis-2013-205095.

Disclosure of Interests: None declared