Background Septic arthritis (SA) is a rheumatologic emergency as joint destruction occurs rapidly and can lead to significant morbidity and mortality. Accurate diagnosis can be particularly challenging in patients with underlying inflammatory joint disease. Crystal-induced arthritis (CA) is a risk factor for its appearance. When both conditions appear simultaneously, CA may mask diagnosis of infection and delay the antibiotic treatment.

Objectives To describe the characteristics of patients with concurrent septic and CA.

Methods Retrospective analysis of patients with coexistence of septic and CA attended between 1985 and 2015 in a university hospital with a reference area of 850,000 inhabitants. We collect demographic, clinical, laboratory and imaging data as well as patient medical treatment, complications and evolution records. All patients had positive bacterial culture (blood and/or joint fluid) and crystals in synovial fluid.

Results A total of 123 patients with SA were identified. 20.3% (n=25) of them had concomitant CA, with mean age of 67 years (SD 14), 17 (68%) males and 8 (32%) females. Risk factors were: diabetes (24%), diuretic drugs (24%), chronic renal failure (16%) -2 of them undergoing hemodialysis and 4 kidney transplant patients with immunosuppressive treatment. In only 2 cases there was a previous arthrocentesis. The mean diagnostic delay was 14 days (SD 13) (data available in 14 cases). The most commonly affected joint was the knee (48%), followed by the foot (20%) and the hip (12%). In 2 cases several joints were involved at the same time. In synovial fluid cytological studies, the most frequently identified crystals were: urate (60%), calcium pyrophosphate (20%) and hydroxyapatite (8%). In 32% of cases gram staining was positive, but 88% of patients had a positive joint fluid culture, with the most frequently isolated germs being methicillin-sensitive S. aureus (48%), methicillin-resistant S. aureus (MRSA) (12%) and M. tuberculosis (12%). 32% of patients presented positive blood cultures (12% with negative synovial fluid culture), although 48% of patients had fever at the time of diagnosis. It should be noted that 48% had radiological baseline damage. Surgical debridement was performed in 32% of patients. Evolution was successful in 56% of patients; although intercurrent complications were usual (40%). Mortality was 8% -one case due to acute pulmonary edema and the other because of septic shock.

Conclusions Coexistence of infectious and CA represents a diagnostic challenge and requires a high suspicion index. It usually appears in elderly patients with comorbidities. Gout was the most prevalent CA. S. aureus was the most commonly causative pathogen, with a high rate of MRSA infection. If it's treated early the evolution is usually favorable, which makes synovial fluid microbiological study imperative.

Disclosure of Interest None declared