identification of bacterial antigens and super antigens in synovial fluid of patients with arthritis: a cross sectional study

background : an accurate and prompt diagnosis of bacterial arthritis is essential for earlier treatment and a good outcome. superantigens produced by staph. aureus are among the most lethal toxins. the paper objective was identification of common bacterial antigens and s.aureus superantigens in synovial fluid (sf) of children with negative culture and direct smear for other bacteria except for s.aureus. methods : in this cross-sectional study a total of 62 patients with a mean age of 11 ± 3.8 years (range: 5 months-16 years) with acute arthritis in pediatric and orthopedic wards of rasoul hospital (2008-2010) were studied. three common bacterial antigens (e.g. s.pneumonia, h.influenza, n. meningitis) using lpa (latex particle antigen) and staphylococcal superantigens (tsst1 enterotoxin a b c) using elisa method (abcam usa) were identified in 60 adequate sf samples with negative culture and negative direct smears ) for other bacteria except for s.aureus. staphylococcal superantigens were compared with s.aureus infection (positive culture or direct smear).   results : positive bacterial antigens (lpa test) were found in 4 cases including two s. pneumonia, one n.meningitis, and one h.influenza. s.aureus was diagnosed in 7 cases including 4 positive cultures and 3 positive smears. staphylococcal superantigens (toxins) were found in 73% of sf samples. some cases had 2 or 3 types of toxins. s.aureus toxins were reported in 47% of culture negative sf samples. positive tsst1, enterotoxin b, enterotoxin a, and enterotoxin c were found in 47% (n= 28), 18% (n= 10), 39% (n= 22), and 39% ( n= 21) of cases respectively. the most common type of superantigens was tsst1 and enterotoxin a was the less common type. except for enterotoxin a, no relation between positive s.aureus culture and positive tests for superantigens in sf was found.   conclusion: s.aureus has a prominent role in septic arthritis. s.aureus toxins might have a prominent role in arthritis with negative sf culture. rapid identification of bacterial antigens (lpa) or s.aureus superantigens (toxins) are valuable for diagnosis in cases with negative cultures. we recommend usage of complementary methods (e.g. antigen detection tests) in children. those tests are cheaper and easier in comparison with pcr as a complex and time-taking method. identification of s.aureus superantigens in sf of all cases with negative culture, or treatment with antagonist drugs needs further clinical trial studies.