Dissociation of acetylsalicylic acid in blood and joint fluid.

Assay method in blood of 30 patients, who had ingested 0.6 g buffered aspirin, showed earlier appearance of acetylsalicylate (ASA) than of de-acetylated salicylate (SA); the values for ASA averaged 7.7 min, and for SA, 10.9 min. Similarly in joint fluid, ASA appeared earlier than SA; the values of ASA averaged 19.4 min, and those for SA, 21.9 min. Transport times did not differ significantly between ASA and SA in most types of synovitis. Close to maximum concentrations in blood averaged 18.9 mg/l for total salicylates (TSA), 3.3 mg/l for ASA, and 23.3 mg/l for SA, whereas maximum concentrations in joint fluid averaged 15.7 mg/l for TSA, 2.5 mg/l for ASA, and 14.5 mg/l for SA. ASA in blood initially consituted 65% of TSA and decreased to less than measurable amounts 75 min after intake. Joint fluid initially contained a somewhat smaller maximal proportion of ASA, but de-acetylation progressed more slowly than in blood; ASA decreased to undetectable levels within 140-145 min after intake.