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Binding of osteocalcin to hydroxyapatite was determined in nine pairs of samples from patients with OA and in 10 sample pairs from patients with RA, as described by Price et al.2Serum or synovial fluid for osteocalcin measurement was divided into two portions. One aliquot was extracted with hydroxyapatite (20 mg to 200 p1 serum or synovial fluid), the mixture vortexed, turned end over end at 4°C for 30 minutes, and centrifuged for five minutes in a table top clinical centrifuge. The resulting supernatant containing non-hydroxyapatitebound osteocalcin and the other aliquot containing the total amount of osteocalcin were assayed for osteocalcin in the same radioimmunoassay. The hydroxyapatite bound osteocalcin was obtained by subtraction, and the lower limit of sensitivity for osteocalcin was 0-35 ng/ml. ResultsSignificant amounts of total osteocalcin were detected in the synovial fluid, and dilution of synovial fluid samples showed linearity when the osteocalcin values were compared with those in serum. The serum total osteocalcin values in RA and OA were normal (5 8 (SD 2 08) and 6-5 (2-5) ng/ml respectively), whereas the values in RA synovial fluid were significantly lower than those in OA synovial fluid (3-24 (2 05) and 5-1 (2-1) ng/ml respectively, p<0 04).Up to 25% of the osteocalcin in normal, OA, and RA serum did not bind to hydroxyapatite. The osteocalcin in OA synovial fluid, however, bound almost completely to hydroxyapatite (0-1-9% unbound), whereas a significant proportion of the osteocalcin in RA synovial fluid 229 on 10 May 2018 by guest. Protected by copyright.