OBJECTIVE The aim of this study was to develop a specific immunoassay for PIIANP and measure its serum concentration in healthy controls and in patients with osteoarthritis (OA) and rheumatoid arthritis (RA). In addition, we investigated circulating forms recognized by antiserum IIA in pools of serum from healthy adults, patients with OA and patients with RA. DESIGN Using as immunogen and standard the recombinant human Glutathione S-Transferase (GST)-exon 2 fusion protein of type II collagen, we developed a competitive polyclonal antibody-based ELISA. We compare serum PIIANP levels in 43 patients with knee OA (23 women and 20 men; mean age: 62.6+/-9.6 yr), 63 women with RA (mean age: 54+/-16 yr) and 88 healthy controls (67 women, mean age: 53+/-13 yr and 21 men, mean age: 63+/-7 yr). We randomly selected serum in each group for analyze circulating forms. RESULTS The immunoassay we developed demonstrated adequate intra and inter-assay precision (CV<10%) and dilution recovery (mean: 96%), allowing accurate measurements of serum PIIANP from 1.13 to 40 ng/ml. No significant cross-reactivity of the ELISA was observed with purified intact human procollagen type I N-propeptide, circulating thrombospondin and von Willebrand factor, proteins which exhibit significant sequence homology with PIIANP. Western blot analysis showed that antiserum IIA recognized two circulating immunoreactive forms of approximately 80 and 100 KDa respectively in serum from healthy adults, patients with OA and RA but also in a pool of synovial fluids from patients with OA. Serum PIIANP levels were markedly decreased in patients with knee OA (12.0+/-3.2 vs 25.8+/-7.5 ng/ml for OA and controls respectively, P<0.0001) and RA (14.1+/-2.5 ng/ml vs 21.7+/-7.6 ng/ml for RA and controls respectively, P<0.0001). In patients with RA, serum PIIANP levels were higher in those taking low-dose prednisone compared to non-users (15.0+/-2.4 vs 13.5+/-2.4 ng/ml, P<0.05). CONCLUSIONS We have developed the first specific immunoassay for serum PIIANP which exhibits adequate technical performances. This assay detects specifically two immunoreactive forms both in healthy adults and patients with arthritis and does not cross react with other proteins with sequence homology with PIIANP. Levels of PIIANP were significantly decreased in patients with knee OA and RA suggesting that type IIA collagen synthesis may be altered in these arthritic diseases. The measurement of type IIA collagen synthesis with this new molecular marker may be useful for the clinical investigation of patients with joint diseases.