Objective To investigate the diagnostic value of glucose-6-phosphate isomerase (GPI) in rheumatoid arthritis (RA).  Methods ELISA was used to measure the serum concentration of GPI in 108  patients with RA

42 patients with systemic lupus erythematosus (SLE)

50 patients with other autoimmune diseases and 112 normal controls. The levels of rheumatoid factor (RF) and anti-CCP antibodies were also assayed for RA patients. Results Serum GPI level in RA patients were (1.840±2.821) μg/ml

while (0.143±0426) μg/ml in SLE patients

(0.074±0.279) μg/ml in other autoimmune diseases and (0.092±0.232) μg/ml in normal controls. There was significant statistical difference among RA

SLE

other autoimmune diseases and the normal controls (P0.01). Spearman rank correlation analysis showed the concentration of GPI was positively correlated with RF level (r=0.622

P0.01)

but not with other parameters.  Conclusion Serum GPI might be a new marker in diagnosis of RA. The serum concentions of GPI over the cut off value 0.209 μg/ml has a remarkable specificity in diagonising RA.