Establishment of an in-house ELISA and the reference range for serum amyloid A (SAA). Complementarity between SAA and C-reactive protein as markers of inflammation

Introduction: Serum amyloid A (SAA) and C-reactive protein (CRP) are both acute-phase reactants synthesized by the liver upon stimulation by proinflammatory cytokines reflecting both the acute and chronic inflammatory states. Methods: We have established a one-step, sandwich ELISA on microplate for SAA using commercial antibodies for coating and detection. Results: This in-house ELISA has a sensitivity of 0.12 mg/l. Both within-day and between-day CVs were < 10%. The in-house assay correlated well with the commercial ELISA kit from Anogen (r = 0.95). We also established the reference range for apparently healthy Chinese. Statistically higher SAA values were found in those > 50 years old. No difference was found between genders. We found only slightly increased levels of SAA in early stage of type 2 diabetics, but highly increased levels of SAA were detected in patients with acute myocardial infarction, generally associated with intense inflammation. At the early stage of type 2 diabetes associated with low inflammation, SAA was found to be complementary to CRP in test sensitivity. Conclusions: Based on our data and reports from the literature we believe that SAA responds differently than CRP in inflammatory diseases such as in type 2 diabetes and acute myocardial infarction, and is complementary to CRP in test sensitivity.