[Inflammatory reaction and laboratory tests: serum amyloid A (SAA) protein]

Serum amyloid A protein (SAA) is a sensitive acute phase reactant. Here, the assay of SAA in serum and its clinical significance are reviewed. SAA was measured simply by radial immunodiffusion and enzyme immunoassay with rabbit anti-amyloid A antibodies, however further investigation is necessary because SAA is an insoluble apolipoprotein. The concentration of SAA was 1.5-3.0 folds higher at physiological states, and 3.0-10.0 folds higher at inflammatory states than that of C-reactive protein (CRP). Therefore, SAA might be a sensitive and useful method for full observation of diseases. At the acute phase such as myocardial infarction, SAA changed at the same time as CRP. Most inflammatory disorders, for example, rheumatoid arthritis and malignant tumors which show elevation of CRP, showed elevation of SAA. These two proteins were strongly correlated, but showed no disease specificity. Also at secondary amyloidosis which was caused by deposition of SAA fragments, the level of SAA did not indicate the presence of amyloid. Only at the time of kidney allograft rejection for a recipient, was SAA elevated markedly in comparison with CRP. Recently, we developed a method of quantitative analysis of SAA isotypes and applied it in a few cases. Although significant features for diseases have not been obtained yet, such analysis might become useful for the physiological and pathological understanding of SAA.