Measurement of serum amyloid A4 (SAA4): Its constitutive presence in serum

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Abstract
Newly described human serum amyloid A4 (SAA4) was measured in serum by an enzyme-linked immunosorbent assay using anti-SAA4 monoclonal antibody and recombinant human SAA4 as the assay standard. Interference by elevated acute phase SAA (aSAA) was abolished by the addition of rabbit anti-human aSAA antiserum prior to the addition ofanti-SAA4 to the test samples. Normal levels of SAA4 ranged between 80–140 mg/L, substantially higher than those ofaSAA. No statistically significant difference in SAA4 levels between normal subjects and patients with Mycoplasma pneumonia was seen, indicating that SAA4 does not behave as an acute phase reactant. Low SAA4 levels were found in some patients with high aSAA, although no significant relationship between SAA4 and aSAA was apparent. The constitutive presence of SAA4 in serum suggests a physiologic function different from that ofaSAA.