Radioimmunoassay of human C-reactive protein and levels in normal sera.

C-reactive protein (CRP) has been considered an acute-phase protein which appears only during reactions of tissue injury or inflammation. We report here the quantitation of CRP levels in normal adults and neonates made possible by the development of a sensitive and precise radioimmunoassay for human CRP which enables the detection of 3 ng. per milliliter. Serum levels in 153 healthy blood donors ranged from 68 to 8,200 ng. per milliliter, with a median value of 580 ng. per milliliter (mean = 1,340 ng. per milliliter). CRP levels in 24 normal cord serum samples ranged from 10 to 370 ng. per milliliter, with a median value of 70 ng. per milliliter (mean = 109 ng. per milliliter). CRP levels in 246 individuals evaluated for autoimmune diseases ranged to 256,000 ng. per milliliter with a median value of 13,000 ng. per milliliter (mean = 38,000 ng. per milliliter). No individual lacking CRP was detected. Thus, CRP can be considered a component of normal serum which only increases dramatically in concentration during inflammation.