Serum C-reactive protein is rarely lost into urine in patients with secondary amyloidosis and proteinuria

We studied whether the low serum C-reactive protein (S-CRP) level in patients with inflammatory arthritis and proteinuria was due to the loss of CRP into urine. In 19 patients with secondary amyloidosis (14 with rheumatoid arthritis and five with juvenile chronic arthritis), S-CRP was measured with both immunoturbidimetric and radioimmunoassays. The concentration of urinary CRP was measured with a double-antibody radioimmunoassay. One patient with the most extensive proteinuria (12 g/24 h) excreted CRP at 14 mg/24 h, while in 18 of 19 patients only negligible, if any, amounts of CRP were found in 24-h urine samples. Proteinuria of <8 g/24 h did not reduce the S-CRP level. Proteinuria exceeding this level may result in increased excretion of CRP into urine and consequently may result in a reduced S-CRP level.