A new turbidimetric method for assaying serum C-reactive protein based on phosphocholine interaction

Abstract

Background: C-reactive protein (CRP) is able to bind phospholipids (mainly phosphocholine) in the presence of calcium ions. We investigated the use of this property for developing an affordable turbidimetrical CRP assay based on diluted soy oil. Methods: Serum (or heparinized plasma) was mixed with Intralipid® 20% in Tris-calcium buffer (pH 7.5). After 30 min of incubation at 37°C, the CRP-phospholipids complexes were measured by turbidimetry (660 nm/700 nm) with a Cobas 6000 analyzer (Roche). Results were compared with those obtained using a typical immunoturbidimetric method (Roche). Results: Good correlation (r2=0.931) was obtained between the functional and the immunoturbidimetric CRP assay. Within-run and between-run %CV values for the functional assay were 2.4% (100 mg/L); 6.0% (50 mg/L); 10% (20 mg/L), and 3.6% (100 mg/L); 8.0% (50 mg/L); 11% (20 mg/L), respectively. The limit of detection was 7 mg/L. Results were not affected by serum calcium, triglyceride, or phospholipid concentrations. Conclusions: The functional CRP assay allowed measurement of CRP in serum and plasma in the range of 7 mg/L–400 mg/L. The assay is particularly suited in conditions where resources are restricted. Since the assay is species independent, the described functional CRP assay could be used for veterinary purposes as well. Clin Chem Lab Med 2009;47:1417–22.