Laboratory Methods to Measure CRP
The 3 clinical laboratory methods used to measure serum CRP levels are as follows:
All 3 tests are based on the ability of CRP to bind to a variety of biologic ligands forming CRP-ligand complexes. When a reagent containing anti-human CRP antibodies is added to a serum sample containing CRP, the CRP binds to the antibodies forming insoluble CRP-ligand complexes that clump and precipitate, which can then be visualized and measured. Table 2 provides a comparison of these methods.
The qualitative latex agglutination test is the first laboratory method developed to measure CRP.[12,29,43] This method measures the presence or absence of agglutination and precipitation, indicating only whether CRP is present or absent in the serum sample.[12,29] A positive test result indicates the presence of CRP-ligand complexes formed when CRP binds to cause agglutination and precipitation, whereas a negative test result occurs when no agglutination is present.[12,44] Positive test results indicate a CRP level greater than 6 mg/L or more than 10 mg/L, depending on the specific testing kit and reagent being used. Qualitative tests can be performed rapidly at the bedside within 10 to 15 minutes.
Purely qualitative CRP testing methods typically are not used to measure CRP levels because they have a low sensitivity, with positive results occurring with any condition involving inflammation or tissue destruction.[12,29,35] A positive qualitative CRP test should always be followed by a semiquantitative test, which is a more sensitive measuring method to quantify the concentration of CRP.
The semiquantitative latex agglutination assay involves the use of serial dilutions of serum and saline. Each is mixed with a latex reagent and observed for the presence of agglutination.[29,44,45] The highest dilution in which agglutination is visualized corresponds to an approximate titer or concentration of CRP-ligand complexes.
Like the qualitative test, the semiquantitative test can be performed in 15 to 30 minutes and has a reported upper detection level between 6 and 10 mg/L.[35,45] Results are reported as a ratio and/or an approximate mg/L concentration of CRP.[44,45] The reported cost of a Murex semiquantitative testing kit (Murex Diagnostic, Norcross, GA) is approximately $21.00.
Quantitative immunoassay is the most rapid, sophisticated, and sensitive method of detecting and measuring CRP.[12,43,46,47] The enzyme-linked immunoabsorbent assay (ELISA) and immunofluorescence quantitative test uses monoclonal anti-CRP antibodies marked with an enzyme or fluorescent tracer fixed to microwells located within a microtiter plate or test tube. When diluted human serum is added, CRP binds to the immobilized marked anti-CRP antibody forming bound CRP-ligand complexes. Unbound antibodies are then washed from the test tube. The fluorescent-marked CRP-ligand complexes can be visualized and measured under a fluorescence microscope. A substrate is added to the enzyme-marked CRP-ligand complexes, which reacts with the enzyme causing a color reaction. The specimen is read with a spectrophotometer, which calculates the net color absorbency. This method of testing can be performed within 10 to 15 minutes and is capable of detecting CRP levels between 1 to 40 mg/L.[43,46,47]
Laser and rate nephelometry quantitative immunoassays use infrared light-emitting diodes and detectors passed through test tubes containing fixed amounts of anti-CRP monoclonal antibody mixed with human serum, resulting in the formation of CRP-ligand complexes.[29,46,47,48] Because the antibody concentration remains constant, the extent of light scatter is determined from the amount of CRP-ligand complexes. This method can be performed within 30 to 60 minutes. When using a fully automated analyzer, however, it takes 10 minutes.[29,47] The cost of the fully automated quantitative CRP test is reported to be around $50.00.
Qualitative and semiquantitative methods of measuring CRP, although less expensive, are not accurate enough to be used for infants.[12,27,29] The limited detection level of 6 mg/L in some kits could easily miss infants with true sepsis and result in longer antibiotic therapy. It is imperative that practitioners be aware of which CRP testing method is used in their own institution and the sensitivity and specificity of that test when evaluating measured CRP levels in the clinical setting.
Adv Neonatal Care. 2003;3(1) © 2003 W.B. Saunders
Cite this: The Role of C-Reactive Protein in the Evaluation and Management of Infants With Suspected Sepsis - Medscape - Feb 01, 2003.