Turbidimetric and Nephelometric Assays

The analytical performance of nephelometric or turbidimetric assays can be significantly improved by increasing the reaction rate by addition of water-soluble linear polymers. This allows the use of much lower reactant concentrations and results in a more stable immune complex suspension. 

Nephelometric methods in general are more sensitive than turbidimetric assays and have an average lower limit of detection of 0.1 to 10 mg/L for a serum protein. Lower detection limits are obtained in fluids, such as cerebrospinal fluid and urine, because of their lower lipid and protein concentrations, which results in a better signal-to-noise ratio. In addition, for low molecular weight proteins (e.g., myoglobin, MW 17,800), assay detection Emits can be lowered using a latex-enhanced procedure based on antibody-coated latex beads.  

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A number of immunochemical techniques have been used to quantify analytes of clinical interest. They include radial diffusion (RID) and electroimmunoassays, turbidimetric and nephelometric assays, and labeled immunochemical assays. 

Turbidimetric and nephelometric assay Nephelo-metry and turbidimetry, because of their speed and ease of use, are most widely used. These techniques can be used either by measuring the amount of Ag-Ab complex formation (endpoint methods) or by measuring the rate of complex formation (kinetic methods). The kinetic methods are more rapid because measurements are accomplished within 20 s, and are more precise because sample blanks are not necessary. Kinetic assays are, however, somewhat less sensitive because low-affinity antibodies do not have time to react. The occurrence of Ag-Ab formation is related to the amount of light scattering and is used as the basis for antigen quantification. This approach has been accompanied by the development of... 

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