The measurement of urine albumin now has a well-established role in the monitoring of patients with diabetes mellitus. We have developed a particle-enhanced immunoturbidimetric inhibition assay for urine albumin on the Dade aca® analyzer. The inhibition approach removes any of the potential antigen excess difficulties that could be expected from the wide clinical range of urine albumin, but retains the sensitivity advantages of latex-enhanced immunoturbidimetry. Human serum albumin (HSA) is covalently attached to 40-nm poly(chloromethyl)styrene-modified latex particles. This reagent, along with monoclonal antibody to HSA, is aliquoted into the aca reagent pack along with polyethylene glycol 8000 in a tablet form (giving a final reaction concentration of 15 g/L). A 150 mmol/L phosphate buffer, pH 7.8, is used to fill the reagent pack in the instrument and the agglutination reaction is monitored at 340 nm. The sample volume is 100 μL and the calibration curve covers the range 2–250 mg/L. Evaluation of commercial scale reagents against the Beckman Array nephelometric immunoassay system gave a Deming regression correlation of aca = 0.87 × Beckman + 8.5,r = 0.995, n = 145. Mean analytical recovery was 104 ± 4.5%, n = 20, and there was no evidence of a lack of parallelism. Interassay precision was 8.8% at 10.0 mg/L and <2.5% at >65 mg/L. Calibrator stability was in excess of 60 days. A small reference range study (24-h urine collections, n = 27) gave a mean of 5.6 mg/L with a range of 0.5–16.2 mg/L. Analytical sensitivity (2.5 SD from zero) was 0.40 mg/L.

Recent innovations in particle design have led to the development of highly sensitive and reproducible immunoassay methods for the Du Pont aca discrete clinical analyzer. Key advances include the synthesis and use of particles less than 1 micron in diameter with high refractive index cores surrounded by thin, chemically reactive shells. The cores are prepared by emulsion polymerization to a well-defined size that depends on the choice of monomer and the requirements for turbidimetric signal. Methods for measuring therapeutic drugs (e.g., theophylline) involve particles with polystyrene cores; other methods require cores with higher refractive indices such as polyvinylnaphthalene. The shells are critical for overall method performance because they bind covalently the immunochemicals of interest. Polyglycidyl methacrylate shells have been used effectively to attach antigens and haptens to the particle surface.

We report kinetic studies on the reaction of a latex agglutination immunoassay used to quantify phenytoin in serum. In this assay, polystyrene particles with a covalently attached analog of phenytoin react with an antiphenytoin monoclonal antibody to form light-scattering aggregates, with the rate of this reaction being decreased by addition of phenytoin from sample. In the absence of free (sample) phenytoin, this reaction did not exhibit a maximum rate of agglutination in the presence of excess antibody, i.e., an equivalence point. Furthermore, agglutination was inhibitable by free phenytoin even when the latter was added after agglutination of particles with antibody had begun. Most significantly, the immunoagglutination proceeded in an identical fashion with monovalent F(ab) fragment. These data are consistent with low-affinity immunospecific particle–antibody complexation, which then induces colloidal aggregation, without requiring immunospecific bridging by antibody molecules. The described mechanism is not generalizable to all latex agglutination immunoassays, although disturbance of colloidal stability may be a component in most assays.

Background: Monitoring of the concentration of gentamicin in serum and plasma during therapy is widely recommended and practiced in hospitals. Our aim was to develop a homogeneous immunoassay based on particle-enhanced turbidimetric inhibition immunoassay technology to quantify gentamicin on the Dimension® clinical chemistry system. Methods: Assay performance was assessed on each of the Dimension models in a 15-instrument interlaboratory comparison study. A split-sample comparison (n = 1171) was also performed between the gentamicin methods on the Dimension system and the Abbott®TDx® analyzer, using multiple reagent and calibrator lots on multiple instruments. Results: The Dimension method was linear to 25.1 μmol/L (12.0 μg/mL) with a detection limit of 0.63 μmol/L (0.3 μg/mL). Calibration was stable for 30 days. The within-run imprecision (CV) was <1.3%, and total imprecision ranged from 1.8% to 3.2% between 4.2 μmol/L (2.0 μg/mL) and 16.7 μmol/L (8.0 μg/mL) gentamicin. Linear regression analysis of the results on the Dimension method (DM) vs the Abbott TDx yielded the following equation: DM = 0.98TDx − 0.42; r = 0.987. Minimal interference was observed from structurally related compounds such as sagamicin, netilmicin, and sisomicin. Conclusion: The monoclonal antibody used in this method has similar reactivities toward the individual gentamicin subspecies C1, C1a, and C2, thus providing analytical recovery not significantly dependent on relative subspecies concentrations.

Turbidimetric immunoassay is commonly used to quantify serum proteins. Latex-particle enhancement of this type of assay has been primarily associated with increasing assay sensitivity. However, covalent coupling of an antibody to a latex particle can offer other advantages that are also pertinent in measurement of high concentrations of analytes. By using a common antibody with IgG as a model analyte, we describe the development of a nonenhanced and a latex-particle-enhanced turbidimetric assay for measuring serum IgG. Both assays show adequate analytical recovery and parallelism, and results compare well with those by rate nephelometry. The latex-enhanced assay has equivalent sensitivity, working range, and interassay precision, but much greater signal change and calibration stability than the nonenhanced assay. In addition, with latex particles, less antiserum is needed. Coupling antibodies to latex particles offers considerable advantages, even when an improved assay detection limit is not required.