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Background: Available assays for cardiac troponin I (cTnI) yield numerically different results. The aim of this study was to compare patient values obtained from four cTnI immunoassays., Methods: We studied the Stratus(R) II assay, the Opus(R) II assay, the Access(R) assay, and a research-only cTnI heterogeneous immunoassay that uses the Dade Behring aca(R) plus immunoassay system equipped with two new noncommercial monoclonal antibodies. Because the aca plus cTnI assay is for research only, we first evaluated and analytically validated it for serum and citrated plasma. Initially, each method was calibrated using the method-specific calibrator supplied by each manufacturer; however, the aca plus cTnI assay was calibrated using patient serum pools containing cTnI and selected on the basis of increased creatine kinase MB isoenzyme and with values assigned by use of the Stratus cTnI assay. For method comparisons, individual patient sample cTnI values were determined and compared with the Stratus II assay., Results: Passing and Bablock regression analysis yielded slopes of 1.44 (r = 0.96; n = 72) for the Opus II vs Stratus II assays; 0.07 (r = 0.91; n = 72) for the Access vs Stratus II assays; and 0.90 (r = 0.91, n = 72) for the aca plus vs Stratus II assays. The recalibration of each method with a Stratus II-assigned serum pool improved, but did not entirely eliminate, the slope differences between the different assays (range, 1.00-1.16). The observed scatter in the correlation curves remained., Conclusion: There is a need to further explore the specificities of these assays with respect to the different circulating forms of cTnI.

The measurement of circulating prealbumin has been shown to be clinically useful in the assessment of nutritional status, both as an initial screen and in the monitoring of nutritional recovery. We describe a fully automated, noncompetitive, homogeneous, light-scattering immunoassay that has been developed for this analyte on a Dimension (Dade) analyzer. A sheep anti-prealbumin IgG fraction was covalently coupled to 40-nm chloromethyl styrene particles and, after the addition of sample, polyethylene glycol-assisted immunoagglutination was monitored by turbidimetry. The prealbumin working assay range was 8-550 mg/L at a sample volume of 2 microL and a reaction time of 6.5 min. When data were analyzed using ANOVA, total and within-run assay imprecision values (CVs) were 1-5%, and calibration and reagent stabilities were in excess of 40 days. Mean analytical recoveries were 102% +/- 4% (SD), and there was no lack of parallelism. Hemolysis, lipemia, and bilirubin did not interfere. Both plasma anticoagulated with heparin or EDTA and serum from plain or serum-separation tubes were acceptable as sample matrices. Comparison with the Beckman Array method gave a Passing and Bablok regression of: Dimension analyzer = 1.01Beckman + 7.1 (n = 103), using a common calibrator. We conclude that the prealbumin method is appropriate for clinical use according to the analytical criteria used in this study.

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.

Serum cystatin C has been suggested as a new marker of glomerular filtration rate (GFR). We describe a fully automated and rapid particle-enhanced nephelometric immunoassay (PENIA) for measuring serum cystatin C on the Behring nephelometer systems (BNA, BN II). Each sample is analyzed in 6 min with as many as 75 samples per batch. The assay covers the range 0.23-7.25 mg/L, up to seven times the upper limit of normal. The intra- and interassay imprecision are < 3.3% and < 4.5%, respectively. There is absolute linearity across the assay range (r2 = 0.997), with analytical recovery by cystatin C addition between 95% and 109% (mean 102%). Hemoglobin (< or = 8.0 g/L), bilirubin (< or = 488 microL), triglycerides (< or = 23 mmol/L), rheumatoid factor (< or = 2000 kIU/L), and myeloma paraprotein (< or = 41 g/L) do not interfere with the assay. This assay agreed well with an in-house particle-enhanced turbidimetric immunoassay (PETIA) (mean difference = 1.73 +/- 2.10) and a commercial PETIA (mean difference = 1.13 +/- 0.86). This is a new assay by which cystatin C may be effectively used as a marker of GFR estimation.

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 microL 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 x 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.

At present no method for glycohemoglobin (%HbA1c) is automated on a main-line analyzer to allow joint measurement with other indicators of diabetic control such as glucose and cholesterol. We describe an adaptation of a latex-enhanced competitive immunoassay for quantifying %HbA1c to the Dade International Dimension analyzer. After a manual hemolysis step, HbA1c and total hemoglobin (Hb) are determined separately. The concentration of glycated beta-subunit is obtained from the immunoassay, whereas Hb is assessed colorimetrically from a derivatized form. Both reactions were fully optimized for accuracy, precision, and specificity on the Dimension; stabilities of reagents and calibration were established; and potential interferences were assessed. The analyzer gave reliable results over the required clinical range of 1-15% HbA1c. Within-run and total assay variation were within 5% of the target CV limits, as determined by ANOVA with three representative sample pools across 20 days. Close agreement with an established HPLC procedure and a commercially available enzyme immunoassay was observed for 140 samples from clinically defined patient groups. Additional samples from patients with hemoglobinopathies (n = 20) demonstrated a more complex relationship between methods. We conclude that adaptation of the method for use with the Dimension analyzer is a valid method for quantifying %HbA1c.

An immunosensor is a device comprising an antigen or antibody species coupled to a signal transducer, which detects the binding of the complementary species. An indirect immunosensor uses a separate labeled species that is detected after binding by, e.g., fluorescence or luminescence (i.e., a heterogeneous immunoassay). A direct device detects the binding by a change in potential difference, current, resistance, mass, heat, or optical properties (i.e., a homogeneous immunoassay). Although indirect sensors may encounter fewer problems due to nonspecific binding effects, the direct sensors are capable of real-time monitoring of the antigen-antibody reaction. A wide range of molecules can be detected with detection limits ranging between 10(-9) and 10(-13) mol/L. However, there are only a few successful commercial applications of direct immunosensors, these being of the optical type. This review describes the principles underlying the technologies, their merits, limitations, and applications.

We report a fully automated method for determining dibucaine number (DN) in a single-run procedure involving Dimension cholinesterase (CHE) Flex pseudo-(P)CHE reagents. The method was developed and optimized with the "open channels" and "kinetic" software facilities of the Dimension-ES instrument, where the DN is calculated automatically by an algorithm from the ratio of the uninhibited and inhibited rates, measured bichromatically, from a single analysis. The protocol was satisfactorily assessed for substrate depletion, linearity, reagent stability. and the effects of different dibucaine concentrations. Validation was performed across a range of CHE activities (1.5-22 kU/L) representing the three main genotypes, UU, UA, and AA. The respective DNs (mean +/- SD), determined on the Dimension-ES, were 82.0 +/- 1.6 (n = 32), 71.0 +/- 3.1 (n = 10), and 23.0 +/- 2.7 (n = 14), with corresponding imprecisions (CV) of 0.3%, 0.6%, and 5.2% (intraassay) and 0.7%, 0.7%, and 8.6% (interassay). Comparisons with reference (x) laboratory values and the DuPont aca (x') procedure (n = 53) gave regression equations of: y = 0.88x + 11.2, r = 0.99, and y = 0.85x' + 11.9, r = 0.99. A separate trial conducted with a Dimension-AR instrument gave similar performances. We conclude that the new DN method is fast, efficient, and appropriate for clinical use.

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.

A particle-enhanced turbidimetric immunoassay (PETIA) for human sex-hormone-binding globulin (SHBG) is described. The method involves use of antibody covalently coupled to latex particles and is almost fully automated, with sample processing being complete in less than 20 min. The working reagents are stable for at least three months, and full calibration of the assay each day is not essential. A particular advantage is that pretreatment of samples is rarely required because the working range of the assay is from 2.0 to 320 nmol/L for nondiluted serum. Intra- and interassay CVs were less than 4.5% and 8.5%, respectively, and mean analytical recovery was 101.5%. SHBG concentrations of 129 serum samples determined by this method and by a commercially available immunoradiometric assay correlated highly.

We describe a rapid particle-enhanced turbidimetric immunoassay for albumin in urine. Intra- and interassay CVs were less than 5% and less than 10%, respectively, the detection limit is 2 mg/L, and the working range extends to 200 mg/L. Mean analytical recovery of albumin added to centrifuged urines was 100% (SD 10.6%), and, when results were compared with those by the Pharmacia RIA, the correlation coefficient was 0.99. The working reagents are stable for at least six months; thus this assay is suited for both batch and urgent analysis.