Automated nephelometric immunoassays with novel shell/core particles

Acoustofluidics 21: ultrasound-enhanced immunoassays and particle sensors

In part 21 of the tutorial series "Acoustofluidics--exploiting ultrasonic standing wave forces and acoustic streaming in microfluidic systems for cell and particle manipulation", we review applications of ultrasonic standing waves used for enhancing immunoassays and particle sensors. The paper covers ultrasonic enhancement of bead-based immuno-agglutination assays, bead-based immuno-fluorescence assays, vibrational spectroscopy sensors and cell deposition on a sensor surface.

Fully automated immunoassay for quantitative determination of FXIII

Coagulation factor XIII (FXIII) is essential for clot stabilization. Deficiency of FXIII is associated with a risk of bleeding and impaired wound healing. Substitution therapy with FXIII remedies for patients with low plasma levels of FXIII requires diagnostic quantification of the factor before and during therapy. Here, we describe a prototype of a preliminary research immunoassay for quantification of FXIII antigen on automated coagulation instruments. The prototype assay is based on a monoclonal antibody (mAb) directed against FXIII A chain, whereas the mAbs are coupled to latex particles. FXIII in a plasma specimen causes agglutination of the latex particles, which can be quantified turbidimetrically. Performance data of the assay prototype processed on BCS® XP and Sysmex® CA-1500 instruments demonstrate a good correlation to the Berichrom® factor XIII activity assay1 from Siemens Healthcare Diagnostics (r = 0.94). Results: Comparability of instruments was excellent (r = 0.98). Coefficients of variation of total imprecision measurements ranged from 2.2 to 3.4%. Linearity was excellent over the range tested (12–121% FXIII). Analytical sensitivity was 0.51% FXIII on BCS XP and 0.44% FXIII on Sysmex CA-1500, respectively. No interference (> 10% bias) was observed with haemoglobin (up to 400 mg/dl), cholesterol (up to 300 mg/dl), bilirubin (up to 60 mg/dl) or triglycerides (up to 3000 mg/dl). Conclusion: The preliminary research assay prototype has the potential for excellent analytical sensitivity, precision, and dynamic range suitable to measure reliably FXIII antigen levels in human plasma.

Latex of immunodiagnosis for detecting the Chagas disease. I. Synthesis of the base carboxylated latex

This article investigates the synthesis of two (monodisperse, carboxylated, and core-shell) latexes, through a batch and a semibatch emulsion copolymerizations of styrene (St) and methacrylic acid (MAA) onto polystyrene latex seeds. A mathematical model of the process was developed that predicts conversion, average particle size, and surface density of carboxyl groups. The model was adjusted to the batch reaction measurements, and then it was used in the design of the semibatch experiment. The semibatch reaction involved an initial homopolymerization of St followed by instantaneous addition of MAA-St-initiator. Compared with the batch reaction results, the semibatch policy more than doubled the surface density of carboxyl groups. The second part of this series describes the development of an immunodiagnosis latex-protein complex for detecting the Chagas disease, by coupling an antigen of Trypanosoma cruzi onto the produced carboxylated latexes.

Ultrasonic enhancement of bead-based bioaffinity assays

Ultrasonic radiation forces can be used for non-intrusive manipulation and concentration of suspended micrometer-sized particles. For bioanalytical purposes, standing-wave ultrasound has long been used for rapid immuno-agglutination of functionalized latex beads. More recently, detection methods based on laser-scanning fluorometry and single-step homogeneous bead-based assays show promise for fast, easy and sensitive biochemical analysis. If such methods are combined with ultrasonic enhancement, detection limits in the femtomolar region are feasible. In this paper, we review the development of standing-wave ultrasonic manipulation for bioanalysis, with special emphasis on miniaturization and ultrasensitive bead-based immunoassays.

Fluorescence-microscopy-based image analysis for analyte-dependent particle doublet detection in a single-step immunoagglutination assay

A novel fluorescence-microscopy-based image analysis method for classification of singlet and doublet latex particles is demonstrated and applied to a particle-based immunoagglutination assay for quantification of biomolecules in microliter-volume bulk samples. The image analysis method, verified by flow cytometric agglutination analysis, is based on a pattern recognition algorithm employing Gaussian-base-function fitting which allows robust identification and counting of singlets, doublets, and higher agglomerates of fluorescent microparticles. The immunoagglutination assay is experimentally modeled by a biotin-streptavidin interaction, with the goal of both theoretically and experimentally investigating the performance of a general immunoagglutination-based assay. For this purpose a theoretical model of the initial agglutination kinetics, based on particle diffusion combined with a steric factor determined by the level of specific and nonspecific agglutination, was developed. The theoretical model combined with the experimental data can be used to optimize an agglutination-based assay with regard to sensitivity and dynamic range and to estimate the affinity, receptor surface density, molecular and binding site sizes, and level of nonspecific binding that is present in the assay. The experimental results are in good agreement with the theoretical model, indicating the usefulness of the model for immunoagglutination assay optimization.

A review of factors affecting the performances of latex agglutination tests

The present review describes the different strategies followed to improve the performance of latex agglutination tests. The analysis is mainly focused on the diverse parameters that affect the final colloidal stability of the immunoprotein-latex system. These parameters include: the surface properties of polymer carriers; the different kind of antibodies usually employed; the use of BSA as stabilizer; the co-adsorption of various macromolecules (BSA, surfactants and lipids) and antibodies; recent approaches to colloidal stability at high ionic strengths due to hydration forces; and the covalent coupling of antibodies on functionalized latex particles. Special emphasis is given to the relation between electrophoretic mobility and the colloidal stability of the sensitized particles and how this knowledge can be utilized for a better understanding of the immunoagglutination kinetic.

A comparative study of optical techniques applied to particle-enhanced assays of C-reactive protein

This work is based on the well-established immunoassay principle of agglutination of latex particles covered by immunoproteins. In our experiments, positively charged particles act as carriers for the F(ab')2 fragment, obtained from rabbit polyclonal IgG, active against C-reactive protein (CRP). The presence of the antigen CRP in the immunolatex system causes agglutination and the aim of the present study was to compare different optical techniques (turbidimetry, nephelometry, angular anisotropy and photon correlation spectroscopy) capable of detecting the agglutination. The sensitivity and detection limit largely depend on the optical method. We have analyzed for each optical technique the following aspects: sensitivity, reproducibility, detection limit, reaction time, amount of sample wasted and availability of the required detection device. The results presented in this paper show that both angular anisotropy and photon correlation spectroscopy offer lower detection limits, and use little reagent, but have longer assay times than the classical optical techniques of turbidimetry and nephelometry.

Development and validation of an automated particle-enhanced nephelometric immunoassay method for the measurement of human plasma C1q

We have developed a sensitive immunoassay based on latex particle agglutination for measuring C1q concentrations in human plasma. In this simple and fast particle-enhanced immunoassay, we used carboxylated latex particles (diameter 210 nm) covalently coated with F(ab')2 fragments of anti-C1q antibodies. These particles are incubated with diluted sample (400-fold) for 6 min at room temperature, with the resulting agglutination quantified by measuring the change of light-scatter produced. The assay has been automated on the Behring nephelometer analyzer with a sampling rate of 150 samples/hr. This assay generates a standard curve in the range of 24-775 mg/L, showing intraassay and interassay precision of < 8% and < 10%, respectively. Dilution linearity was validated throughout the dynamic range of the assay. There were no interferences from bilirubin, Intralipid, haemoglobin, and rheumatoid factor. Results obtained in 45 clinical samples correlated well with those obtained by a commercial radial immunodiffusion method (r = 0.936), and with those obtained by the Behring immunoprecipitation nephelometric test (r = 0.950). The mean concentration in plasma from healthy subjects was 180 mg/L and the reference interval was from 128 to 237 mg/L. This latex nephelometric procedure is a convenient method and an interesting alternative to other immunoassays for routine measurement of human C1q.

Plasma myoglobin in the early diagnosis of acute myocardial infarction

Serum and plasma myoglobin and creatine kinase-MB catalytic activity were analysed in 157 patients admitted within 2 hours of the onset of chest pain (58 were retrospectively recognized as acute myocardial infarction). Serum and plasma values were highly correlated for both myoglobin and creatine kinase-MB. Plasma myoglobin appeared to be more sensitive than creatine kinase-MB for the early diagnosis of acute myocardial infarction; using a cut-off value of 100 micrograms/l, 90% of acute myocardial infarction cases were correctly recognized by plasma myoglobin 6 hours after the onset of chest pain, with a diagnostic specificity of 100% for non-acute myocardial infarction chest pain subjects. Plasma creatine kinase-MB showed a diagnostic sensitivity of 62% and a diagnostic specificity of 95% in the same group of patients. We suggest the inclusion of the plasma myoglobin immunonephelometric assay together with plasma creatine kinase-MB activity analysis in protocols for the early diagnosis of acute myocardial infarction.

The relationship between serum levels of lipoprotein(a) and proteins associated with the acute phase response

The association of serum lipoprotein(a) (Lp(a)) with inflammation was investigated in a primarily rheumatologic study group (n = 570; 202 males and 368 females) by studying the relationship between serum levels of Lp(a) and a panel of acute phase proteins (C-reactive protein (CRP), alpha 1-antitrypsin (AAT), alpha 1-acid glycoprotein (AGP), haptoglobin (HPT), complement components 3 and 4 (C3, C4), prealbumin (PAL), albumin (ALB) and transferrin (TRF)). Lp(a) data were adjusted for age and sex, but not clinical condition as no significant differences in Lp(a) levels were observed, using analysis of variance, among the 15 diagnostic categories in the study group. Univariate analyses revealed significant positive associations between Lp(a) levels and levels of C4, AGP, C3 and HPT. Multivariate analysis revealed that C4 and AGP (in descending order of significance) were significant independent predictors of Lp(a) concentration, together accounting for 2.9% of the variability in Lp(a) concentration in the present study group. The data indicate that confounding effects of an acute phase response should be considered in epidemiologic studies, if a high prevalence of inflammation is suspected.

Quantitative turbidimetric assay for determining myoglobin evaluated

A recently introduced turbidimetric immunoassay using shell/core particles for determination of myoglobin (Turbiquant Myoglobin, Behringwerke, Marburg, Germany) was evaluated on the Behring Turbitimer analyzer. Intra-assay CV varied between 1.0% and 3.2%; interassay CV was between 2.0% and 3.6%. Linear results were obtained between 2.8 nmol/L and 36.7 nmol/L. Manual dilution in saline solution allowed measurement of myoglobin concentrations up to 25,400 nmol/L. In the tested range, no high-dose 'hook' effect was observed. Haemolysis interfered with the assay when haemoglobin concentrations exceeded 2 g/L. The occurrence of error codes due to the presence of triglycerides was shown to depend on particle size. Repeat assays on diluted samples were necessary at triglyceride concentrations of 2.3 mmol/L for post-prandial chylomicrons (200-1000 nm), and at 11-31 mmol/L for very-low density lipoprotein-triglycerides (80-200 nm). No significant interferences of haptoglobin, bilirubin, iodine containing contrast media, and rheumatoid factors were detected for the assay. Treatment of lipaemic samples with Lipoclean (Behringwerke) resulted in lower myoglobin values. Simultaneously drawn serum and heparin-plasma samples gave comparable myoglobin results. However, values obtained in EDTA- or citrate-treated plasma samples occasionally differed significantly from serum values. The upper reference limit for myoglobin concentration was 4.2 nmol/L. The turbidimetric assay correlated well with a radioimmunoassay (Byk-Sangtec; r = 0.892) and with the automated nephelometric assay (Behringwerke; r = 0.944). Values obtained by turbidimetry were comparable to those obtained by the latex agglutination test (Behringwerke).

Quantitative automated latex nephelometric immunoassay for determination of myoglobin in human serum

We have evaluated a new latex nephelometric test for the quantitation of myoglobin in human serum. The assay consists of incubating the diluted serum sample (20-fold) for 12 min at room temperature with latex particles covalently coated with anti-myoglobin antibodies and then quantifying the change of light-scatter produced. The assay is fully automated on the Behring nephelometer analyzer with a sampling rate of 150 samples/hour. There is no interference from bilirubin (up to 340 mumol/l), haemoglobin (up to 7,000 mg/l), or rheumatoid factor (up to 1,100 int. units/ml). Myoglobin standard curve extends from 20 to 380 micrograms/l. Assay detection limit lies around 6 micrograms/l. Coefficient of variation ranged from 2.7 to 7.6%. Correlation coefficient between latex immunoassay and an RIA method was 0.987, calculated from the assay of 37 samples. A statistically significant difference was found between the distribution for females and males. The serum level of myoglobin showed an age-dependent variation. Concentrations up to 60 micrograms/l are considered to be normal.