Quantitative, Wide-Range, 5-Minute Point-of-Care Immunoassay for Total Human Chorionic Gonadotropin in Whole Blood

Accurate detection of β-hCG in women's serum and cervical secretions for predicting early pregnancy viability based on time-resolved luminescent lanthanide nanoprobes

Sensitive and specific detection of β-hCG in women's serum and cervical secretions is of great significance for early pregnancy evaluation. However, the accurate detection of trace amounts of β-hCG in cervical secretions remains challenging because of its low level. Herein, we report a unique strategy for β-hCG detection in a heterogeneous sandwich-type bioassay by using LiLuF₄:Ce,Tb nanoparticles as time-resolved photoluminescence (PL) nanoprobes. By taking advantage of the intense and long-lived PL of the nanoprobes, the short-lived background autofluorescence can be completely eliminated, which enables the sensitive detection of β-hCG with a linear range of 0-10 ng mL^-1 and a detection limit down to 6.1 pg mL^-1, approximately two orders of magnitude improvement relative to that of a commercial β-hCG assay kit. Furthermore, we demonstrate the application of the nanoprobes for accurate detection of β-hCG in clinical serum and cervical secretion samples and unveil that the ratio of β-hCG levels in cervical secretions and serum can be a good indicator of early pregnancy viability in unknown locations. These findings bring new opportunities in perinatal medicine by employing luminescent lanthanide nanoprobes, thus laying a foundation for future development of luminescent nanoprobes for versatile biomedical applications.

Advances in human chorionic gonadotropin detection technologies: a review

Human chorionic gonadotropin (HCG) is a glycoprotein secreted by placental trophoblast cells in pregnancy. HCG is a heterodimer composed of two different α- and β-subunits, with the latter being unique to HCG. As well as being the most important diagnostic markers for pregnancy, HCG is also a tumor marker, therefore, quantitative detection of HCG is of great value. Numerous advanced technologies have been developed for HCG concentration detection including electrochemical immunoassay, chemiluminescent immunoassay, fluorescence immunoassay, resonance scattering spectrometry, atomic emission spectrometry, radioimmunoassay, MS and so on. Some have pursued simple and easy operation, while others have emphasized on accuracy and applications in clinical medicine. This review provides a comprehensive summary of various methods of detecting HCG.

Micro-scale blood plasma separation: from acoustophoresis to egg-beaters

Plasma is a rich mine of various biomarkers including proteins, metabolites and circulating nucleic acids. The diagnostic and therapeutic potential of these analytes has been quite recently uncovered, and the number of plasma biomarkers will still be growing in the coming years. A significant part of the blood plasma preparation is still handled manually, off-chip, via centrifugation or filtration. These batch methods have variable waiting times, and are often performed under non-reproducible conditions that may impair the collection of analytes of interest, with variable degradation. The development of miniaturised modules capable of automated and reproducible blood plasma separation would aid in the translation of lab-on-a-chip devices to the clinical market. Here we propose a systematic review of major plasma analytes and target applications, alongside existing solutions for micro-scale blood plasma extraction, focusing on the approaches that have been biologically validated for specific applications.

Quantification of cystatin C by time-resolved fluorometry-based immunoassays

Plasma cystatin C is increasingly used as a marker of glomerular filtration rate. Most assays for cystatin C are based on turbidimetric or nephelometric detection and studies of other rapid methods are limited. This study aimed to develop and compare differently configured immunoassays for quantification of plasma cystatin C, using recombinant cystatin C and two cystatin C-specific antibodies. Method 1 was a two-step sandwich assay with polyclonal antibody as capture and europium chelate-labeled monoclonal antibody as tracer. Method 2 was a one-step heterogeneous competitive assay using immobilized polyclonal antibody and europium-labeled cystatin C. Method 3 was a one-step homogeneous competitive assay with europium-labeled polyclonal antibody as donor and cyanine 5-labeled cystatin C as acceptor. All three assays were evaluated with plasma samples and their performance was compared to a conventional particle-enhanced turbidimetric immunoassay (PETIA). Method 3 was the easiest to perform, with incubation at ambient temperature for 10 min and 20 μL of sample, while methods 1 and 2 had washing steps, took 40 min and 15 min at 37°C, respectively, but used only 10 μL of 100- or 10-fold diluted sample, respectively. The working ranges for methods 1, 2 and 3 were 0.0005-0.2, 0.05-1.0 and 0.25-20mg/L, respectively. Kinetics for method 3 was the fastest with >95% binding completion and for method 2 the slowest with 60% binding completion. All three methods showed good correlation to PETIA, but produced higher cystatin C levels than PETIA. Methods 1 and 3 offered the most favorable performance characteristics and especially method 3 enabled rapid and simple measurement of circulating cystatin C.

Dry-Reagent Double-Monoclonal Assay for Cystatin C

BACKGROUND

Cystatin C is a low molecular weight cysteine proteinase inhibitor whose plasma or serum concentrations have been shown to be better correlated with glomerular filtration rate than serum creatinine concentrations. Routine assays for cystatin C are based on use of polyclonal antibodies and immunoturbidimetric and nephelometric designs. This study aimed to develop a double-monoclonal immunoassay for cystatin C.

METHODS

We tested functionality of 42 2-site antibody combinations involving 7 monoclonal antibodies with recombinant and plasma cystatin C. We developed a heterogeneous assay using 2 antibodies selected to give the best analytical performance. The assay used a dilution step and was based on a dry-reagent, all-in-one immunoassay concept with time-resolved fluorometry. The assay was performed on an automated immunoanalyzer in single wells that contained all the required assay components. We used heparin-derived plasma samples for methodological evaluation of the assay.

RESULTS

From a relative epitope map involving 7 cystatin C–specific antibodies, we selected a pair of antibodies for a 2-site sandwich-type dry-reagent assay. Total assay time was 15 min, and 10 μL of a 100-fold diluted sample was used. The analytical detection limit (background + 3SD) and functional detection limit (CV 20%) were 0.01 mg/L and 0.02 mg/L, respectively. Within-run and total assay imprecision were <4.7% and <5.6% (at 0.84–3.2 mg/L), respectively, and plasma recoveries of added cystatin C were 94%–110%. Regression analysis with the Roche particle-enhanced immunoturbidimetric method yielded the following (SD): slope, 1.391 (0.029); y-intercept, −0.152 (0.045) mg/L; Sy⊻x=0.294 mg/L (n=131).

CONCLUSIONS

The developed assay enables rapid and reliable measurement of cystatin C.

Magnetic relaxation switch detection of human chorionic gonadotrophin

Functionalized nanoparticle contrast agents, also known as magnetic relaxation switches (MRS), were prepared to detect protein A and the beta subunit of human chorionic gonadotrophin (hCG-beta). Antibodies were attached to cross-linked iron oxide (CLIO) nanoparticles using standard peptide chemistry. Protein A was used as a simple model analyte, as it is naturally multivalent and can bind multiple CLIO-IgG simultaneously. The addition of PA to CLIO-IgG resulted in transverse relaxation time (T2) shortening compared to a blank control as seen by NMR relaxometry measurements. Analyte-induced aggregation was confirmed by light scattering particle size analysis. A two-particle system was designed to measure hCG-beta, as it is not multivalent and requires conjugation of a matched pair of monoclonal antibodies to CLIO (referred to as C95 and C97). Measurement of hCG-beta is important, as elevated serum levels are associated with malignancies including testicular and ovarian cancers. The addition of hCG-beta to C95 and C97 resulted in T2 shortening with a linear dynamic concentration range of 0.1 to 1 molecules of analyte per nanoparticle. Similar data were obtained for the hCG dimer. Observations with higher stoichiometric ratios of analyte to nanoparticle and increased nanoparticle valency were also made. This method can potentially be adapted to detect other biomarkers in solution.

Red blood cells do not attenuate the SPCE fluorescence in surface assays

We describe the positive effect of surface plasmon-coupled fluorescence emission (SPCE) on the detection of a signal from a surface immunoassay in highly absorbing or/and scattering samples. A model immunoassay using fluorescently labeled anti-rabbit antibodies that bind to rabbit immunoglobulin on a silver surface was performed, and the signal was detected in the presence of various highly absorbing and/or scattering solutions or suspensions, such as hemoglobin solution, plastic beads, and red blood cells. The results showed that a highly absorbing solution consisting of small molecules (dye, hemoglobin) attenuates the SPCE signal approximately 2-3-fold. In contrast, suspensions with the same absorption containing large particles (large beads, red blood cell suspension) attenuate the SPCE signal only slightly, approximately 5-10%. Also, a suspension of large undyed, highly scattering beads does not reduce the SPCE signal. The effects on the immunoassay signal of the sample background absorption and scattering, the size of the background particles, and the geometry of the experimental set-up are discussed. We believe that SPCE is a promising technique in the development of biosensors utilized for surface-based assays, as well as any assays performed directly in highly absorbing and/or scattering solutions without washing or separation procedures. Figure Red blood cells (unlike hemoglobin) do not attenuate the SPCE fluorescence in surface assays.

Upconversion fluorescence resonance energy transfer in a homogeneous immunoassay for estradiol

We recently described a novel homogeneous assay principle based on upconversion fluorescence resonance energy transfer (UC-FRET), where an upconverting phosphor (UCP) is utilized as a donor. The UC-FRET has now been applied to a competitive homogeneous immunoassay for 17beta-estradiol (E2) in serum, using a small-molecular dye as an acceptor. The assay was constructed by employing an UCP coated with an E2-specific recombinant antibody Fab fragment as a donor and an E2-conjugated small-molecular dye, Oyster-556, as an acceptor. Standard curves for the assay were produced both in buffer and in male serum. Sensitized acceptor emission was measured at 600 nm under continuous laser diode excitation at 980 nm. In buffer, the IC50 value of the assay was 1 nM and in serum 3 nM. The lower limits of detection (mean of zero calibrators, 3 SD) were 0.4 and 0.9 nM, respectively. The measurable concentration range extended up to 3 nM in buffer and 9 nM in serum. Equilibrium in the assay was reached in 30 min. The novel principle of UC-FRET has unique advantages compared to present homogeneous luminescence-based methods and can enable an attractive assay system platform for clinical diagnostics and for high-throughput screening approaches.

Microwave-Accelerated Surface Plasmon-Coupled Directional Luminescence: application to fast and sensitive assays in buffer, human serum and whole blood

The applicability of a new technique, Microwave-Accelerated Surface Plasmon-Coupled Luminescence (MA-SPCL) for fast and sensitive bioassays in buffer, serum and whole blood using quantum dots as luminescence reporters is demonstrated. In this regard, a model bioassay based on the well-known interactions of biotin and streptavidin is used. Using MA-SPCL, the bioassay was kinetically completed within 1 min with the use of low power microwave heating as compared to the identical bioassay which took in excess of 30 min to reach >95% completion at room temperature, a 30-fold increase in assay kinetics. The luminescence emission from the quantum dots was coupled to surface plasmons of the gold film, enabling the detection of the luminescence emission in a highly directional fashion as compared to the normal isotropic emission, for enhanced sensitivity and detection. The combined effect of microwaves for faster assay kinetics, with surface plasmon-coupled luminescence for sensitive luminescence measurements, has also made possible the demonstration of the use of the MA-SPCL technique for assays run in complex media, such as human serum and whole blood, where the same assay could not be performed at room temperature due to the coagulation of blood. In the MA-SPCL assay run in serum and whole blood, the luminescence intensity from 33 nM quantum dots was 75% and 20% that of the luminescence intensity from the assay run in buffer, with a signal to noise ratio of 12.5 and 3, respectively.

Immunofluorometric Point-of-Care Assays for the Detection of Acute Coronary Syndrome-Related Noncomplexed Pregnancy-Associated Plasma Protein A

Background: We recently reported that the pregnancy-associated plasma protein A (PAPP-A) form specifically related to acute coronary syndromes (ACS) is not complexed with the proform of eosinophil major basic protein (proMBP). The aim of this study was to develop rapid point-of-care immunoassays for the measurement of the noncomplexed PAPP-A.

Methods: We developed immunofluorometric noncompetitive dry-reagent assays for total PAPP-A with 2 PAPP-A subunit-specific monoclonal antibodies and for PAPP-A/proMBP complex with 1 PAPP-A subunit-specific antibody and 1 proMBP subunit-specific antibody. The concentration of noncomplexed PAPP-A was determined as the difference of the results obtained with the 2 assays.

Results: The assays were linear from 0.5 to 300 mIU/L. The analytical detection limit and functional detection limit (CV <20%) were 0.18 mIU/L and 0.27 mIU/L for total PAPP-A assay and 0.23 mIU/L and 0.70 mIU/L for PAPP-A/proMBP assay, respectively. The total assay imprecisions were <10%, and recoveries were 88%–107% for both assays. The mean difference (95% limits of agreement) between the new total PAPP-A assay and a previously reported total PAPP-A assay was −3.2% (−45.7% to 39.3%; n = 546; P = 0.0019). In serum samples from 159 non-ACS individuals, median concentrations (interquartile range) were 2.42 (1.14) mIU/L for total PAPP-A, 2.20 (1.18) mIU/L for PAPP-A/proMBP, and 0.18 (0.63) mIU/L for noncomplexed PAPP-A. Total PAPP-A and PAPP-A/proMBP, but not noncomplexed PAPP-A, correlated with age (r = 0.290, P = 0.0002; r = 0.230, P = 0.0035; r = 0.075, P = 0.3483, respectively).

Conclusions: The new assays described revealed that noncomplexed PAPP-A is found only in negligible amounts in non-ACS samples.

Plastic versus glass support for an immunoassay on metal-coated surfaces in optically dense samples utilizing directional surface plasmon-coupled emission

We compared plastic (polycarbonate) and high-quality glass support materials for gold-coated slides, when performing a model immunoassay against rabbit IgG using fluorescently labeled (AlexaFluor-647) anti-rabbit IgG, and detecting surface plasmon-coupled emission (SPCE) signals. Both, glass and plastic slides were simultaneously coated with a 48-nm layer of gold and protected with a 10-nm layer of silica. The maximum SPCE signal of AlexaFluor-647 was only two- to three-fold smaller on plastic slides than on glass slides. A small difference in the SPCE angles on glass (theta (F) = 55 degrees ) and plastic (theta (F) = 52.5 degrees ) slides was observed and can be explained with a slightly smaller refractive index of the plastic. We have not found any difference in the angle distribution (sharpness of the fluorescence signal at optimal SPCE angle) for the plastic slide compared to the glass slide. The kinetics of binding was monitored on the plastic slide as well as on the glass slide. Optically dense samples, a 4% red blood cell suspension and a 15% hemoglobin solution, are causing a reduction in the immunoassay SPCE signal by approximately 15% and three times, respectively, and the percentage of the reduction is the same for plastic and for glass slides. We believe that plastic substrates can be readily used in any SPCE assay, with only marginally lower total signal compared to high-quality glass slides.

Directional surface plasmon-coupled emission: application for an immunoassay in whole blood

We present a new approach for performing fluorescence immunoassay in whole blood using fluorescently labeled anti-rabbit immunoglobulin G (IgG) on a silver surface. This approach, which is based on surface plasmon-coupled emission (SPCE), provides increased sensitivity and substantial background reduction due to exclusive selection of the signal from the fluorophores located near a bioaffinity surface. This article describes the effect of an optically dense sample matrix, namely human whole blood and serum, on the intensity of the SPCE. An antigen (rabbit IgG) was adsorbed to a slide covered with a thin silver metal layer, and the SPCE signal from the fluorophore-labeled anti-rabbit antibody, binding to the immobilized antigen, was detected. The effect of the sample matrix (buffer, human serum, or human whole blood) on the end-point immunoassay SPCE signal was studied. It was demonstrated that the kinetics of binding could be monitored directly in whole blood or serum. The results showed that human serum and human whole blood attenuate the SPCE end-point signal and the immunoassay kinetic signal only approximately two- and threefold, respectively, as compared with buffer, resulting in signals that are easily detectable even in whole blood. The high optical absorption of the hemoglobin can be tolerated because only fluorophores within a couple of hundred nanometers from the metallic film contribute to SPCE. Excited fluorophores outside the 200-nm layer do not contribute to SPCE, and their free space emission is not transmitted through the opaque metallic film into the glass substrate. We believe that SPCE has the potential of becoming a powerful approach for performing immunoassays based on surface-bound analytes or antibodies for many biomarkers directly in dense samples such as whole blood with no need for washing steps.

Sensitive and quantitative, 10-min immunofluorometric assay for D-Dimer in whole blood

INTRODUCTION

Normal concentrations of D-Dimer can be used to exclude venous thromboembolism (VTE). However, methods for sensitive and quantitative D-Dimer measurements at the point-of-care (POC) are still limited.

MATERIALS AND METHODS

We developed a 10-min, non-competitive immunofluorometric assay for D-Dimer in citrated whole blood and plasma using pre-dispensed reagents dried in single assay wells. The simple, automated assay procedure comprises a 1:50 sample dilution, one-step incubation, washing, and time-resolved fluorometric measurement directly from the wet well surface.

RESULTS

The limits of detection (background + 3SD) and quantification (CV <15%) were 0.05 and 0.2 mg/L D-Dimer, respectively, and the assay was linear up to 400 mg/L. Correlations to Roche TinaQuant (r=0.726, n=200) and Biopool Auto.Dimer (r=0.190, n=149) were carried out using citrated plasma. Diagnostic sensitivity, specificity, and negative (NPV) and positive (PPV) predictive values were 98.7%, 64.4%, 99.1% and 55.1%, and 92.2%, 81.0%, 95.9% and 68.3%, respectively, using cut-off values of 0.6 and 1.0 mg/L, respectively, in outpatients with deep vein thrombosis (DVT) and/or pulmonary embolism (PE) (n=77) compared with outpatients with various other diseases (n=174). The within- and between-run CVs near the cut-off values were < or =10% in both whole blood and plasma. The 95th percentile upper range in apparently healthy individuals was 0.68 mg/L of whole blood (n=101).

CONCLUSIONS

The high sensitivity and NPV suggest that the rapid immunofluorometric assay could be valuable for rapid exclusion of VTE in outpatients. With appropriate cut-offs, the assay could potentially be used as a stand-alone test or combined with clinical probability assessment, but further studies are required.

Point-of-care immunotesting: Approaching the analytical performance of central laboratory methods

The use of point-of-care (POC) immunoassays has increased significantly and the menu of analytes continues to expand. Most of the rapid immunoassays are currently based on simple manual assay devices such as the immunochromatographic, agglutination, and immunofiltration assays. Although automated readers have recently been introduced at an increasing pace, the major benefit of these genuinely hand-portable assay devices is that they do not usually necessitate instrumentation but can be performed anywhere. Significant advances in assay and detection technologies have, however, recently facilitated the introduction of truly quantitative, sophisticated immunoassay methods to POC settings as well, with the analytical performance characteristics approaching those of conventional laboratory assays. Furthermore, innovative assay technologies such as those based on immunosensors have been introduced to POC testing (POCT) without ever being employed in clinical laboratories. However, further simplification of the assay procedures and analyzers is still feasible, and strong efforts are directed towards the development of miniaturized and simple, yet sensitive and quantitative, novel assay technologies to keep up with the increasing expectations set on future POC immunotesting.

Simultaneous Use of Time-Resolved Fluorescence and Anti-Stokes Photoluminescence in a Bioaffinity Assay

A bioaffinity assay is described where anti-Stokes photoluminescence of inorganic lanthanide phosphors and time-resolved fluorescence of lanthanide chelates are measured from a single microtitration well without any disturbance from these label technologies to each other. Up-converting phosphor (UPC-phosphor) bioconjugate was produced by grinding the commercial, micrometer-sized UPC-phosphors to colloidal, submicrometer-sized phosphor particles and by attaching these phosphors to biomolecules. Experiments were carried out in standard 96-well microtitration plates to determine detection limits, linearity, and cross-talk of UPC-phosphor and europium chelate. In numbers of molecules the lower limits of detection for UPC-phosphor were roughly 3 x 10(3) particles in solution and 1 x 10(4) particles in solid phase, and for europium label same values were 9 x 10(6) and 9 x 10(7) molecules. Linearity of detection was for UPC-phosphor 5 orders of magnitude in solution and over 4 orders of magnitude in solid phase and for europium label over 5 orders of magnitude in solution and over 4 orders of magnitude in solid phase. The cross-talk between the two labels was practically nonexistent. In this study we show that up-converting anti-Stokes photoluminescent phosphors could be employed in bioaffinity assays as very potential labels with significant advantages either alone or together with long-lifetime lanthanide chelates.