Development of a competitive immunoassay for the determination of cortisol in human saliva

Diagnostic performance of an automated immunoassay for salivary cortisol

Automated immunoanalysis (AI) is an interesting alternative for measuring salivary cortisol, as the gold standard HPLC-MS/MS method is not yet readily available. The aim of this study was to evaluate the diagnostic performance of salivary cortisol immunoassay on the iSYS immunoanalyzer in adrenal dynamic tests. Cortisol was measured on iSYS and on HPLC-MS/MS in saliva samples collected after 1mg-dexamethasone suppression test (DST) in 115 patients suspected of Cushing syndrome, and during Synacthen® stimulation test (SST) in 108 patients suspected of adrenal insufficiency. Concentrations on AI correlated well with HPLC-MS/MS (Spearman r=0.9496; P<0.0001), but with a significant positive bias. ROC analysis of salivary cortisol identified optimal cut-off values on AI and HPLC-MS/MS of respectively 3.5 and 0.77nmol/L for DST and 32.6 and 13.8nmol/L at T60 after SST. Automated immunoassays for salivary cortisol are suitable in daily practice but require determination of specific cut-off and reference values.

Portable Chemiluminescence-Based Lateral Flow Assay Platform for the Detection of Cortisol in Human Serum

In this study, we developed the portable chemiluminescence (CL)-based lateral flow assay (LFA) platform for the detection of cortisol in human serum. Cortisol is well-known as a stress hormone due to its high relevancy for human mental and physical health, such as hypertension or depression. To date, a number of optical devices have provided the sensitive determination of levels of analytes. However, this modality type still requires costly optical modules. The developed CL platform is simply composed of two detection modules along with a loading part for the LFA strip. The LFA membrane contains gold nanoparticle probes conjugated with antibodies against cortisol and horseradish peroxidase (HRP), which can also efficiently increase the luminescent signal by providing many areas for anti-cortisol antibody and HRP. The measured voltage signals coming from the photodiode in a CL reader were compared with a standard microplate reader for the evaluation of accuracy. The linear range observed for cortisol was measured to be 0.78–12.5 μg/dL (R² = 0.99) with a limit of detection (LOD) of 0.342 μg/dL. In addition, the CL-LFA reader showed a high correlation (R² = 0.96) with the standard cortisol console (COBAS 8000, Roche), suggesting that our developed CL-based LFA platform can be usable in situ.

Surface-specific washing-free immunosensor for time-resolved cortisol monitoring

Cortisol is a steroid hormone that regulates a wide range of vital processes. Its level changes with diurnal rhythm and reacts to stress. Measurement of cortisol levels is still a complex multi-step process. A reversible washing-free registration method is required. Here we describe metal-enhanced fluorescence assay based on a displacement of a dye labeled BSA-cortisol conjugate from the immune complex immobilized on the golden islands by free cortisol. This competitive approach allows time-resolved monitoring of the fluorescent signal, surface-enhanced by the gold film, and provides the possibility of continuous real-time cortisol monitoring based on the implantable surface-enhanced immunosensor, which was not demonstrated so far even in vitro.

Simultaneous capillary electrophoresis of anions and cations in a single injection using an anion exchanger-modified capillary for determination of salivary ions in combination with statistical analyses

This paper describes the simultaneous capillary electrophoresis (CE) of anions and cations using an anion exchange-modified capillary, which was prepared by chemical coating with a cationic silylating reagent, and its application to saliva analysis. The CE method provides high-throughput (5 min for a single sample injection) analysis by generating a high-velocity electroosmotic flow at pH 3.0-3.5. The detection limits at a signal-to-noise ratio of 3 ranged from 1.2 to 18 μM for anions and 1.0 to 2.7 μM for cations. The relative standard deviations for the migration times and peak areas of analytes (n = 4) ranged from 0.05% to 0.40% and 0.94% to 4.7%, respectively. The CE system was used to analyze 11 common ions in saliva samples collected from long-distance runners and sedentary university students before and after running for a set distance or a set time. Interestingly, the SCN^- concentrations decreased in the saliva samples of all 14 athletes and 16 sedentary students after running. Furthermore, when the concentrations of the analyzed ions were compared with that of cortisol as a typical stress marker by multiple regression analysis, SCN^- and NO₃^- in saliva samples from the two subject groups strongly correlated with cortisol levels, as determined by an electrochemiluminescence immunoassay. This study improves our knowledge of both the analytical methodology for CE and statistical methods for identifying common ions that could be used as physical stress markers.

Current methods for stress marker detection in saliva

•

Introduction of relevant biomarkers in stress conditions.

•

Reference ranges of biomarkers in normal conditions.

•

Saliva as easy-accessible specimen.

•

Review of analytical methods for biomarker determination in saliva.

•

Possibilities for design of point-of-care devices.

Stress and stress-related diseases are leading to drastic consequences in private and professional life. Therefore, the need for stress prevention strategies is of personal and economic interest. Especially during the recent period related to covid-19 outbreak and lock-down, an ongoing discussion of increasing stress etiology is reported. Biomarker analysis may help to assist diagnosis and classification of stress-related diseases and therefore support therapeutical decisions. Due to its non-invasive sampling, the analysis of saliva has become highly attractive compared to the detection methods in other specimen. This review article summarizes the status of research, innovative approaches, and trends. Scientific literature published since 2011 was excerpted with concentration on the detection of up to seven promising marker substances. Most often reported cortisol represents the currently best evaluated stress marker, while norepinephrine (noradrenaline) or its metabolite 3-methoxy-4-hydroxyphenylglycol is also a quite commonly considered stress marker. Other complementary stress marker candidates are testosterone, dehydroepiandrosterone (DHEA) and its sulfonated analogue DHEA-S, alpha-amylase, secretory immunoglobulin A, and chromogranin A. Several working groups are researching in the field of stress marker detection to develop reliable, fast, and affordable methods. Analytical methods reported mainly focused on immunological and electrochemical as well as chromatographic methods hyphenated to mass spectrometric detection to yield the required detection limits.

A Simple Ultraperformance Liquid Chromatography-Tandem Mass Spectrometry Method for Measurement of Cortisol Level in Human Saliva

A simple ultraperformance liquid chromatography-tandem mass spectrometry assay for measurement of cortisol level in human saliva was developed and validated. Saliva samples containing cortisol were spiked with tolperisone as internal standard (IS) and extracted with a mixture of methyl tert-butyl ether and hexane (8:2, v:v). After solvent evaporation, residue was reconstituted in 100 μl mobile phase. Analysis was performed on Atlantis dC18 column (2.1 × 100 mm, 3 μm particle size) with a mobile phase composed of acetonitrile and 2 mM ammonium acetate (50:50, v:v) and delivered at a flow rate of 0.3 ml/minute. Mass spectrometry acquisition was performed with multiple reaction monitoring in positive-ion mode for cortisol and IS (m/z: 363.1 → 121.0 and 246.0 → 97.9, respectively). Retention times of cortisol and IS were about 1.35 and 2.45 minutes, respectively. The relationship between cortisol level and peak area ratio of cortisol to IS was linear in the range of 0.5-100 ng/ml. Intra- and interday coefficient of variation and bias were ≤ 9.0% and ≤12.0%, respectively. Mean extraction recoveries of cortisol and IS from saliva samples were 92% and 94%, respectively. Using the method, cortisol was found to be ≥ 86% stable in processed (24 hours at room temperature or 48 hours at -20°C) and ≥ 91% stable in unprocessed (24 hours at room temperature or 20 weeks at -20°C) saliva samples. Further, the method was successfully applied to determine daily cortisol profile in saliva samples of a healthy volunteer.

Development of competitive lateral flow immunoassay coupled with silver enhancement for simple and sensitive salivary cortisol detection

Cortisol is known as a stress biomarker. The measurement of cortisol levels is an early warning indicator for health conditions and diagnosis of stress-related diseases. Herein, a lateral flow immunoassay using a gold nanoparticle label with a silver enhancement system was developed for the simple, sensitive and rapid detection of cortisol. The developed assay was based on a competitive platform of which cortisol-BSA conjugate was immobilized at the test zone to compete with an analyte. The quantitative analysis was performed using gold nanoparticles (AuNPs) as signal labeling. Sequentially, the silver enhancement solution was applied in order to enhance the sensitivity of the assay with the results easily seen by the naked eye. Using this system, the limit of detection (LOD) was found to be 0.5 ng/mL with a 3.6 fold more sensitive detection than without the enhancement system (LOD = 1.8 ng/mL). The salivary cortisol analysis was in the range of 0.5-150 ng/mL (R² = 0.9984), which is in the clinical acceptable range. For the semi-quantitative analysis, the intensity color of the results was analyzed using an image processing program. The proposed method was successfully applied to detect cortisol in saliva. In addition, the results from our method also complied with the ones of those obtained by using the commercial enzyme-linked immunosorbent assay (ELISA). This developed assay offers great promise for a non-invasive screening test of salivary cortisol.

Oriented, molecularly imprinted cavities with dual binding sites for highly sensitive and selective recognition of cortisol

Novel, molecularly imprinted polymers (MIPs) were developed for the highly sensitive and selective recognition of the stress marker cortisol. Oriented, homogeneous cavities with two binding sites for cortisol were fabricated by surface-initiated atom transfer radical polymerization, using a cortisol motif template molecule (TM1) which consists of a polymerizable moiety attached at the 3-carbonyl group of cortisol via an oxime linkage and an adamantane carboxylate moiety coupled with the 21-hydroxyl group. TM1 was orientationally immobilized on a β-cyclodextrin (β-CD)-grafted gold-coated sensor chip by inclusion of the adamantane moiety of TM1, followed by copolymerization of a hydrophilic comonomer, 2-methacryloyloxyethyl phosphorylcholine, with or without a cross-linker, N,N'-methylenebisacrylamide. Subsequent cleavage of the oxime linkage leaves the imprinted cavities that contain dual binding sites-namely, the aminooxy group and β-CD-capable of oxime formation and hydrophobic interaction, respectively. As an application, MIP-based picomolar level detection of cortisol was demonstrated by a competitive binding assay using a fluorescent competitor. Cross-linking of the MIP imparts rigidity to the binding cavities, and improves the selectivity and sensitivity significantly, reducing the limit of detection to 4.8 pM. In addition, detection of cortisol in saliva samples was demonstrated as a feasibility study.

Diffraction-Free Bloch Surface Waves

Here, we demonstrate a diffraction-free Bloch surface wave sustained on all-dielectric multilayers that does not diffract after being passed through three obstacles or across a single mode fiber. It can propagate in a straight line for distances longer than 110 μm at a wavelength of 633 nm and could be applied as an in-plane optical virtual probe both in air and in an aqueous environment. Its ability to be used in water, its long diffraction-free distance, and its tolerance to multiple obstacles make this wave ideal for certain applications in areas such as the biological sciences, where many measurements are made on glass surfaces or for which an aqueous environment is required, and for high-speed interconnections between chips, where low loss is necessary.

Measuring cortisol in serum, urine and saliva - are our assays good enough?

Cortisol is a steroid hormone produced in response to stress. It is essential for maintaining health and wellbeing and leads to significant morbidity when deficient or present in excess. It is lipophilic and is transported bound to cortisol-binding globulin (CBG) and albumin; a small fraction (∼10%) of total serum cortisol is unbound and biologically active. Serum cortisol assays measure total cortisol and their results can be misleading in patients with altered serum protein concentrations. Automated immunoassays are used to measure cortisol but lack specificity and show significant inter-assay differences. Liquid chromatography - tandem mass spectrometry (LC-MS/MS) offers improved specificity and sensitivity; however, cortisol cut-offs used in the short Synacthen and Dexamethasone suppression tests are yet to be validated for these assays. Urine free cortisol is used to screen for Cushing's syndrome. Unbound cortisol is excreted unchanged in the urine and 24-h urine free cortisol correlates well with mean serum-free cortisol in conditions of cortisol excess. Urine free cortisol is measured predominantly by immunoassay or LC-MS/MS. Salivary cortisol also reflects changes in unbound serum cortisol and offers a reliable alternative to measuring free cortisol in serum. LC-MS/MS is the method of choice for measuring salivary cortisol; however, its use is limited by the lack of a single, validated reference range and poorly standardized assays. This review examines the methods available for measuring cortisol in serum, urine and saliva, explores cortisol in disease and considers the difficulties of measuring cortisol in acutely unwell patients and in neonates.

Microfluidic immunosensor for rapid and highly-sensitive salivary cortisol quantification

This paper presents a novel poly(dimethylsiloxane) (PDMS) microfluidic immunosensor that integrates a complementary metal-oxide-semiconductor (CMOS) optical detection system for a rapid and highly-sensitive quantification of salivary cortisol. The simple and non-invasive method of saliva sampling provides an interesting alternative to the blood, allowing a fast sampling at short intervals, relevant for many clinical diagnostic applications. The developed approach is based on the covalent immobilization of a coating antibody (Ab), a polyclonal anti-IgG, onto a treated PDMS surface. The coating Ab binds the capture Ab, an IgG specific for cortisol, allowing its correct orientation. Horseradish peroxidase (HRP)-labelled cortisol is added to compete with the cortisol in the sample, for the capture Ab binding sites. The HRP-labelled cortisol, bonded to the capture Ab, is measured through the HRP enzyme and the tetramethylbenzidine (TMB) substrate reaction. The cortisol quantification is performed by colorimetric detection of HRP-labelled cortisol, through optical absorption at 450nm, using a CMOS silicon photodiode as the photodetector. Under the developed optimized conditions presented here, e.g., microfluidic channels geometry, immobilization method and immunoassay conditions, the immunosensor shows a linear range of detection between 0.01-20ng/mL, a limit of detection (LOD) of 18pg/mL and an analysis time of 35min, featuring a great potential for point-of-care applications requiring continuous monitoring of the salivary cortisol levels during a circadian cycle.

Reference ranges for cortisol and α-amylase in mother and newborn saliva samples at different perinatal and postnatal periods

This paper describes a reliable analytical method based on Ultra High-Performance Liquid Chromatography coupled to tandem mass spectrometry to determine cortisol in saliva samples from healthy mothers (n=87) and newborns (n=65) at different time points: (i) 38 weeks of gestation, (ii) in the immediate postnatal period (48h) after a term delivery and, (iii) 3 months after delivery. The procedure is characterized by a simple sample treatment employing a sample volume of 25μL. In addition to this, salivary α-amylase was determined using a commercial kit. We have proposed potential reference ranges in saliva for cortisol (0.7-35nmolL(-1)) and α-amylase (2-500UmL(-1)) in mothers, and for cortisol (0.1-56nmolL(-1)) and α-amylase (0.1-500UmL(-1)) in newborn infants. In addition, statistical differences between the two sensitive population groups (mothers and newborns) at the perinatal and postnatal periods were studied. A lower concentration for maternal cortisol was found at 38 weeks of gestation than at 48h (p=0.048) or 3 months after delivery (p=0.021). Similar results were found for the α-amylase determinations. Hence, higher concentrations than could be expected from a chronic stress marker were found at 3 months after delivery than at 38 weeks of gestation (p<0.001) or 48h after delivery (p<0.001). We conclude that this analytical method could be applied to further clinical research on perinatal and postnatal stress, such as threatened preterm labor and/or parenting stress, respectively.

Sensitive time-resolved fluoroimmunoassay for quantitative determination of clothianidin in agricultural samples

Europium (Eu(3+))-labeled antibody was used as a fluorescent label to develop a highly sensitive time-resolved fluoroimmunoassay (TRFIA) for determination of clothianidin residues in agricultural samples. Toward this goal, the Eu(3+)-labeled polyclonal antibody and goat anti-rabbit antibody were prepared for developing and evaluating direct competitive TRFIA (dc-TRFIA) and indirect competitive TRFIA (ic-TRFIA). Under optimal conditions, the half-maximal inhibition concentration (IC50) and the limit of detection (LOD, IC10) of clothianidin were 9.20 and 0.0909 μg/L for the dc-TRFIA and 2.07 and 0.0220 μg/L for the ic-TRFIA, respectively. The ic-TRFIA has no obvious cross-reactivity with the analogues of clothianidin except for dinotefuran. The average recoveries of clothianidin from spiked water, soil, cabbage, and rice samples were estimated to range from 74.1 to 115.9 %, with relative standard deviations of 3.3 to 11.7 %. The results of TRFIA for the blind samples were largely consistent with gas chromatography (R (2) = 0.9902). The optimized ic-TRFIA might become a sensitive and satisfactory analytical method for the quantitative monitoring of clothianidin residues in agricultural samples.