Number Concentration Measurements of Polystyrene Submicrometer Particles

The number of techniques to measure number concentrations and size distributions of submicrometer particles has recently increased. Submicrometer particle standards are needed to improve the accuracy and reproducibility of these techniques. The number concentrations of fluorescently labeled polystyrene submicrometer sphere suspensions with nominal 100 nm, 200 nm and 500 nm diameters were measured using seven different techniques. Diameter values were also measured where possible. The diameter values were found to agree within 20%, but the number concentration values differed by as much as a factor of two. Accuracy and reproducibility related with the different techniques are discussed with the goal of using number concentration standards for instrument calibration. Three of the techniques were used to determine SI-traceable number concentration values, and the three independent values were averaged to give consensus values. This consensus approach is proposed as a protocol for certifying SI-traceable number concentration standards.

Development and Validation of Measurement Traceability for In Situ Immunoassays

BACKGROUND

Immunoassays for protein analytes measured in situ support a $2 billion laboratory testing industry that suffers from significant interlaboratory disparities, affecting patient treatment. The root cause is that immunohistochemical testing lacks the generally accepted tools for analytic standardization, including reference standards and traceable units of measure. Until now, the creation of these tools has represented an insoluble technical hurdle.

METHODS

We address the need with a new concept in metrology-that is, linked traceability. Rather than calculating analyte concentration directly, which has proven too variable, we calculate concentration by measuring an attached fluorescein, traceable to NIST Standard Reference Material 1934, a fluorescein standard.

RESULTS

For validation, newly developed estrogen receptor (ER) calibrators were deployed in tandem with an array of 80 breast cancer tissue sections in a national external quality assessment program. Laboratory performance was assessed using both the ER standards and the tissue array. Similar to previous studies, the tissue array revealed substantial discrepancies in ER test results among the participating laboratories. The new ER calibrators revealed a broad range of analytic sensitivity, with the lower limits of detection ranging from 7310 to 74 790 molecules of ER. The data demonstrate, for the first time, that the variable test results correlate with analytic sensitivity, which can now be measured quantitatively.

CONCLUSIONS

The reference standard enables precise interlaboratory alignment of immunohistochemistry test sensitivity for measuring cellular proteins in situ. The introduction of a reference standard and traceable units of measure for protein expression marks an important milestone.

Abstract PS5-46: Introduction and clinical validation of metrology standards for immunohistochemistry (IHC); New tool for standardization of estrogen receptor (ER) IHC assay in breast cancer

Traceability of measurement to a higher order reference standard is a foundation of laboratory testing. There is as yet no method for creating reference standards for cellular proteins in situ in an analogous fashion as for soluble analytes. At present, IHC laboratories produce results for breast cancer hormone receptors without connection to a reference standard. Not surprisingly, high rates of testing variation as well as discrepancies among IHC laboratories have been reported. To address this need, we developed a system of measurement traceability using a linked fluorescein tag for creating reference standards for any cellular analyte and, as a first test, validate it for estrogen receptor (ER) testing. In this study, the newly developed ER standard defines and compares the thresholds separating “high positive”, “low positive”, and “negative” tests according to updated ASCO/CAP guidelines as detected by clinical IHC laboratories in a national external quality assessment survey. This reference standard utilizes NIST Standard Reference Material (SRM) 1934 as a universal IHC standard. We calculated ER concentration based on a linked fluorescence measurement traceable to NIST SRM 1934 as each ER is linked to a single fluorescein, and fluorescein concentration equals ER concentration. Each laboratory’s lowest detected ER concentration (i.e. “limit of detection”, LOD) was compared to their results with 80 tumor samples enriched for triple negative breast cases. For the Canadian Immunohistochemistry Quality Control (CIQC) ER proficiency testing run, calibrator sets with peptides for the SP1, EP1, and 6F11 epitopes were created. The various concentrations were pipetted onto histology slides used by CIQC to place its 80-case breast cancer tissue microarray. These slides were stained by participating laboratories using their routine ER IHC protocols and returned to the CIQC. For the purpose of this study, Histology Score and ASCO/CAP categorical scoring recommendation was used for the readout. Results with SP1 clone are reported here because it was employed by overwhelming majority of laboratories. A total of 3,038 readouts were included in the analysis. Most IHC laboratories had a LOD between 10,000 – 25,000 molecules ER per microbead. Highly sensitive ER assays (low LODs) detected more positive cases while those with poorly sensitive assay detected fewer. The LOD correlated with the percent positive cases (R2= -.767, p < 0.0001, Spearman Correlation) as well as with cumulative individual laboratory Histology Score (R2= -.612, p < 0.0001, Spearman Correlation). The tumor samples accounting for this difference were principally “low positive” (ASCO/CAP classification) while ER-high positive tumors were less affected. Although the concept of a non-quantitative IHC LOD has been introduced with critical assay performance controls (iCAPCs, e.g. germinal center cells in the tonsil for ER), there were no tools until now to actually measure LOD for ER IHC testing. With the calibrators introduced here, it is now possible to measure the LOD for ER IHC assays. Furthermore, our data show that ER low positive and some cases of ER high positive breast cancers are highly affected by the variability of analytic sensitivity between clinical IHC laboratories. These data argue for the urgent need to standardize testing to a defined analytic sensitivity range, which is now possible for the first time. Beyond inter-laboratory standardization, known measured LOD also enables: i) methodology transfer from clinical trials to clinical laboratories; ii) determination of optimal analytical sensitivity to produce assays with highest clinical and analytical validity, and iii) daily monitoring of analytical sensitivity for each tested IHC slide.

Citation Format: Emina Emilia Torlakovic, Seshi R. Sompuram, Kodela Vani, Lili Wang, Anika K Schaedle, Paul C. DeRose, Steven A. Bogen. Introduction and clinical validation of metrology standards for immunohistochemistry (IHC); New tool for standardization of estrogen receptor (ER) IHC assay in breast cancer [abstract]. In: Proceedings of the 2020 San Antonio Breast Cancer Virtual Symposium; 2020 Dec 8-11; San Antonio, TX. Philadelphia (PA): AACR; Cancer Res 2021;81(4 Suppl):Abstract nr PS5-46.

Measurement and standardization challenges for extracellular vesicle therapeutic delivery vectors

Extracellular vesicles (EVs), such as exosomes and microvesicles, are nonreplicating lipid bilayer particles shed by most cell types which have the potential to revolutionize the development and efficient delivery of clinical therapeutics. This article provides an introduction to the landscape of EV-based vectors under development for the delivery of protein- and nucleic acid-based therapeutics. We highlight some of the most pressing measurement and standardization challenges that limit the translation of EVs to the clinic. Current challenges limiting development of EVs for drug delivery are the lack of: standardized cell-based platforms for the production of EV-based therapeutics; EV reference materials that allow researchers/manufacturers to validate EV measurements and standardized measurement systems for determining the molecular composition of EVs.

Stochastic Reaction-Diffusion Model of the Binding of Monoclonal Antibodies to CD4 Receptors on the Surface of T Cells

A stochastic reaction-diffusion model was developed to describe the binding of labeled monoclonal antibodies (mAbs) to CD4 receptors on the surface of T cells. The mAbs diffused to, adsorbed on, and underwent monovalent and bivalent binding to CD4 receptors on the cell surface. The model predicted the time-dependent nature of all populations involved in the labeling process. At large time, the populations reached equilibrium values, giving the number of antibodies bound to the T cell (ABC) defined as the sum of monovalently and bivalently bound mAbs. The predicted coefficient of variation (CV%) of the (ABC) values translated directly to a corresponding CV% of the measured mean fluorescence intensity (MFI). The predicted CV% was about 0.2% from the intrinsic fluctuations of the stochastic reaction process, about 5% after inclusion of the known fluctuations in the number of available CD4 receptors, and about 11% when fluctuations in bivalent binding affinity were included. The fluorescence detection process is expected to contribute approximately 7%. The abovementioned contributions to CV% sum up to approximately 13%. Work is underway to reconcile the predicted values and the measured values of 17% to 22%.

A Model for the Binding of Fluorescently Labeled Anti-Human CD4 Monoclonal Antibodies to CD4 Receptors on Human Lymphocytes

The CD4 glycoprotein is a component of the T cell receptor complex which plays an important role in the human immune response. This manuscript describes the measurement and modeling of the binding of fluorescently labeled anti-human CD4 monoclonal antibodies (mAb; SK3 clone) to CD4 receptors on the surface of human peripheral blood mononuclear cells (PBMC). CD4 mAb fluorescein isothiocyanate (FITC) and CD4 mAb allophycoerythrin (APC) conjugates were obtained from commercial sources. Four binding conditions were performed, each with the same PBMC sample and different CD4 mAb conjugate. Each binding condition consisted of the PBMC sample incubated for 30 min in labeling solutions containing progressively larger concentrations of the CD4 mAb-label conjugate. After the incubation period, the cells were re-suspended in PBS-based buffer and analyzed using a flow cytometer to measure the mean fluorescence intensity (MFI) of the labeled cell populations. A model was developed to estimate the equilibrium concentration of bound CD4 mAb-label conjugates to CD4 receptors on PBMC. A set of parameters was obtained from the best fit of the model to the measured MFI data and the known number of CD4 receptors on PBMC surface. Divalent and monovalent binding had to be invoked for the APC and FITC CD4 mAb conjugates, respectively. This suggests that the mAb binding depends on the size of the label, which has significant implications for quantitative flow cytometry. The study supports the National Institute of Standards and Technology program to develop quantitative flow cytometry measurements.

Primary Determination of Particle Number Concentration with Light Obscuration and Dynamic Imaging Particle Counters

Accurate number concentrations of particles in liquid media are needed to assess the quality of water, pharmaceuticals, and other liquids, yet there are limited reference materials or calibration services available with clear traceability to the International System of Units. We describe two methods, based on very simple modifications of commercial particle counter instruments, that can provide traceable number concentration measurements. One method used a light obscuration counter. Fitting a model to the data enabled correction for timing and coincidence errors, and gravimetric calibration of the syringe pump gave a traceable determination of measured volume. Other potential biases were diagnosed by analysis of the particle size distribution. The other method used a dynamic imaging particle counter (a flow imaging microscope). The instrument was intentionally configured so that each particle passing through the flow cell was imaged multiple times. Following the particle image acquisition runs, runs with a rinse solution released and counted microspheres adsorbed to tubing or flow-cell walls. Software assembled the redundant particle images into tracks, and the total number of tracks was assigned as the number of particles counted. Both light obscuration and dynamic imaging methods, when applied to polystyrene microspheres of approximately 4 μm diameter, achieved expanded uncertainties (k = 2) of approximately 2 % of number concentration and agreed to within a difference of 1.1 %.

Beetroot-pigment-derived colorimetric sensor for detection of calcium dipicolinate in bacterial spores

In this proof-of-concept study, we describe the use of the main red beet pigment betanin for the quantification of calcium dipicolinate in bacterial spores, including Bacillus anthracis. In the presence of europium(III) ions, betanin is converted to a water-soluble, non-luminescent orange 1∶1 complex with a stability constant of 1.4 × 10(5) L mol(-1). The addition of calcium dipicolinate, largely found in bacterial spores, changes the color of the aqueous solution of [Eu(Bn)(+)] from orange to magenta. The limit of detection (LOD) of calcium dipicolinate is around 2.0 × 10(-6) mol L(-1) and the LOD determined for both spores, B. cereus and B. anthracis, is (1.1 ± 0.3)× 10(6) spores mL(-1). This simple, green, fast and low cost colorimetric assay was selective for calcium dipicolinate when compared to several analogous compounds. The importance of this work relies on the potential use of betalains, raw natural pigments, as colorimetric sensors for biological applications.

State-of-the art comparability of corrected emission spectra. 1. Spectral correction with physical transfer standards and spectral fluorescence standards by expert laboratories

The development of fluorescence applications in the life and material sciences has proceeded largely without sufficient concern for the measurement uncertainties related to the characterization of fluorescence instruments. In this first part of a two-part series on the state-of-the-art comparability of corrected emission spectra, four National Metrology Institutes active in high-precision steady-state fluorometry performed a first comparison of fluorescence measurement capabilities by evaluating physical transfer standard (PTS)-based and reference material (RM)-based calibration methods. To identify achievable comparability and sources of error in instrument calibration, the emission spectra of three test dyes in the wavelength region from 300 to 770 nm were corrected and compared using both calibration methods. The results, obtained for typical spectrofluorometric (0°/90° transmitting) and colorimetric (45°/0° front-face) measurement geometries, demonstrated a comparability of corrected emission spectra within a relative standard uncertainty of 4.2% for PTS- and 2.4% for RM-based spectral correction when measurements and calibrations were performed under identical conditions. Moreover, the emission spectra of RMs F001 to F005, certified by BAM, Federal Institute for Materials Research and Testing, were confirmed. These RMs were subsequently used for the assessment of the comparability of RM-based corrected emission spectra of field laboratories using common commercial spectrofluorometers and routine measurement conditions in part 2 of this series (subsequent paper in this issue).

State-of-the art comparability of corrected emission spectra. 2. Field laboratory assessment of calibration performance using spectral fluorescence standards

In the second part of this two-part series on the state-of-the-art comparability of corrected emission spectra, we have extended this assessment to the broader community of fluorescence spectroscopists by involving 12 field laboratories that were randomly selected on the basis of their fluorescence measuring equipment. These laboratories performed a reference material (RM)-based fluorometer calibration with commercially available spectral fluorescence standards following a standard operating procedure that involved routine measurement conditions and the data evaluation software LINKCORR developed and provided by the Federal Institute for Materials Research and Testing (BAM). This instrument-specific emission correction curve was subsequently used for the determination of the corrected emission spectra of three test dyes, X, QS, and Y, revealing an average accuracy of 6.8% for the corrected emission spectra. This compares well with the relative standard uncertainties of 4.2% for physical standard-based spectral corrections demonstrated in the first part of this study (previous paper in this issue) involving an international group of four expert laboratories. The excellent comparability of the measurements of the field laboratories also demonstrates the effectiveness of RM-based correction procedures.

Fluorescence standards: Classification, terminology, and recommendations on their selection, use, and production (IUPAC Technical Report)

Chromophore-based fluorescence standards for the characterization of photo-luminescence measuring systems and the determination of relevant fluorometric quantities are classified according to their scope and area of application. General and type-specific requirements for suitable standards are derived for each class of standards. Metrological requirements linked to the realization of comparable measurements are addressed and recommendations on selecting, using, and developing fluorescence standards are given.

Recommendations for fluorescence instrument qualification: the new ASTM Standard Guide

Aimed at improving quality assurance and quantitation for modern fluorescence techniques, ASTM International (ASTM) is about to release a Standard Guide for Fluorescence, reviewed here. The guide's main focus is on steady state fluorometry, for which available standards and instrument characterization procedures are discussed along with their purpose, suitability, and general instructions for use. These include the most relevant instrument properties needing qualification, such as linearity and spectral responsivity of the detection system, spectral irradiance reaching the sample, wavelength accuracy, sensitivity or limit of detection for an analyte, and day-to-day performance verification. With proper consideration of method-inherent requirements and limitations, many of these procedures and standards can be adapted to other fluorescence techniques. In addition, procedures for the determination of other relevant fluorometric quantities including fluorescence quantum yields and fluorescence lifetimes are briefly introduced. The guide is a clear and concise reference geared for users of fluorescence instrumentation at all levels of experience and is intended to aid in the ongoing standardization of fluorescence measurements.

Qualification of a fluorescence spectrometer for measuring true fluorescence spectra

New analytical methods using fluorescence detection are becoming increasingly quantitative and require easy-to-use material standards for fluorometer qualification and method validation. NIST is responding to this need by developing and producing such standards. Reported here is the first step in this process, which is to qualify a research-grade fluorescence spectrometer for measuring true fluorescence spectra of reference material candidates. "True" spectra are defined here as those with fluorescence intensity, either relative or absolute as required, and wavelength both being reported with high accuracy and known precision, after wavelength has been calibrated and corrections for excitation intensity and detection system response have been applied. The uncertainties determined in relative and absolute intensity-corrected fluorescence spectra using both calibrated source (CS)- and calibrated detector (CD)-based methods were compared. The CS-based method gave uncertainties, typically about +/-5% for relative spectral correction, that were about half that of the CD-based method for determining both relative and absolute spectral correction factors. Absolute spectral correction factors can be determined using either method without knowing the optical geometry of the instrument. The absolute spectral correction factors were found to have much larger uncertainties than the corresponding relative correction factors with uncertainties for the CS-based method of +/-10% to +/-15% being typical and +/-20% or more not being uncommon, particularly for excitation and emission wavelengths below 400 nm. Uncertainties arising from detection system nonlinearity and instrument polarization ratios were also explored.

Excitation-emission matrix fluorescence spectroscopy for natural organic matter characterization: a quantitative evaluation of calibration and spectral correction procedures

The influence of different data collection procedures and of wavelength-dependent instrumental biases on fluorescence excitation-emission matrix (EEM) spectral analysis of aqueous organic matter samples was investigated. Particular attention was given to fluorescence contours (spectral shape) and peak fluorescence intensities. Instrumental bias was evaluated by independently applying excitation and emission correction factors to the raw excitation and emission data, respectively. The peak fluorescence intensities of representative natural organic matter and tryptophan were significantly influenced by the application of excitation and emission spectral correction factors and by the manner in which the raw data was collected. Humification and fluorescence indices were also influenced by emission correction factors but were independent of reference (excitation) intensity normalization or correction. EEM surface contours were dependent on normalization of the fluorescence intensity to the reference intensity but were not influenced by either excitation or emission spectral correction factors. Authors should be explicit in how excitation and emission spectral correction procedures are implemented in their investigations, which will help to facilitate intra-laboratory comparisons and data sharing.

Impact of reclaimed water on select organic matter properties of a receiving stream-fluorescence and perylene sorption behavior

Surface water samples obtained from the Bull Run tributary upstream and downstream of the Upper Occoquan Sewage Authority(UOSA) advanced wastewater reclamation facility (WRF) were characterized by fluorescence excitation-emission matrix (EEM) spectroscopy, and sorption coefficients (Kmoc) of macromolecular organic carbon isolates were quantified by fluorescence quenching. The EEM data revealed a signature fluorescence distribution in the downstream samples that was attributed to the presence of proteinlike material. The Kmoc values for upstream samples were consistently and significantly higher than those of corresponding downstream samples. There was a moderate correlation (R2 = 0.67) between log Kmoc and the molar extinction coefficient at 280 nm (E280) and a strong correlation (R2 = 0.96) between Kmoc and the proteinlike fluorescence region for macromolecular isolates with negligible quantum yields. This study demonstrates that organic matter downstream of the UOSA-WRF has unique fluorescence and perylene sorption characteristics compared to those of upstream organic matter during summer baseflow conditions. This implies that wastewater treatment facilities, including those advanced facilities designed to reclaim wastewater for indirect potable reuse, can influence the composition and behavior of organic matter in a receiving stream.