Abstract
Low-cost imaging systems that utilize exogenous fluorescent dyes, such as acridine orange (AO), have recently been developed for use as point-of-care (POC) blood analyzers. AO-based fluorescence imaging exploits variations in emission wavelength within different cell types to enumerate and classify leukocyte subpopulations from whole blood specimens. This approach to leukocyte classification relies on accurate and reproducible colorimetric features, which have previously been demonstrated to be highly dependent on the cell staining protocols (such as specific AO concentration, timing, and pH). We have developed a light-sheet-based fluorescence imaging spectrometer, featuring a spectral resolution of 9 nm, with an automated spectral extraction algorithm as an investigative tool to study the spectral features from AO-stained leukocytes. Whole blood specimens were collected from human subjects, stained with AO using POC methods, and leukocyte spectra were acquired on a cell-by-cell basis. The post-processing method involves three steps: image segmentation to isolate individual cells in each spectral image; image quality control to exclude cells with low emission intensity, out-of-focus cells, and cellular debris; and the extraction of spectra for each cell. An increase in AO concentration was determined to contribute to the red-shift in AO-fluorescence, while varied pH values did not cause a change in fluorescence. In relation to the spectra of AO-stained leukocytes, there were corresponding red-shift trends associated with dye accumulation within acidic vesicles and at increasing incubation periods. The system presented here could guide future development of POC systems reliant on AO fluorescence and colorimetric features to identify leukocyte subpopulations in whole blood specimens.
Highlights
A leukocyte differential count is a subset of a complete blood count (CBC), which is among the most frequently ordered blood tests in clinical laboratories [1,2,3]
A leukocyte differential count is commonly reported as a proportion of leukocyte subpopulations, where a three-part leukocyte differential reports the proportion of lymphocytes, monocytes, and granulocytes [3]
This predominately occurs within acidic vesicles in the cells where there is a decrease in pH causing the protonation of acridine orange (AO) resulting in the entrapment and accumulation of AO [25,27]
Summary
A leukocyte differential count is a subset of a complete blood count (CBC), which is among the most frequently ordered blood tests in clinical laboratories [1,2,3]. The manual enumeration approach involves the identifying and classifying 100 stained cells using a light microscope by a trained technician, which can be time and labor intensive, and can result in subjective operator error [5,6,7,8] To overcome these limitations, automated hematology analyzers based on flow cytometry techniques (electrical impendence, forward and side light scattering and immunostaining) have become the “gold standard” due to their high-throughput (10,000–25,000 cells per sec), high specificity and sensitivity, reliability, and rapid diagnostics [5,9,10,11]. Due to their large size, cost at upwards of $20,000, requirement of several proprietary reagents, and the requirement of trained technicians, these analyzers are unavailable in both developed and developing rural and remote areas and economically-challenged countries [2,3,7,9]
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