Abstract
Plasmodium falciparum infection causes structural and biochemical changes in red blood cells (RBCs). To quantify these changes, we apply a novel optical technique, quantitative phase spectroscopy (QPS) to characterize individual red blood cells (RBCs) during the intraerythrocytic life cycle of P. falciparum. QPS captures hyperspectral holograms of individual RBCs to measure spectroscopic changes across the visible wavelength range (475–700 nm), providing complex information, i.e. amplitude and phase, about the light field which has interacted with the cell. The complex field provides complimentary information on hemoglobin content and cell mass, which are both found to dramatically change upon infection by P. falciparum. Hb content progressively decreases with parasite life cycle, with an average 72.2% reduction observed for RBCs infected by schizont-stage P. falciparum compared to uninfected cells. Infection also resulted in a 33.1% reduction in RBC’s optical volume, a measure of the cells’ non-aqueous components. Notably, optical volume is only partially correlated with hemoglobin content, suggesting that changes in other dry mass components such as parasite mass may also be assessed using this technique. The unique ability of QPS to discriminate individual healthy and infected cells using spectroscopic changes indicates that the approach can be used to detect disease.
Highlights
P. falciparum is the primary cause of malaria, which is responsible for over 500,000 deaths each year world-wide[1]
Quantitative phase microscopy (QPM) has previously been applied to the study of P. falciparum biophysics extensively over the past decade[8,9,10,11,12,13,14]
QPM is highly sensitive to microscopic morphology and refractive index changes, the approach lacks molecular specificity, an important feature in many diagnostic approaches
Summary
P. falciparum is the primary cause of malaria, which is responsible for over 500,000 deaths each year world-wide[1]. Rapid diagnostic tests (RDTs) are inexpensive, convenient, and do not require highly-skilled microscopists They suffer from reduced diagnostic accuracy and provide no information on parasite staging[2]. One research group has even performed a proof-of-concept study to use holographic phase images to develop an automated algorithm capable of distinguishing individual infected cells from uninfected cells[8]. These studies all rely on literature values of RI determined at a particular, single wavlength to derive RBC metrics, which may not accurately determine Hb content. Spectral variation in the phase of transmitted light, i.e., phase dispersion, provides knowledge of the complex refractive index which can be used to provide molecular information
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