White blood cell counts can give vital information about the body’s immune response. The HemoCue® WBC DIFF Analyzer is a commercial off the shelf (COTS) device to be utilized for a technology demonstration for counting white blood cells in a microgravity environment. This analyzer further differentiates the cell counts into 5 major types of White Blood Cells (WBC): Neutrophils (NEU), Lymphocytes (LYM), Monocytes (MON), Eosinophils (EOS), and Basophils (BAS). The body recruits these cells in varying amounts when needed. Thus, changes observed in the quantities and percentages of these 5 types of cells can be used to help identify infections, radiation effects, and responses to treatment.
The ability to measure white blood cells is currently an unmet need on the International Space Station (ISS) and for future exploration missions. Most in-flight blood tests on ISS are currently limited to blood draws taken just prior to undock being sent back to Earth for lab analysis within 48 hours utilizing instruments not practical for use in space. It was highlighted that spaceflight missions over extended time durations can have adverse effects on the human body that elevate the health risks of those astronauts involved. Prior to the completion of this technical demonstration, no instrument has been capable of measuring a WBC count and differential to meet NASA's medical requirements.
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1. Flight hardware certification
To certify the instrument for spaceflight, it was subjected to a standardized battery of ground evaluation tests. This included vibration sensitivity, toxicology evaluation, materials evaluation, touch temperature evaluation, electromagnetic interference, and acoustics. Battery and power requirements were also evaluated. Assessing the effects of radiation on instrument performance was not required for this technical evaluation, as the instrument was not left onboard ISS for a prolonged period.
2. Control blood samples
For this evaluation commercial hematology standards were obtained for three different levels of cellular concentration (‘HC WBC Diff Hematology Controls’; RnD Systems, Minneapolis, MN). Controls were utilized, and the reagents were shipped to NASA within days after manufacture was completed. Control solutions were stored in refrigerated conditions until use, including ground, ascent, and onboard ISS, and all work was completed prior to the expiration date. Per the manufacturer, levels 1 ‘low’, 2 ‘normal’ and 3 ‘high’, each possessed increasing concentrations of WBCs with increasing percentages of neutrophils. The samples were supplied in a small eyedropper bottle, which was deemed appropriate for sample access and delivery to the analyzer in spaceflight conditions. The manufacturers stated WBC concentration for each sample was as follows: low-2.7 K cells/uL; normal-8 K cells/uL, high-17 K cells/uL.
3. Fingerstick blood sample
Strictly to evaluate sample collection, handling, and loading onto the instrument, a fingerstick sample was obtained by a single astronaut and analyzed. For this collection, Personal Protective Equipment (PPE) was donned by the astronaut, benzalkonium wipes were used to cleanse the skin, and a lancet was used to obtain the capillary blood sample. The sample was collected into the ‘cuvette’ supplied by the instrument manufacturer. PPE was required due to the trace amounts of reagent within the cuvette, which was considered a potential irritant in the unlikely event it escaped the cuvette.
4. WBC count and differential analysis
For the evaluation instrument, samples were aspirated into a small cuvette containing both a lysing reagent and a DNA dye (methylene blue). Analysis was then performed by the instrument via nuclear morphology. All control samples were assayed on a terrestrial ‘gold standard’ hematology analyzer (Sysmex, Kobe Hyogo, Japan), and on the evaluation instrument during ground control data collection and aboard ISS (Fig. 1 gold, green, blue respectively). The flight assessment consisted of technical replicates performed immediately in sequence. The approximate time from procurement of the control solutions at the NASA Glenn Research Center until the assessments onboard ISS was approximately 10 and 13 weeks. Ground data was collected using the same lot of control solution, and within days of the flight data collection. The low, normal, and high control solutions were analyzed 4, 6 and 4 times, respectively. As many replicates were performed onboard ISS as could be completed in the limited time allowed for this activity (approximately 2 hours). Analysis consisted of white blood cell count (WBC) with differential (neutrophils, lymphocytes, monocytes, eosinophils, and basophils). Due to logistical considerations, the astronaut blood sample was an add-on test objective, and only analyzed during spaceflight.
Assessment of all control samples was performed in a collapsible glove bag in Node 2 aboard ISS. Several methods were examined aboard ISS to obtain a single drop of control solution from the bottle and properly fill a cuvette without bubbles forming in the microgravity environment. The best technique was found to be deploying a single drop onto the foil packaging in which the cuvette is supplied. This somewhat recreates the appearance of a single fingerstick drop on a fingertip. With the drop adhering to the foil due to surface tension, the astronauts were then free and able to fill the cuvette via capillary action. The operation was found to be mildly challenging, as fluids are difficult to control in space, but astronauts quickly mastered the technique after several practice runs.
For the WBC analysis, there was excellent homology between the terrestrial analysis (both ‘gold standard’ hematology analyzer, and the evaluation instrument), as well as the data collected aboard ISS. The mean values for WBC for ground were 3.1±.13, 8.4±.29, 19.3±.64 K/ul, for control solutions 1, 2, 3, respectively. This compares favorably to mean values of 2.9±.14, 8.6±.38, 18.8±.78 for the ISS data. This homology was consistent across the three levels of control samples analyzed, and also matched the WBC values provided by the manufacturer. For the differential analysis, aboard the ISS the neutrophil count was consistently above both the manufacturer provided mean value, and the ground control data. This disparity was most notable in the ‘normal’ and ‘high’ control samples. In each instance, a near-corresponding decrease in the eosinophil count was observed. Concentrations of lymphocytes, monocytes and basophils were generally in agreement between ground and flight data. Analysis of a single fingerstick blood collection was successfully performed and yielded WBC count and differential findings well within the established normal ranges for each parameter for a healthy human subject.