As reliability, blood safety, and ease of use of RDT test kits will be fundamental to the success of large-scale implementation of parasite-based diagnosis, optimizing blood transfer devices is important. The five blood transfer devices assessed here, four of which are representative of the devices commonly used with commercially-available malaria RDTs, and one of a more novel design, exhibited a wide range of performance characteristics.
In this study, the inverted cup device provided the best overall performance, and was considered the most appropriate choice for use with RDTs by the majority of participating health workers. The loop also performed well, with similar accuracy and precision of blood volumes transferred but slightly lower scores for other characteristics. The other devices evaluated, including a squeezable straw-pipette, squeezable plastic calibrated pipette, and glass capillary, provided lower accuracy and precision in blood transfer, higher risk of blood exposure, and lower scores on ease-of-use assessment and user preference. The main advantages of the cup design appear to result from the ease of uptake or collection of blood (achieved by contact with the blood drop surface) and presentation of a broad surface area of retained blood for release onto the absorbent surface. In contrast, the loop presents a narrow contact surface unless tilted sideways (which some participants noted is more challenging when depositing blood into the well of an RDT rather than onto the flat surface of the filter paper used in this evaluation). The straw-pipette and calibrated pipette also required further manipulation to ensure contact between the blood and the absorbent surface.
The need for improved blood transfer methods for use with RDTs has been identified in previous reports[7–12]. The only previously published direct comparison of malaria RDT blood transfer devices, by Luchavez et al , assessed blood volumes and ease of use of devices available at the time and assessed the importance of variation in blood volume to RDT accuracy. Although excess blood did not affect accuracy, it was noted that blood staining of the RDT test strip may obscure test results. Inadequate blood volume is likely to become critical at low parasite densities, when low volume reduces RDT sensitivity. In the assessment by Luchavez et al sensitivity was considerably reduced at parasite densities of 200 parasites/μL with volumes less than 3 μL for RDTs designed for 5 μL of blood. In the present study, for all devices at all sites, both the mean volume transferred and the mean minus one standard deviation fell above 3 μL (Table 2), although the range fell below 3 μL in some cases. The reason for differences among the three sites in volumes transferred is not clear, as identical methods and materials were used at all sites; it may be speculated that differences in blood samples and ambient conditions, or possibly inadvertent differences in technique, may have led to variation in how the blood was absorbed by the filter paper.
A limitation of this evaluation is the somewhat artificial condition created by using absorbent filter paper, rather than RDT pads, for blood deposition. This method allowed us to estimate the blood volumes transferred by each device in the hands of health workers, but provided unreliable results for transfer volume with the glass capillary due to the observed difference in the absorbent qualities of filter paper compared to an RDT pad. The same phenomenon, and the resultant difficulty in depositing blood onto the filter paper from a glass capillary, may also have contributed to health workers' less favorable perception of this device, although many participants also commented on unrelated features. Another artificial condition created for this study was the placement of a blood drop on a gloved finger, rather than pricking a patient's finger, to obtain a source of blood for each transfer. The devices tested may perform slightly differently when collecting blood from skin rather than the latex surface.
The current study was specifically aimed at assessing the suitability of the devices for use by front-line health workers with limited laboratory training and limited facilities. Some workers may also have lesser capacity for fine manipulation. Clearly, specific training may overcome some of the apparent deficiencies illustrated here. For example, the capillary tube and calibrated pipette may perform better in laboratories with technicians accustomed to manipulating such devices. The results of this study show significant variation between study sites; differences in prior training or experience using certain types of devices are likely explanations for these variations. For example, Ugandan health workers who have considerable experience with RDT use made, in general, fewer observed errors with all devices than did less-experienced participants in the other two countries (Table 3), although patterns across the sites were not always consistent and differences were not often large. Where health workers are already trained and accustomed to a particular device design, the process of change may have a detrimental impact on overall performance, which would outweigh the potential advantages of a new design. However, when considering large-scale implementation of RDTs in remote areas, essential to fulfill the new global WHO policy of parasite-based diagnosis prior to anti-malarial treatment, performance, safety and ease of use in the hands of front-line health workers will be critical to success.
As RDT cassette design is essentially standard, blood transfer devices should be an exchangeable accessory. Careful selection of the blood transfer device should be a part of overall RDT kit selection. Flexibility in matching the most appropriate RDT (in terms of detection threshold, species detection, stability and cost) to the most appropriate transfer device for the field of intended use could improve the outcome of RDTs' introduction into malaria programmes.
In addition to malaria RDTs, point-of-care tests based on sampling of small quantities of blood or serum are currently used at peripheral health care levels for a number of diseases (e.g. dengue, syphilis), and will be of increasing importance as new tests are developed for other diseases. The results of this study are likely to be applicable to these tests, though they should be confirmed on different sample types and with different cadres of health or laboratory worker. It seems illogical to use a wide variety of device designs aimed at achieving essentially the same task, when required transfer volumes are similar, and accuracy, blood safety and ease of use are universally important issues. Establishing quality and consistency in the design of point-of-care diagnostic kits for different diseases will improve both diagnostic performance on the front lines of the health care system and ease of training and implementation.