Spreading and wetting behaviour of blood during dried blood spot sampling

Dried blood spot (DBS) is a blood collecting and storage method, which has been widely applied for newborn screening, therapeutic drug monitoring (TDM), and others. The concept of DBS involves the use a porous filter paper to absorb blood drops and preserve them as dried blood samples for further analysis. However, there are some disadvantages of the DBS process, such as, unequal distribution of the analyte of interest, haematocrit effects and difficulties in accurate analyte extraction, all of which have limited their applications for highly sensitive analysis. In addressing these issues, the spreading/imbibition of blood during the DBS sampling process has been investigated from both experimental and theoretical points of view in this work. DBS is described as a spreading process of a non-Newtonian drop (blood) over porous substrates (filter paper). Porcine blood with different haematocrit levels of spreading/penetrating over different porous substrates, including, commercial DBS filter papers and nitrocellulose membranes, have been used for the investigation via our spreading experiment. The time evolution of spreading parameters of DBS sampling process, such as, contact angles, droplet base radius and wetted region radius, were recorded and analysed to describe the spreading/imbibition behaviour of DBS sampling. The experimental results have shown that the spreading parameters of DBS sampling on each porous substrate fall into a universal curve under the dimensionless scale. According to our observations, the spreading behaviour of blood can be described as a complete wetting on the commercial DBS cards (Whatman 903) and partial wetting on the nitrocellulose membrane. Simulating models have been developed to describe the spreading/imbibition behaviours of DBS sampling process over porous substrate in complete wetting case. In the case of complete wetting, a system of two differential equations is derived, which describes the time evolution of radii of both the drop base and the wetted region inside the porous medium. The results show a good agreement while validating the spreading parameter, known as, droplet base radius, wetting region radius and contact angle with experimental data. These models provide simple and reliable methods to describe the blood spreading behaviour during DBS sampling process. In this project, the investigation of spreading behaviour of DBS analysis has been developed and presented. Reliable theoretical prediction of DBS sampling result can improve the defect of current detecting method and enhance the accuracy of the DBS analysis. In the consideration of the practical application of DBS analysis, the theoretical investigation of spreading/imbibition behaviour of DBS sampling should allow us to better control and predict the performance and outcome of pharmaceutical and analytical studies while using DBS method