What is the most appropriate way to collect a liquid bloodstain at a crime scene?

Collecting a liquid bloodstain requires careful consideration to preserve the integrity of the sample for analysis. Storing the blood in a paper bag after air drying is the most appropriate method. This approach allows the blood to dry, which helps to prevent degradation and contamination from moisture. Paper is preferable because it is breathable; it enables the sample to dry out while also allowing any residual moisture from the blood to evaporate. This helps ensure that the evidence is not compromised.

Choosing to use a plastic bag for storage could trap moisture, potentially causing bacterial growth and deterioration of the sample. Similarly, while swabbing can be an effective collection method, it does not provide a full representation of the liquid sample available in the stain. Using a syringe could extract the liquid efficiently, but it may also increase the risk of contamination or mishandling.

Thus, air drying the bloodstain before sealing it in a paper bag represents the best practice for maintaining the viability of the sample while also adhering to evidence collection protocols.