Buyers must prioritize the filtration accuracy, typically measured in microns. A high-quality filter straw should have a hollow fiber ultrafiltration membrane with a pore size of 0.1 microns or smaller to effectively remove 99.9999% of bacteria and protozoa. Additionally, check the filtration capacity, which usually ranges from 1,000 to 5,000 liters depending on the model, and ensure the flow rate is sufficient (e.g., 200-500ml/min) to provide a good user experience without excessive suction effort.