Comparison of results obtained in different laboratories is usually difficult because of the different methods used. The concentration and size distribution of the detected microplastics are mainly determined by the methods and tools used during sampling and the separation and purification procedures applied in sample preparation. For this reason, there are ongoing efforts in the scientific community researching microplastics to standardize the protocols used in studies [10], [170]. Another problem arises from the non-uniform definition of “microplastic”. Most commonly, polymer particles smaller than 5 mm are referred to as microplastics and particles smaller than 100 nm are referred to as nanoplastics, but there may be differences between different laboratories [171], [172]. Although this definition is generally consistent among researchers, the pore sizes of available sampling nets and filters have a large deviation, and in many cases the results are not comparable for this reason alone. Another problem is that it is insufficient for accurate determination of microplastic particles because it is based only on length and does not take shape into account. In terms of behavior in natural environments and thus biological effects, surface area and volume are more important. For this reason, a unified three-dimensional definition would be needed that could also provide information on shape [172]. A practical review of analytical methods for river water sampling, identification, quantification, and characterization of microplastics [173] highlights that the lack of uniform and standardized methods and protocols for the assessment and quantification of microplastic pollution makes the comparison of results from different studies questionable.