Microplastic pollution is a pervasive global issue, and its measurement is often undermined by methodological bias, incomplete validation, and inconsistent reporting. Each limit comparability and reduce its value for environmental monitoring and evaluating the efficacy of treatment technologies. After two decades of research, standardised methods for microplastic identification and quantification remain elusive but are now critical to understand their fate and behaviour in surface waters. Improved reporting through combined analytical techniques, appropriate controls, and rigorous validation are essential for producing reliable, comparable microplastic data.

Factors affecting the recovery of known microplastic polymers during density separation were systematically evaluated to identify methodological biases, including polymer type, separation equipment, and salt solution. Subsampling, a common strategy to reduce counting time, proved a major source of error with fixed spatial subsampling (12.5% of a filter) producing inaccuracies exceeding 50%, while randomised bootstrapping performed better. Nile Red staining for detecting suspected microplastics substantially overestimated microplastic abundance, while co staining with DAPI revealed high false positive rates due to Nile Red positive particles actually organic and not plastic material.