Residual protein assessment in medical device processing: from analytical sensitivity to measurement system reliability.

Residual protein testing is widely used to assess cleaning efficacy during processing of reusable medical devices and serves as a surrogate marker for organic contamination. Advances in analytical technologies have enabled detection of increasingly small quantities of residual protein; however, uncertainty remains regarding the interpretation of results, the selection of analytical methods, and the clinical relevance of accepted cleanliness thresholds. This review examines residual protein assessment from a measurement-systems perspective, evaluating commonly used analytical methods, including Bradford, o-phthaldialdehyde, bicinchoninic acid, adenosine triphosphate (ATP) bioluminescence, and liquid chromatography-tandem mass spectrometry. Although ATP bioluminescence does not directly measure residual protein, it is included due to its widespread use in cleaning verification. The evidence reviewed indicates that reliable interpretation of residual protein measurements obtained using commonly employed indirect elution-based methods depends not only on analytical sensitivity but also on factors including recovery efficiency, extraction procedures, sampling strategy, and assay performance. Residual protein assessment should therefore be considered a measurement-system challenge rather than solely an analytical challenge. While emerging highly sensitive direct in-situ methods overcome many of the recovery limitations associated with indirect techniques, they introduce new challenges regarding the interpretation of very low residual protein levels and the establishment of clinically meaningful cleanliness thresholds. Acceptance criteria and reporting approaches should be interpreted within the broader framework of cleaning validation, where residual protein represents one of several complementary indicators used to support assessment of cleaning effectiveness.