Critical evaluation of ninhydrin for monitoring surgical instrument decontamination

New Department of Health (England) Choice Framework for Local Policies and Procedures guidance (CFPP 0101) still states that ninhydrin can be used to check for efficient protein removal from surgical instruments processed in sterile services departments (SSDs).

With the potential transfer of variant Creutzfeldt-Jakob disease (vCJD) via surgical procedures it is necessary to re-evaluate recommended methods for protein detection.

This paper reports studies on the sensitivity and applicability of ninhydrin for detecting proteins in laboratories and SSDs. The efficiency of protein removal by swabbing was also evaluated.

Ninhydrin showed poor sensitivity toward proteins. Limits of detection for bovine serum albumin (BSA) in solution were 205 microg/mL compared with arginine 6 microg/mL. A commercial kit could detect neither rat brain homogenate nor BSA at <1000 microg protein pipetted directly into the vials. Swabbing with water-wetted rayon swabs was inefficient at removing protein (50 microg) from instruments (N = 6) with 32 ± 4% BSA and 61 ± 5% fibrinogen remaining bound. Swabs dipped in 0.5% detergent (Triton X-100) solution had slightly better removal efficiency with 20 ± 3% BSA and 24 ± 2.8% fibrinogen remaining.

Ninhydrin kits, currently used in SSDs, are ineffective at detecting residual proteins due not only to the insensitivity of ninhydrin towards proteins but also to the poor desorption of adhered proteins by swabbing. Overall ninhydrin, either as a laboratory reagent or as supplied in protein detection kits, does not provide sensitive detection of proteins and generates high numbers of false negatives when used in decontamination practices.