Recent regulatory additions (ANSI/AAMI/ISO 10993-5:2009) on biocompatibility for devices state that the qualitative cytotoxicity tests (direct contact, mem elution, agar diffusion) are appropriate for screening purposes, but that quantitative evaluation is preferable.

Annex C of ISO 10993-5:2009 refers to the MTT cytotoxicity assay, which can accurately quantify as few as 950 cells. The MTT is a colorimetric method that measures the reduction of yellow 3-(4,5-dimethylthiazol-2-yl)-2,5-diphenyl tetrazolium bromide by mitochondial succinate dehydrogenase. Because the cellular reduction is only catalyzed by living cells, it is possible to quantify the percentage of living cells in a solution.

The MTT can be used to evaluate the cytotoxicity of:

• Extractable materials of medical devices
• Toxic compounds
• Toxins and environmental pollutants
• Potential anti-cancer drugs
• Antibodies to examine growth inhibiting potential

The major advantages of the MTT are its quantitative ability, that it can be done on either extracts or by direct contact, and that the results are not subject to analyst interpretation. Additionally, the MTT can be performed on 96-well microplates in a standard reader (such as a Bio-Tek ELx808) allowing for fast screening of multiple samples.

However, it should be noted that while the MTT is recommended, the MTT assay does not discriminate a specific cellular death mechanism – such as apopotosis vs. induced cell death. Additionally, it may underestimate cellular damage and only detect death at the last stages of the cellular dying process.