A Neutral Red Uptake (NRU) assay proficiency study was conducted to evaluate the cytotoxicity of a mainstream smoke extract of three cigarettes using a common basic study protocol, and using each participating laboratory's preferred cell lines and protocols.  Laboratories were able to evaluate their method proficiency, to compare results with those of other laboratories and to obtain an external audit of documentation procedures that might identify potential areas for improvement.

The proficiency study showed that the sensitivity of the Neutral Red Uptake assay is good enough to differentiate the test pieces. The median variability of the three replicates was below 15 %. The main ranking in terms of descending cytotoxicity was Flue-Cured > 3R4F > Burley.