I am doing cell viability assays using alamar blue and the following calculation: 

The percentage difference in the amount of resazurin reduced by treated cells when compared to a positive growth control (untreated cells) was calculated using the following equation:

 ((εox)λ2 Aλ1- (εox)λ1 Aλ2 of test agent dilution )/((εox)λ2 A°λ1- (εox)λ1 A°λ2 of untreated positive growth control)× 100

Where:             εox = molar extinction coefficient of alamarBlue® oxidized form (blue)

A = absorbance of test wells

A° = absorbance of positive growth control well

λ1 = 570nm

λ2 = 600nm

In one or two cases I have gotten results such as -50% reduction (which is proportional to cell viability but there cant be negative cell viability). What does this mean or how do I report these kinds of values? 

Any help you guys can give me would be greatly appreciated. Thank you in advance. 

Catriona