Why can a negative ICER be difficult to interpret?

• A. It always indicates the new intervention is worse.
• B. It may represent either a good or bad outcome depending on cost and effectiveness direction.
• C. It means costs exceed benefits.
• D. It is not interpretable and should be ignored.

Answer: (B)

Negative ICER arises when costs and effects move in opposite directions. This happens when the incremental cost change and the incremental effect change have opposite signs, so the ratio is below zero. That can mean two different realities. If the new option costs less and delivers more effect, you get both savings and a bigger benefit—this is favorable. If it costs more and yields less effect, you pay more for worse outcomes—this is unfavorable. Because the negative sign alone doesn’t tell you which situation you’re in, you must check the directions of both cost and effect differences to interpret it correctly. In practice, this is viewed in terms of dominance: a negative ICER indicates either dominance (cheaper and more effective) or being dominated (more costly and less effective). Also be aware that if the effect difference is very small or due to sampling variability, the ICER can be unstable, so confidence intervals and plots on the cost-effectiveness plane are often consulted, along with thresholds for willingness to pay.