Converting from ICD-9 to ICD-10 may be apples to oranges when it comes to hospital safety assessments, according to a new report in the Journal of the American Medical Informatics Association.

The ICD-10 codes may actually complicate, not simplify, the assessment of hospital safety, according to study author Andrew Boyd, an assistant professor of biomedical and health information sciences at the University of Illinois at Chicago.

Although some ICD-9 indicator codes that assess hospital safety might translate well, many more have very convoluted mappings, and some simply don’t map at all.

For example, a hemorrhage coded in ICD-9 may have any of several different codes in ICD-10 depending on which organ system is involved—making it difficult to tell whether a hospital’s safety record is actually improving. What’s more, it’s possible to select “accurate” new ICD-10 codes that make you look safer than you are because of the differences in the design of the ICD-10 system.

At the same time, some hospitals may look less safe than they really are because of apparent increases in patient safety indicators that are actually the same indicators calculated differently, the new report states.

The ICD-10 compliance deadline is now set for October 1, 2015.