You cannot assume either acute or chronic if it is not documented unless so directed in the ICD-10 instructions. The default, if there is one, can vary depending on the condition or diagnosis, and for some codes, 'acute' or 'chronic' will be listed in parenthesis in the code description, which means that the term in not required in order to use that code. The code book will take you to the appropriate code to select if follow the steps of going to alphabetic index to look up your code based on what is documented. So, for example, 'pharyngitis' is coded J02.9 which is also the code for 'acute pharyngitis', but 'chronic pharyngitis' has its own separate code J31.2. On the other hand, for a condition like cholecystitis, acute and chronic each have their own codes (K81.0 and K81.2), but if it is not documented one way or the other, you are directed to assign the unspecified code K81.9. Hope that makes sense.