ICD-9-CM: NOS vs. NEC
Within ICD-9-CM, you may select codes defined as “Not Otherwise Specified” (NOS) when there isn’t enough documentation to select a more specific code. In other words, a deficiency in documentation prevents you from coding to a higher level of specificity.
NOS codes are never favored, and claims submitted with such diagnoses may be rejected for lack of medical necessity and/or specificity. When possible, you may wish to ask the documenting provider for additional information and/or to append the record, so that a more precise diagnosis may be selected.
Codes defined as “Not Elsewhere Classifiable” (NEC) may be selected when specific information is documented for the diagnosis, but there isn’t an existing ICD-9-CM code to report it. In this case, the ICD-9-CM manual—not the documentation—lacks additional specificity.