Researchers suggest that we can differentiate PMDD and premenstrual exacerbation of another disorder by going ‘symptom by symptom’ with daily ratings of the severity of each symptom. To do this, they explain that “symptoms of the ongoing disorder that worsen premenstrually should not be counted as symptoms of PMDD, but should instead be considered evidence for [PME]… of the current disorder. For example, depressed mood that increases premenstrually in a woman with [major depression] is considered to be evidence for [PME] of the woman’s major depression, rather than considered as a possible symptom of PMDD.”
You can see how this gets tricky. Depressed mood is a symptom of both PMDD and major depression. So, is depressed mood severe premenstrually because of PMDD or premenstrually exacerbated major depression? Also, could the cyclic depressed mood of PMDD have caused the now co-occurring major depression? However, despite the theoretical controversy, this method is often effective in diagnosing PMDD and PME respectively, although it is conservative and could lack some sensitivity (i.e. with this method, it is difficult to receive diagnoses of both major depression and PMDD because the conditions share some of the same symptoms!