ICHD criteria
The ICHD diagnostic criteria are:
Headache that, within 3 days of onset, fulfils criteria 2-4
Headache is present daily, and is unremitting, for > 3 months
At least two of the following pain characteristics:
bilateral location
pressing/tightening (non-pulsating) quality
mild or moderate intensity
not aggravated by routine physical activity such as walking or climbing
Both of the following:
no more than one of photophobia, phonophobia or mild nausea
neither moderate or severe nausea nor vomiting
Not attributed to another disorderNotes:

Headache may be unremitting from the moment of onset or very rapidly build up to continuous and unremitting pain. Such onset or rapid development must be clearly recalled and unambiguously described by the patient. Otherwise it is coded as 2.3 chronic tension-type headache. History and physical and neurological examinations do not suggest any of the disorders listed in groups 5-12 (including 8.2 medication overuse headaches and its subforms), or history and/or physical and/or neurological examinations do suggest such disorder but it is ruled out by appropriate investigations, or such disorder is present but headache does not occur for the first time in close temporal relation to the disorder. Criteria revision
Although the original Silberstein–Lipton criteria and the original description by Vanast make no suggestion for the exclusion of migrainous features in NDPH, the current ICHD criteria exclude patients with migrainous features. When migraine features are present, classification thus becomes problematic. It has been reported that migraine symptoms may be present in over 50% of NDPH patients. The current criteria definition thus excludes more than half of patients with new onset of daily headache.
Headache: The Journal of Head and Face Pain. 51 (1): 145–154. doi:10.1111/j.1526-4610.2010.01812.x. PMID 21198576. Robert, Teri (2004). "New Daily Persistent Headache - The Basics". Health Central.