Calculate Pain diagnostic questionnaire (DN-4 - Interview) sum score

Calculate sum score of DN-4

Patients are asked "Does the pain have one or more of the following characteristics?"

calculate_dn4_sum(dn4_1, dn4_2, dn4_3, dn4_4, dn4_5, dn4_6, dn4_7)

Arguments

dn4_1: vector for item "Burning"
dn4_2: vector for item "Painful cold"
dn4_3: vector for item "Electric Shocks" Patients were asked "Is the pain associated with one or more of the following symptoms in the same area?"
dn4_4: vector for item "Tingling"
dn4_5: vector for item "Pins and Needles"
dn4_6: vector for item "Numbness"
dn4_7: vector for item "Itching"

A numeric vector with sum score of dn4