WHO Drug Information Vol. 20, No. 4, 2006 - World Health ...

WHO Drug Information Vol 20, No. 4, 2006
Rational Use of Medicines
The role of prescribed and defined
daily doses in pharmacoepidemiology
In pharmacoepidemiological studies, it is important to use one single measurement
for the volume of drugs prescribed, dispensed or consumed. For many years, the
defined daily dose (DDD) has been the globally accepted standard unit for such
assessments. A DDD is the assumed average maintenance dose per day for a drug
used for its main indication in adults (1). However, the DDD for a drug may deviate
from the prescribed daily dose (PDD) (2–4). This may limit its usefulness unless the
DDD/PDD ratio is known. Both DDD and PDD have been particularly useful in studying
patient adherence since reports increasingly indicate that adherence to longterm
drug therapy is low (5).
As an example, prescription refill adherence to corticosteroids used for asthma/chronic
obstructive pulmonary disease (COPD) treatment has been found to be only 34%
(6). This case provided the background for the study described below, where refill
adherence was determined by comparing patient prescribed doses to data from pharmacies
in Jämtland, Sweden (6). Although data bases of pharmacy records are preferred
compared to prescription collection, many data bases only have information
about drug volumes actually dispensed and not the prescribed doses. Furthermore,
a large proportion of the prescriptions issued are not dispensed (7). When prescribed
doses are not listed in the available data bases, dispensed drug volumes in DDDs
can serve as a proxy for patient dosing and refill adherence, providing that the PDDs
do not deviate from the DDDs to any great extent.
Asthma/COPD drugs: PDDs are
critical to determining DDDs
The present study* was conducted with
the aim of improving the rational use of
medicines through application of DDDs
and PDDs to data available concerning
the treatment of asthma and COPD in
Sweden. DDDs for different drug substances
are available from the WHO
Collaborating Centre of Drug Statistics
Methology (1). The prescribed doses for
the dispensing of asthma/COPD drugs
used in Sweden were obtained from
the pharmacy record data base of the
County of Jämtland with approximately
120 000 inhabitants. The data base holds
information on all drugs dispensed to a
representative sample of 17 000 inhabitants
(9, 10). Information relating to
dispensing include: age and sex of the
patient, dispensation date, name of the
drug, its ATC-code, drug volume dispensed,
and prescribed dose with code
numbers for patients and prescribers. The
identity of the patients and prescribers, as
well as medical conditions, remain
confidential.
*Dan Haupt, Department of Health Sciences, Luleå Technical University, 971 87 Luleå, Sweden
and J. Lars G. Nilsson, NEPI Foundation, 18 81 Stockholm, Sweden. Correspondence to:
lars.nilsson@nepi.net
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