E-DRUG: DDD of antibiotics (contd.)
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Dear Dr. Riewpaiboon and E-druggers:
I just came from the II Congress of the European Society of Clinical
Pharmacology and the Scientific meeting of EUR-DURG (European Drug
Utilization Research Group) when I got your question about the DDD
of antibiotics an the answers provided by the list. This is in fact
a key issue which was discussed in the two above mentioned fora.
DDD (Defined Daily Dose) is (ref: the "Guidelines for DDD" WHO
Collaborating Centre for Drug Statistics Methodology. Oslo 1991):
"the *assumed average* dose per day for a drug used on its *main
indication* in adults".
The guidelines continue with:
"It should be emphasized that the defined daily dose is a
*technical* unit of measurement and does not necessarily reflect the
recommended or actual used dose. Many drugs are for instance used in
different dosages on different indications and this must be taken
into consideration when evaluating drug consumption statistics.
Sales or prescription data monitored and presented in DDDs will thus
only give a rough estimate of consumption and not a real picture of
actual use." (end of quotation).
So DDD are just a *common* unit to compare sales or prescriptions
from one country/region to another, or in different periods of time.
The whole concept of DDD has proved extremely useful in the beginning
of the Drug Utilization Studies, and it is the most appropriate way to
compare sales and drug utilization data from one country to another with
very different settings, conditions, pharmaceutical market, etc... Its
use should also be linked to the use of the WHO-ATC Classification
system for drugs. The use of costs, or volumes of packages sold in
different countries is far more difficult and easy to misinterpret than
the use of DDD.
Furthermore for drugs with a very different dosage, the DDD help in
bringing together meaningful associations with the "actual dose". For
example, the DDD of digoxin (0.25 mg) is different from the DDD of
propanolol (160 mg), but when the sales are converted into DDD / 1.000
inh. / day, the "intensity" of use could be assessed in a more practical
way.
It is quite clear that if you wish to compare different countries, with
different "pharmaceutical" backgrounds, you need to have the same unit
of measurement, the DDD. That is why the DDD is *defined* and not
*obtained* from a particular setting or country. This makes it possible
to use DDD in international comparisons, but limits the interpretation
of the results. DDD is defined internationally by the WHO Collaborating
Centre.
WHO Collaborating Centre for Drug Statistics Methodology
P.O.Box 100 veitvet
0518 Oslo
Norway
Tel.: +47 22 16 98 11
Fax.: +47 22 16 98 18
e-mail: marit.ronning@nmd.no
Please, contact the Centre if you want to use DDD (or me if you only
need copy of the relevant pages), and do not try to *define* the DDD
for yourself, because it will have meaning for you only.............
(that actually happens in many places, and it's unfortunately reflected
in publications).
Maybe it is important to also mention concept of ***PDD***:
(Quote from the guidelines...):
"The Prescribed Daily Dose (PDD) can be determined from prescription
studies, medical records and patient interviews. It is important to
relate the PDD to the diagnosis which the dosage is based on. The PDD
will give the average daily amount of a drug which is actually
prescribed. When there is a substantial discrepancy between the PDD and
the DDD, caution is needed when comparison is made. (...) The PDD can
vary according to both the illness treated and national therapy
traditions" (end of quote).
The PDD must be obtained from direct sampling of prescriptions, either
by random samples, or by using computerized databases. It normally
varies a lot from one country, or setting (hospital v PHC, etc) or
according to the indications. It would be even desirable that the range
of the PDD would be quite wide, because it would represent the adequate
dosage according to the personal characteristic of the patients... So,
PDD could help in understanding the therapeutic trends, and even in
assessing the prevalence of use of a particular drug (or disease). It is
not *defined* but *obtained* for a particular group of prescribers /
patients, in a particular study.
The DDD only seldom coincide with the PDD in actual setting. One
major reason is the existence of various indications for the same drug.
A good example is aspirin. The dosage of aspirin varies from 60/75/125
mg /day for prevention of MI, or 500-1.500 mg as analgesic, to 3 gr. for
the treatment of Rheumatoid Arthritis (RA). Which is the "main
indication" ?; and the "assumed average" ?. In fact the *Defined* DD for
aspirin is 3 gr (because the "main indication" was considered to be RA).
That means that if a given population is receiving 3 gr. of aspirin
daily, for a whole year, for the treatment of RA, with an incidence of
almost 0, and non other use of it, the DDD / 1.000 inh. / day will be
close to the prevalence of use of aspirin. This is quite clearly not the
case for aspirin, but could be for Insulin, for example, in particular
conditions.
For general comparison the DDD is more valuable than using total costs
or number of packages, but has serious limitations if the estimation of
prevalence is intended. Then you have to have more direct data, PDD,
distribution of indicatons, etc... These data could be very good in a
particular study but are really difficult to get,in a comparable way, if
you want to compare with other studies.... Sometime the only possible
comparison is DDD, despite its limitations.
So, my recommendation is that you contact with the WHO Centre for
obtaining the DDD, and use those DDD in your study for international
comparisons. And calculate the PDD in your sample, if you have such a
possibility, which together with the indications of the prescriptions
and other demographic variables could help you in calculating the real
use and the characteristics of it in your area.
Contact me directly if you need detailed information and examples.
Prof. Emilio J. Sanz, MD, Ph.D.
Clinical Pharmacology
School of Medicine
University of La Laguna
38071 La Laguna. Tenerife. Spain
EURO-DURG Executive Committee Member
Email: esanz@ull.es
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