The current population of Tayside is approximately 400 000 people and is comparatively small, even for the study of commonly prescribed drugs.
WEAKNESSES
The
current population of Tayside is approximately 400 000 people and is
comparatively small, even for the study of commonly prescribed drugs. However,
drug exposure data in Tayside are only available from 1989 and cover only a
limited set of drugs until January 1993 from when all dispensed prescriptions
have been collected. Scottish doctors are conserva-tive prescribers of new
drugs, so new agents tend to penetrate the market a few years after their
launch. This limits the ability to study new chemical enti-ties, arguably the
most important and interesting drug group to study. Offsetting these
disadvantages, the profile of certain diseases, for example cardiovascular
disease, is higher in Scotland than in other popula-tions and consequently the
prescribing of drugs used in the prevention and treatment of these diseases is
proportionately higher.
Another
weakness, but one that is common to many drug safety databases, is the
inability to capture directly exposure to over-the-counter drugs or drugs
prescribed in hospital. Perhaps more importantly, the diagnostic indication for
prescribing is not available to the researcher. In some cases, the indication
for drug use may be clear; for example, glyceryl trinitrate is used primarily
for angina. However, difficulties arise when a drug has more than one
indication for use, leading to misclassification of exposure or outcome. For
example, beta andreoceptor-blocking drugs can be given for indications varying
from anxiety to hypertrophic cardiomyopathy. This may be a potential source of
error called confounding-by-indication that is difficult to adjust for in
pharmacoepidemiologic research if the information is not available.
MEMO
cannot contact patients directly to elicit information on possible confounding
factors. However, with the Ethical Committee’s approval GPs can do this in a
collaborative manner. Primary care and hospital records can also be checked and
some data on smoking and alcohol can be retrieved from them, although the
quality does vary (Evans et al.,
1998). This is also a method to identify outpa-tient diagnoses, which are not
available electronically for record-linkage in MEMO. MEMO is therefore
currently best suited for the study of serious drug toxicity that requires
hospital admission.
One
of the criticisms levelled at record-linkage studies is the inaccuracy of
computerised medical diagnoses. The discharge diagnoses for SMR1 are abstracted
from the clinical discharge summaries by specially trained coding clerks. These
clerks on occa-sions have to interpret the ‘soft diagnoses’, such as symptoms,
for which no cause can be found. In addi-tion, non-standard terminology may be
employed to describe an illness, for example eponymous terms, and so the coding
of diagnoses may be imprecise. Computerised algorithms exist to detect and
reject the most glaring errors, but errors of interpretation persist within any
database. Several validation studies of the accuracy of hospital discharge data
in Scotland have been performed comparing the coded diagnoses with diagnoses
inferred by one or more senior doctors who have reviewed the original case
records (Kohli and Knill-Jones, 1992; Park, McCabe and Russell, 1992; Pears et al., 1992). The most pertinent of
those stud-ies carried out on Tayside data found 18% of inter-nal medicine diagnoses
to be clinically unacceptable (Pears et
al., 1992). Since the publication of this study, steps have been taken,
mainly for resource manage-ment reasons, to improve the diagnostic accuracy of
computerised data by involving clinicians in quality control. This initiative
has substantially improved the diagnostic accuracy of records.
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