Primary Health Care - JOHN J. HADDAD, Ph.D.

The ‘ologies’ (underpinning academic disciplines) of primary health care 29
or ‘2% more likely’. The research that produces these numerical estimates is
covered briefly in Sections 3.3 and 8.1–8.4, and in more detail in the specialist
textbooks listed on the previous page.
I have my differences with Dave Sackett about what EBM is, how it should
be promoted and where its limitations lie, but it’s worth acknowledging that
the idea of combining the most elegant and abstracted science – mathematics –
with the one closest to our own vulnerable humanity – biomedicine – was
nothing short of brilliant. Doctors have known for centuries that the ‘causes’ of
a particular disease do not necessarily produce that disease (take smoking and
lung cancer for example) nor does a single ‘abnormal’ diagnostic test mean
that a patient has actually got a particular disease (take a suspicious breast
lump detected on a mammogram for example – which means either genuine
breast cancer or a ‘false positive’with all the attendant anxiety and unnecessary
tests). I explain all this in more detail in Section 8.3, but for now, I just want to
convey the idea that numbers (if they are numbers we can trust) can improve
the quality of information and advice we offer our patients. Before EBM, all
we could say to a woman with an abnormal mammogram was ‘You’ve got a
suspicious lump; it might be cancer but it might be just a lumpy breast’. Using
research studies from epidemiology, we can now say that the overall chance
(based on the entire population of women who have had mammograms) of an
abnormal lump detected on mammography being cancerous is around 1 in 10.
But based on a stratified (i.e. divided up by age) analysis of the same dataset,
we can say that if the woman is 35, the chance of her lump being cancer is
approximately 1 in 35; if she is 55 the chance is 1 in 11; and if she is 75 the chance
is 1 in 5 (Figure 2.1). 14 This information is considerably more helpful to the
20
18
16
14
12
10
8
% chance that
a lump
detected on
mammography
will be cancer
6
4
2
0
30−39 40−49 50−59 60−69 70+
Figure 2.1 Positive predictive value of screening mammography at different ages 14 (see Section
8.3 for an explanation of the term ‘positive predictive value’).