A. Arteriosclerotic renal artery stenosis would fit the picture
A.
Hence, there is reason to suspect that A is true. (after Peirce
1998, 231)
In other words, clinicians begin by knowing nothing definitive about the
origin of the signs and symptoms and search by conjecturing hypotheses
that, if true, would explain the symptoms (Stanley and Campos 2013,
301-302). For example, a physician may auscultate a dyspneic patient’s
chest for crackles. She reasons that if the patient had pulmonary
congestion, then crackles would be a predictable consequence. Therefore,
she conjectures that the patient may have pulmonary congestion. The
specific choice of the type of congestion is based, of course, on the
relevant evidence available at the time: e.g. cardiac murmur,
breathlessness, previous history of cardiac disease, etc. Similarly,
uncontrollable hypertension, in spite of the usual cause, idiopathic,
would suggest a different etiology.
Importantly, however, Peirce emphasizes these hypotheses are only
asserted “problematically or conjecturally.” The mere fact that the
hypothesis explains the symptoms is not yet a reason to accept it.
Rather, the conjectured hypotheses are proposals to be subjected
to further testing. For Peirce, as Kapitan (1997, 477) points
out, the purpose of abduction is both to generate hypothesis but also to
“recommend a course of action” by selecting hypotheses for further
examination. This course of action may be likened to an internal
question and answer format (auto-elenchus.): considering these
observations and tests, what are the likely diagnoses.
For any given set of symptoms, physicians may often conjecture many
possible diagnoses; not all of these can be pursued at once. The second
type of reasoning involves clinician having to prioritize one or perhaps
a few hypotheses for further testing. In other words, the clinician has
to rank the hypotheses in terms of their pursuitworthiness
(McKaughan, 2008, Nyrup 2015). While it is often important to consider
the most likely hypotheses, Peirce emphasized that others factors are
also relevant, including how easy it would be to confirm or eliminate a
hypothesis by further testing, the strategic “value” of the hypothesis
and what we might learn from it, even if the hypothesis turns out to be
false (e.g. Peirce 1932-58, §7.220) and how much is at stake if we do
not follow-up on a suspicion. Even false hypotheses may lead to a
diagnosis, e.g. the hypertension imaging may not show renal artery
stenosis, but the adjacent adrenal gland may be enlarged suggesting
Pheochromocytoma!
In a clinical context, the value of a hypothesis consists in its
consequences for further treatment. Aspects of this Peircean approach
are reflected in the received wisdom of the literature on clinical
decision-making. For instance, Richardson & Wilson (2015) highlight two
relevant considerations (or strategies) in addition to the probability
of hypotheses: first, that clinicians should sometimes prioritize
hypotheses that, if true, would support an effective treatment and,
second, that they should prioritize hypotheses which would be very
harmful if left untreated. Similarly, in the analysis underlying the
so-called ‘threshold approach’, Pauker & Kassirer (1980) consider four
possible consequences of testing a diagnosis: (i) treatment is applied
and the diagnosis is true; (ii) treatment is applied, but the diagnosis
is false; (iii) treatment is withheld, but the diagnosis is true; and
(iv) treatment is withheld and the diagnosis is false. Consequently,
whether to apply the test depends both on the accuracy of the test
(false positive, false negatives etc.), on the potential harms of the
treatment and the test and on how serious the disease would be if
left untreated.
In addition to these sensible strategies, one ought to remember that
more strategic considerations can also play an important role. By
strategic, we mean considerations, which may not in themselves take us
closer to the truth, but which will help our further investigations in
other ways (Hintikka 1998, Paavola 2004) as in the clinical example
noted above. For instance, it can sometimes be valuable to rule out
certain plausible hypotheses, e.g. because this would allow us to ignore
otherwise possible complications related to further treatments or tests,
or simply because it makes our diagnostic space more manageable. We are
not considering wild hypotheses because we have a patient and
circumstances before us in the clinic. Thus, if a hypothesis can be
easily and reliably tested without harm to the patient, this can be a
good reason to prioritize it, even if it seems unlikely or a
priori improbable. Examples of this include serologic testing for Lyme
disease, Chagas cardiomyopathy, serum protein electrophoresis, fat
aspirate for amyloidosis, and ferritin levels \soutand cardiac imaging
for myocardial iron.
While we emphasized above that in the opening stages of inquiry,
considerations besides likeliness of being true play an important role,
i.e. conjectures, medical diagnosis usually demands that we find, as
economically and quickly as possible, the explanation for the patient’s
condition. This is the third type of reasoning, where physicians
conclude that a hypothesis is either ruled out, and can therefore be
ignored in further inquiry, or that it is likely enough to merit further
treatment or exploration. Experienced clinicians should have a
diversified kit of strategies rather than relying on any one approach
every time.11See also Campos & Stanley (2015) on the diversity
of logical strategies employed in clinical reasoning. Thus, while
clinicians may sometimes use something akin to Bayesian reasoning or
frequentist statistical reasoning, they sometimes have to rely on less
formalizable clinical judgments, based on their experience. We
illustrate this in the clinical case below.
-
Clinical Case: Chest Pain
Having introduced our general account of the logic of clinical
diagnosis, we proceed to examine a detailed case. In the example we
distinguish between the description of the clinical details (in
italics) and our logical commentary.
Scene : At home: 09:00. 54 year-old man walking down the
stairs to breakfast. He suffers sudden substernal chest pain radiating
to his left shoulder and back. No previous history of chest pain. No
previous chronic disease. His spouse immediately calls the local
emergency number.