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Inexperienced physicians tend to see diagnoses as absolute. It is more
prominent among surgeons. One who sees during an operation, a nerve root
that is compressed by a herniated disc or the collapse of the femoral
head due to avascular necrosis, would be convinced that he has a good
understanding of the pathophysiology of the disease and the causes that
brought the patient to his attention. Unfortunately the situation is not
always that clear. Rheumatic patients often spend weeks, months or even
years before getting a final diagnosis. Incomplete lupus erythematosus
(latent or ”non-criteria” lupus) is a typical example. In oncology, the
pathologist is not always convinced about the precise type of the tumor
or even if the tumor is benign or malignant. Simple count of mitoses,
that is prone to subjectivity, 1 sometimes is the only
method that determines if chemotherapy is necessary. The purpose of this
paper is to examine the diagnostic process from the viewpoint of the
classical scientific method, based on the work of Karl Popper, and to
shed some light on the meaning of making a diagnosis.
Karl Popper was born in Vienna in 1902 to a converted Jewish family.
After the annexation of Austria by the Nazis, he immigrated to New
Zealand and later to Britain. He wrote several books that had a profound
influence on the philosophy of science and medicine in the previous
century. In his 1959 book ”The Logic of scientific discovery”2 Popper has thoroughly analyzed the scientific
method. He claims that after a tentative creation of a hypothesis that
is ”not justified in any way”; the hypothesis is tested by the way of
”empirical applications of the conclusions that can be derived from it”.
A positive experiment is always temporary as any future negative
experiment can overthrow the theory. In the process of medical
diagnosis, when the patient presents with a symptom, the physician makes
a hypothetic list of diagnoses that can possibly explain the symptom.
The next stage would be to exclude or prove the diagnoses by the means
of history taking, physical examination and different laboratory and
imaging tests. In the same manner as Popper described, a proof or a
disproof of a diagnosis is in effect only until a new test will change
the conclusion. Reproducibility of an experiment is another condition
for the effect to be scientifically significant 2 and
certainly is important in the diagnostic process. Multiple repetitions
of an experiment bring us closer to exemplary proof, which is actually
unreachable, as a single negative experiment can disprove the theory. An
experiment actually measures the probability of a result and almost
never the p-value in the medical field would be zero. Even the most
successful drug or procedure sometimes fails, and patients in the
control group can recover with placebo treatment. Intent-to-treat
analysis necessitates including dropouts in the final statistic, thus
decreasing the final effect of the test, drug or the procedure that is
under experiment. 3 Each medical laboratory test or
imaging modality has its own sensitivity, specificity and positive and
negative predictive values that are never absolute. So actually even at
the end of the diagnostic process we remain with a hypothesis that
is proved both temporarily and in a probabilistic manner . Ilgen et al4 indicate that skillful clinicians work comfortably
when uncertain and acquiring this ability to act in complex settings is
necessary for trainees.
Another subject that is a source of confusion and misunderstanding is
the common use of diagnostic criteria. They are very useful, especially
for the less experienced physician, but one should always keep in mind
that diagnostic criteria cannot be more accurate than the tests they are
based upon, prior to the clinical validation of the criteria for
diagnosis. For example, the diagnostic criteria for a prosthetic joint
infection rely on cultures, histology, elevation of acute phase
reactants and some other tests. Each of these tests has its own
sensitivity and specificity and the combination of tests does not create
100% specific and sensitive method. Eventually validation of each set
of clinical criteria for diagnosis is based on a “gold standard
test ” that also has its sensitivity and specificity that never equals
one. Overdependence on diagnostic criteria will eventually lead to
misdiagnosis. For example, Li et al had found that the number of
patients diagnosed with a prosthetic joint infection in a group, grossly
varies depending on the diagnostic criteria that were used.5
In conclusion, the clinical data should be analyzed without prejudice.
Flexibility and readiness to re-evaluate the conclusions when new
information arrives is necessary. Criteria and classifications are
intended only to be an aid in diagnosis, not the absolute truth.
Understanding the intricacies of the diagnostic process is absolutely
necessary to provide proper care.