Misdiagnosis by intuition: Seven diagnostic axioms to enhance patient care

Misdiagnosis by intuition: Seven diagnostic axioms to enhance patient care

Apr 01, 2002

Most would agree that a correct diagnosis is a key prerequisite to providing safe and effective treatment for various illnesses. However, our diagnoses are often a matter of opinion rather than matter of fact.

In fact, isn't it true that most diagnoses often require us to make decisions in the absence of certainty? Our diagnoses are based on probability. This being the case, we must use caution not to fall into the trap of making diagnoses on the basis of faulty logic or insufficient information. It is one thing to make a diagnosis and another to be able to substantiate it. Though we name the things we know, we do not necessarily know them because we name them.

Audit the diagnosis

Shortcuts in diagnostic reasoning tend to become increasingly prevalent when veterinarians are subjected to the pressures of a high case load in a busy hospital. In this context, short cuts are often defended on the basis of "practicality". Although practicality is a virtue, we must use caution not to use the concept of practicality as an excuse for ignorance.

A misdiagnosis may be more detrimental to the patient than the illness. A wise sage once penned this thought: "Heaven defend me from a busy doctor."

How do we know when our diagnoses are in error? If we do not have a system designed to periodically audit our diagnoses for accuracy, we are unlikely to recognize and correct our errors. If the accuracy of our diagnoses is never questioned, we may become overconfident in our judgments with a tendency to rely less and less on clinical data and more and more on our intuition.

What is the inevitable result? Experience has revealed that diagnosis by intuition is often a rapid method of reaching the wrong conclusion.

Based on the premise that a well-defined problem is half solved, the primary objective of this Diagnote is to review some clinical axioms that foster the diagnostic process. An axiom is a statement universally accepted as true.

Seven diagnostic axioms

Axiom 1: There is a difference between knowledge and wisdom.

Knowledge is facts; it consists of familiarity with information gained by study and observation (that is, empirical experience or investigation). Unfortunately, most of us have been taught to over-emphasize the accumulation of new knowledge to a point where we neglect the development of acquiring wisdom.

Whereas knowledge consists of our familiarity with relevant information (facts), wisdom consists of the ability to properly apply knowledge. It implies sufficient breadth of knowledge and depth of understanding to provide sound judgment. Although essential, facts (knowledge) by themselves are rarely of useful value. Facts are not science, just as the dictionary is not literature.

In context of diagnosis of diseases, facts become useful only to the extent that they can be wisely used to define, solve and prevent problems. If we have knowledge but have not learned how to make practical application of it, we lack wisdom.

Axiom 2: There is a difference between problem definition and problem solution.

We use the term diagnosis in context of defining the cause(s) of clinical signs. The ability to define a patient's medical problems without overstating them is a crucial first step in the diagnostic process, since one must be able to define problems before they can be solved.

No veterinarian has or ever will be trained to single-handedly solve all types of medical problems. No one can recall enough knowledge and be proficient with enough techniques to guarantee that (s)he alone can provide the best care of every patient. Veterinarians can be trained to accurately identify problems, however. They can and should be master "problem definers."

Accurate definition of a patient's clinical problems will permit us in our role as diagnosticians to more efficiently use available resources, such as journals, books, the Internet, consultations and referrals, to help resolve diagnostic problems. A problem well defined is half solved.

Axiom 3: There is a difference between observations and interpretations.

Discernment of the difference between observations (facts) and interpretations of observations (inferences or assessments) is a critical component of the diagnostic process. In the process of defining problems, we must use care not to consider the meanings of observations and interpretations as equal.

Likewise, we should avoid mixing observations and interpretations randomly. Why? Because observations and interpretations represent distinctly separate facets of diagnosis. Consider this example. As veterinarians, we frequently interview clients who confuse observations and interpretations when describing the illness of their animals to us. A classic example is to misinterpret the observation of tenesmus as constipation in a male cat with urethral obstruction.

This type of error in reasoning is not limited to clients. It affects us all at one time or another. For example, when asking for specific laboratory data such as the hematocrit value (an observation), we may be told that it is normal (an interpretation).

But a hematocrit value of 37 percent (an observation), which is interpreted as normal may actually be abnormal in a severely dehydrated patient. Although either observations or interpretations may be erroneous, in our experience misinterpretation of a correct observation is the most common pattern of error.

What is the point? A misinterpreted problem is the worst of all problems. Why? Because if misinterpretations are unknowingly accepted as facts, misdiagnosis followed by misprognosis and formulation of ineffective or contraindicated therapy may result. This is indeed ironic since the patient may then be in a worse condition as a result of having visited us in our roles as doctors. What can we do to minimize this problem? One thing is to put the axioms in this column into practice. An observation or an interpretation is unlikely to mislead us if we learn how to avoid being misled.

Axiom 4: There is a difference between possibilities and probabilities.

The need to discern the difference between diagnostic possibilities and diagnostic probabilities is another key diagnostic axiom.

In general, collection and interpretation of relevant clinical data about a patient's illness allows us to reduce numerous diagnostic possibilities to a few or one diagnostic probability.

However, even after collection of a large quantity of relevant data, many diagnostic probabilities still represent a matter of educated opinion rather than a matter of fact.

Recall that absence of clinical evidence of suspected diseases is not always synonymous with evidence of absence of these diseases. As a corollary, detection of evidence that is consistent with a certain, specific type of disease is not always pathognomonic for a specific disease.

It follows that we as veterinarians should convey to our clients that our diagnoses, prognoses and treatment recommendations are based on probability.

Axiom 5: There is a difference between disease and failure.

Discernment of the conceptual difference between organ disease and organ failure is also fundamental to proper diagnostic refinement. Organ function that is "adequate" to sustain homeostasis is often not synonymous with "total" organ function.

For example, patients with only one kidney have adequate renal function to live a "normal" life without manifestations of renal dysfunction. Even when slowly progressive irreversible lesions occur, signs of organ dysfunction do not develop if adequate quantities of functional parenchyma (i.e. nephrons, hepatic lobules, etc.) remain to sustain homeostasis. This concept is the basis for distinguishing organ disease (such as cardiac valvular insufficiency) from organ failure (such as altered circulation associated with abnormal cardiac rate and rhythm which ultimately occur as a result of irreversible progressive cardiac valvular insufficiency).

Won't you agree that the approach to management of a patient with cardiac valvular insufficiency and adequate cardiac function is very different from management designed for a patient with cardiac valvular insufficiency and congestive heart failure?

Axiom 6: There is a difference between clinical signs induced by diseases and the body's compensatory response to disease-induced signs.

Clinical manifestations of disease can be subdivided into the following two classes: 1) signs directly induced by the disease (such as impaired urine concentrating capacity and obligatory polyuria associated with damage to the countercurrent system in patients with bilateral bacterial pyleonephritis), and 2) the body's compensatory response to these signs (such as compensatory polydipsia needed to maintain fluid balance because of obligatory polyuria).

Other examples of this relationship include compensatory inflammation in response to damaged tissue, fever in response to systemic infectious agents, polychromasia and reticulocytosis in response to anemia, and hyperparathomonemia in response to hypocalcemia. It follows that making a diagnosis of urinary tract infection solely on the basis of pyuria would be an overdiagnosis because pyuria may be a compensatory response to both infectious and noninfectious diseases.

Axiom 7: There is a difference between events that occur consecutively and cause and effect relationships.

The ability to recognize true cause and effect relationships is not an innate characteristic - it must be learned. The important point to be made here is that just because two or more events occur in consecutive order does not prove a cause and effect relationship. Why? Because, unrelated coincidences commonly occur in the lives of all of our patients. Consider this example. In the late 1970's and early 1980's, vesicourachal diverticula were cited as playing an etiologic role in some cats with lower urinary tract disease (LUTD). Treatment by surgical extirpation was recommended in most veterinary textbooks at that time. The observation that clinical signs subsided coincidentally with diverticulectomy, and lack of studies of the biologic behavior of macrosopic diverticula without surgery, reinforced the interpretation that this anatomic abnormality was a cause of LUTD.

However, subsequent studies revealed that vesicourachal diverticula were a sequela, rather than a cause, of LUTDs. Most of them spontaneously resolved with appropriate medical therapy of the underlying problem. Surgery was unnecessary. This example highlights the fact that favorable outcomes associated with our treatments do not prove that our diagnoses were correct, or that our treatments were.