How Physicians Reason

20 May

Diagnosis: How Physicians Reason

A problem defined is a problem half solved.

A diagnosis dictates decisions and action. It can be a simple one, easy and quick to make: the  common cold, sprained ankle, pneumonia, cystitis (bladder infection).  Or it can be more complex, requiring substantial thought and time: bacterial endocarditis, a complex illness caused by a heart valve infection; ulcerative colitis, sometimes beginning as mild diar- rhea instead of the more dramatic bloody bowel movements; dissecting aortic aneurysm, a tearing of the main artery leading from the heart and of- ten mimicking the symptoms of a heart attack. A diagnosis, even a tenta- tive one—the answer to the questions “What is wrong?” or “What is going on?”—allows the physician to make decisions and take action.

Like the medical history, diagnosis is an ongoing process of defining and refining the issues. It is not enough to declare, “She has coronary heart dis- ease.” Over and over, the physician needs to ask these questions:

•  Can the diagnosis be refined further and more precisely? Different varieties of the same illness require different treatment.

•  Does the diagnosis require urgent treatment? Delay in treatment may cause ir- reversible harm.

•  Should I look for other illnesses associated with this one? One diagnosis may be the only clue to an associated illness that requires separate consideration and treatment.

•  What psychological and social factors pertain? Some of those factors may clar- ify the diagnosis, and some diagnoses have substantial impact on the patient and the family.

•  What else could this be? From an incorrect diagnosis, the physician will make invalid decisions about treatment and prognosis.

Our ability to describe the system we use to solve problems is important. This allows us to solve individual diagnostic and therapeutic problems con- sistently and efficiently, mature as problem solvers, and learn from our ex- perience. Regardless of our store of knowledge, we are called upon to solve problems of such variety and complexity in the course of normal practice that we require a dependable problem-solving methodology to make best use of that knowledge and skill.

Physicians constantly build on experience. Experience with other pa- tients helps us address the patient at hand. Knowing about other illnesses associated with a diagnosis, complications, beneficial and adverse reac- tions to drugs and other treatments, and psychosocial issues adds to the quality of the diagnostic process. All of this experience helps us under- stand, anticipate, and empathize.


What a physician does to make a diagnosis, a process called “differential di- agnosis,” begins by giving a problem a name and then exploring the possible diagnostic solutions to the problem. To introduce the students in my seminar to the process of diagnosis, we work together in class on this simple problem:

Case 1

One evening you are called at home by your patient, a 50-year-old man, who says, “I’m having pain in my stomach, low on the right side.” As you are driving over to his home, you begin thinking about what might be wrong with him.

I suggest these questions to help them in their deliberations:

1. What is the name of the problem? I do not mean, “What is the diagnosis?” but rather, “What would you call the problem he described?” A problem name can certainly be a diagnosis, such as pneumonia, diabetes, or duode- nal ulcer. Or the name can be a symptom, such as chest pain or shortness of breath. Name the problem as precisely as you can, I urge them, no more, no less. To name the  problem, one need not  be medically sophisticated. “Headaches,” “stomach cramps,” “no pep,” and “swollen knees” are part of everyone’s vocabulary. The name of this man’s problem is “right lower ab- dominal pain,” but that is not enough to direct treatment, which could run the gamut from a heating pad for a sprained abdominal wall muscle to an operation for appendicitis. My goal is to teach the students the importance of how we name a problem. Giving a problem the wrong name and prema- turely jumping to a diagnostic conclusion can delay treatment, bring about incorrect treatment, or imply a prognosis that may be either too optimistic or pessimistic. And certainly such an error can undermine the patient’s fu- ture confidence in the skill of the physician.

2. Where  in the body is the disease? What organs and structures are in this

area? Even at the undergraduate level, most have a basic knowledge of hu- man anatomy, and during the discussion, they acquire more. I ask them, “From front to back, what’s in the right lower abdomen?” Any of these or- gans or tissues could have an abnormality that causes pain, I tell them.

3. On the basis of your answers to the previous questions and your own

current knowledge, what diagnoses might you consider as you try to answer the question, “What is wrong with this patient?” What could be wrong with each of these organs or tissues? With very little prodding from me, they come up with this list:

Organ                                            The Students’ List

Skin                                                 Burn, cut Muscle       Sprain Small intestine         Ileitis


Large intestine                                 Colitis

Cancer Appendix       Appendicitis Ureter              Kidney stone

4. The medical history is important. On the basis of your answer to the previous question, what questions would you ask the patient that would help to clarify the nature of his illness and make a diagnosis? What do you need to know to choose or eliminate each diagnosis you mention in question 3? Ta- ble 10.1 summarizes the process.

Each of these questions, though not definitive, moves the diagnostic

process along. The presence of blood in the urine, for instance, along with inability of the patient to find a comfortable position would point toward a kidney stone. Weight loss over a period of weeks or longer might suggest co-

Table 10.1

Using the History as the First Step in the Differential Diagnosis of “Right

Lower Abdominal Pain” (in Case 1)

lon cancer, ileitis, or colitis. If the person is younger, it is less likely that cancer is the diagnosis.

It is not hard to see that a problem name may have many different solu-

tions. Pain in the lower right side of the abdomen is often a symptom of ap- pendicitis, but not always. The  initial  symptoms of a problem may be different in different people. A patient with colon cancer may notice pain as an initial symptom, or blood in the stool, or weight loss. Our diagnostic skills become more refined as our knowledge and experience increase. Naming the problem is important because it helps to define the additional questions, the additional data that need to be accumulated, and the fund of knowledge we need to approach a problem thoroughly. A problem defined is a problem half solved.

With the lessons learned from Case 1, the class turns to a more difficult

diagnostic problem, a real one from my own experience (Patient 28, N.P., from chapter 8)

Case 2

In the evening, I receive a telephone  call from the husband of a patient, a 40-year-old woman. He tells me, “She’s talking and she’s not making any sense.” On the way to their home, I begin thinking about what might be wrong with her.

Together the students and I go through the same process.

1. What is the name of the problem? Here are some of their answers and my comments.

•  “Brain tumor.” Too precise, given the information provided. Though that may be the answer ultimately, the data do not warrant it just yet. The method of get- ting to the answer is important in order to be more consistently correct.

•  “Not making any sense.” Not a very technical answer, but not bad. At least it shows me where the student really is in his reasoning and where I need to start my teaching.

•  “Confusion.” Not  bad either.  Like “not making any sense,” it allows for a thoughtful differential diagnosis, so long as each name prods the student into asking, “What are the possible causes?”

•  “Dementia.” Dementia is a permanent state, and her illness just began. Delir- ium, a more temporary state, may be a more appropriate term. Each of the terms, delirium and dementia, has its own differential diagnosis.

•  “Change in mental status.” We settle on this name for it is broad enough to lead to a productive diagnostic process and it does not prematurely restrict the choices.

2. Where  in the body is the disease? The brain is the obvious answer. What organs, tissues, etc., are in this area? Nerve tissue, arteries, veins, blood. What does the brain need in order to function? Blood, oxygen, and glucose, for starters.

Then I introduce a nuance of the above question: What other organs or organ systems of the body might contribute to the patient’s problems? In what ways might they contribute? By now, the students feel authorized to think more originally, broadly, and creatively. They add other items to their list:

•  The circulatory system. The heart, if injured, may not be able to pump blood and glucose to the brain. Narrowed or completely blocked arteries may prevent adequate blood and glucose flow.

•  The respiratory system: In the presence of diseased lungs, the supply of oxygen may be limited and the ability to rid the body of carbon dioxide may be impaired.

•  The pancreas: The pancreas could be producing too much insulin, lowering the blood sugar level and altering brain function.

3. On the basis of your answers to the previous questions and your own current knowledge,  what diagnoses might you consider as you try to answer the question, “What is wrong with this patient?”

Again, we start by organs.

•  The brain: stroke, tumor, subdural hematoma (a blood clot usually following trauma), psychological issues.

•  The heart: myocardial infarction (heart attack), congestive heart failure.

•  The arteries: carotid artery stenosis (narrowing of the artery from cholesterol deposits).

•  The lungs: pneumonia, pulmonary embolus (a blood clot formed elsewhere and lodged in the lung).

•  The pancreas: insulinoma (a tumor that secretes too much insulin).

Then we enlarge the list as we go beyond the confines of organs and or- gan systems. What about drug-induced illness from prescribed drugs? If the patient has diabetes and takes insulin, maybe her blood sugar level is too low. If the patient takes a diuretic medication for heart failure or hyperten- sion, and especially if she has been sweating a lot, maybe the blood sodium concentration is sufficiently diminished to cause a change in mental status. What about illness from nonprescribed drugs—alcohol, marijuana, others? What about toxins? What else?

4. What questions would you ask that would help to clarify the nature of her ill- ness and make a diagnosis? Table 10.2 summarizes the process.

So that the students begin to understand not only the technical aspects of the clinical drama but also the human, psychosocial parts, I ask them, “What do you think this experience is like for her and her family? What do you think this experience is like for the physician?”

Studying the case this way presents many “teachable moments,” oppor- tunities for students to learn about what physicians do, how they do it in a reproducible fashion, the differential diagnosis of “change in mental sta- tus,” some of the  symptoms of heart  attack,  pneumonia,  stroke, and hypoglycemia, and what all of this is like for the patient, the family, and the doctor.

Table 10.2

Using the History as the First Step in the Differential Diagnosis of “Change in Mental Status” (in Case 2)


Naming problems and writing them down allows the physician to study the relationships of one to the others, ponder possible causes and effects, consider the validity of past diagnoses and the effectiveness of past treat-ment, and arrive at a solution to each new problem that makes complete use of all the information available. Beyond the fact that this exercise in precision, completeness, and evaluation tends to expose more information on each patient than we could otherwise get, a list of problems provides us with an extra dimension in problem solving, the clinical context.

For instance, examining the relationship between “abdominal pain” and “diabetes mellitus” stimulates the student to ask: “Does their coexis- tence in the same patient alter the way I look at each one of them? Does ap- pendicitis have different symptoms and manifestations in patients with diabetes? Are there special considerations in the differential diagnosis of abdominal pain present in the patient with diabetes? Are special therapeu- tic considerations necessary in caring for the patient with diabetes who has appendicitis? What is it like for a patient who has diabetes and then devel- ops appendicitis?”

Naming the problem, looking at the relationship between problems, taking advantage of each teachable moment, learning from experience, and identifying gaps in our knowledge come together in the problem-ori- ented system,1 a technique that is technologically simple and yet very, very sophisticated.

I use the following history, a fictitious case, to illustrate how the system works. I integrate into it a series of tasks and questions for the students, and I direct them to construct a problem list and then use the list to identify the issues, make clinical decisions about diagnosis and treatment, discover pos- sible relationships between the problems and their treatments, and teach themselves. I provide additional information, help them formulate the questions, facilitate the discussion, validate their techniques and conclu- sions, and provide a model for reasoning.

Case 3

S.M., age 73, is hospitalized because of diabetes, out of control, on 12/10/98. Diabetes mellitus was first diagnosed in 1965 when she rapidly lost weight.

Blood sugar at that time was 520, a very high concentration. Over the years she has been treated with insulin and currently self-administers NPH insulin 30 units each morning. (NPH insulin may have its maximum effect on the blood sugar level about eight hours after it has been administered, and so a common time for a per- son to have a period of hypoglycemia [low blood sugar or insulin reaction] after a morning injection is late in the afternoon.) Her admission this time is precipitated by nausea. On admission, she is dehydrated, and blood chemistry determinations confirm the presence of diabetic ketoacidosis (a complex disorder of body chemis- try, affecting the concentration of water, sugar, sodium, potassium, and products of metabolism). She is treated with extra insulin and intravenous fluids and by the time of discharge a week later is feeling well, and diabetes is well controlled on a 2,000-calorie diabetic diet and NPH insulin 35 units daily.

Cerebral arteriosclerosis was diagnosed in 1990 following a stroke that left her right arm and leg weak. She had no recurrence. Occasionally she has become con- fused.

She had a duodenal ulcer in 1990 at which time she complained of heartburn. X-rays of her stomach and duodenum confirmed the diagnosis. She has no current symptoms.

She has been depressed in the past and was hospitalized in 1992 for three months, during which time she received electroconvulsive therapy (shock therapy).

Twenty-four years ago, she developed hives after an injection of penicillin. She is a widow, lives alone, and rarely sees her two daughters.

The students follow these steps.

1. Construct a problem list, dating the onset of each problem as precisely as possible.

This is their initial list.

1. Diabetes mellitus, onset 1965

2. Nausea, onset 12/10/98

3. Dehydration, onset 12/10/98

4. Diabetic ketoacidosis, onset 12/10/98

We then talk about the different causes of nausea. In this case, I tell them that the nausea and the dehydration are part of the clinical picture of dia- betic ketoacidosis. We then revise and consolidate the first four problems into one:

1. Diabetes mellitus, onset 1965

A.   Diabetic ketoacidosis, onset 12/10/98

and complete the list as follows:

2. Cerebral arteriosclerosis, onset 1990

A.   Stroke (weakness, right arm and leg), onset 1990

3. Duodenal ulcer, onset 1990

4. Depression, onset 1993

5. Penicillin allergy (hives), onset 1974

Following discharge from the hospital on 12/17/98, she returns home. One week later, on 12/24/98, she is readmitted to the hospital because of nausea, and again her diabetes is found to be out of control. After three days, diabetes is again well controlled on NPH insulin 35 units a day and a 2,000-calorie diabetic diet. On

12/30/98, about 4 p.m., she becomes irritable and makes romantic advances to an orderly half her age. This is unusual behavior for her.

2. Name the new problem and add it to the problem list. Regardless  of the magnitude or duration of the problem, give it a name. Failing to identify and name problems, one is apt to miss diagnostic clues of crucial impor- tance. There are actually two new problems: (1) a repeat episode of dia- betic ketoacidosis, which is added to the list as a second event:

1. Diabetes mellitus, onset 1965

A.   Diabetic ketoacidosis, onset 12/10/98, 12/24/98

and (2) the episode occurring about 4 p.m. I ask, “What is this episode? How shall we name it?” The students come up with these more or less so- phisticated names: irritable, romantic advances, confusion, insulin reac- tion, and change in mental status. As in Case 2, we choose the name that allows the broadest inquiry, “change in mental status,” and add it to the list.

1. Diabetes mellitus, onset 1965

A.   Diabetic ketoacidosis, onset 12/10/98, 12/24/98

2. Cerebral arteriosclerosis, onset 1990

A.   Stroke (weak right arm and leg) 1990

3. Duodenal ulcer, onset 1990

4. Depression, onset 1993

5. Penicillin allergy (hives), onset 1974

6. Change in mental status, onset 4 p.m., 12/30/98

3. Address the possible diagnostic solutions to the new problem by scanning the problem list and asking: “Is the new problem related to any of the other problems? Is the new problem related to the treatment of any other problems?” The differ- ential diagnosis of “change in mental status” differs in this patient from Case 2 because the clinical context is different. Each element of this in- quiry is an opportunity to teach about each of the diseases and supplement the student’s fund of knowledge, using the problem list as a reference point. Table 10.3 summarizes the process.

4. Make a diagnosis. Collect additional information from the history, physical examination and laboratory data.

Table 10.3

Differential Diagnosis of “Change in Mental Status” (in Case 3)

5. Implement therapy. Especially consider what immediate therapeutic step should be taken. As indicated above, the urgent action is to treat the possible hypoglycemic reaction. Had her action, “making romantic ad- vances to an orderly half her age,” been dismissed as unimportant, the phy- sician and the nurse would have overlooked and left untreated the easily treated insulin reaction, a medical emergency.

She is discharged on 1/2/99 on a 2,000-calorie diabetic diet and NPH in- sulin 35 units daily. On 1/9/99 she is readmitted because of nausea, and again diabetes is out of control.

6. What is going on? This is now the third time that the patient has been hospitalized with a similar illness, diabetic ketoacidosis, following ade- quate control on a routine that involves the same diet and the same dose of insulin. Any problem that recurs frequently requires a special inquiry.

We revise the problem statement:

1. Diabetes mellitus, onset 1965

A.   Diabetic ketoacidosis onset 12/10/98, 12/24/98, 1/2/99 (a recurrent prob- lem)

When I ask the students, “What is going on?” I mean, “Why is this prob- lem recurring?”—another teachable moment. Unless we recognize that a recurrent problem requires a separate inquiry, we will fail to address impor- tant issues separate from the basic diagnosis. The students speculate and recognize that psychosocial matters often have real importance. She may not be following her diet because she is depressed and wants to die, because she is alone, lonely, and craves attention, or because she cannot afford the special foods. She may not be taking her insulin correctly because she can- not afford the medicine, the syringes, and the testing material. In addition, I point out that among the possible long-term effects of diabetes are visual problems and neuropathy (various disorders of the nervous system). Maybe she cannot see the insulin syringe because of the eye problems; maybe she no longer has the dexterity to manipulate the syringe because of the neu- ropathy.

Physicians learn in many ways: interactions  with colleagues and pa- tients,  reading, attending  lectures, seminars, and postgraduate courses. When we approach our work systematically, we are better able to learn from a primary source, our own experience, and move on more confidently to treatment and prognosis.

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