Help your doctor formulate an accurate differential diagnosis

Last night, I turned on “House,” a TV show about a physician who is in charge of a team of highly specialized diagnosticians.  Every week the team solves a difficult to diagnose medical case by working through an exhaustive differential (DD) diagnosis list.  While the show is absurd to the extreme (as the team of physicians almost kills the patient each week), eventually the correct diagnosis is made and the patient recovers.

The differential diagnosis (DD) is one of the cornerstones of modern medicine.

The DD is a list of possible causes of an illness/disorder.  It is formulated based on probabilities and ranked according to the severity of the illnesses on your list.  From the time you walk into your physician’s office, the physician begins building a DD.  He begins collecting data the minute he walks into the exam room.  The patient’s demeanor, dress, movements, speech, skin pallor and posture are just a few of the things a physician observes, even before he begins to interview the patient.  During the interview (history taking), a DD begins to take shape.  The DD is refined during the exam.  Often, laboratory and x-ray results help to further illuminate the DD.  Once formulated, the DD serves as the basis for a treatment plan.

Someone once said, “When you hear hoof beats, think horses, not zebras.”  During my training, my residency director would often criticize me by remarking that I was always looking for zebras in a herd of horses.  I would always respond by pointing out what a pity it was that he would never see a zebra.  From my perspective, a gifted diagnostician keeps an open mind and a fluid differential diagnosis.

Case in point:

A 15 year old has a sore throat, fever and enlarged cervical glands.  Her strep test is positive.  She obviously has a strep throat and should be treated with amoxicillin.  Three days after she starts on her antibiotic, she breaks out in a horrendous rash.  Now her diagnosis is acute allergic reaction.  The doctor treats her rash and places her on a different antibiotic.  She gets worse.  Why?

The diagnosis was obvious but only partially correct.  The differential diagnosis of a sore throat with swollen glands and a positive strep test is extensive.  If your physician anchors his diagnosis to the positive strep test, he misses the diagnosis of mononucleosis.  The diagnosis of the rash seems obvious.  The patient was on amoxicillin and must have developed an allergic reaction.   Again, if mononucleosis (mono) is in your DD, amoxicillin is contraindicated.  Ninety nine percent of patients with mono will develop a rash when given amoxicillin.  If mononucleosis is included in the DD, another antibiotic is prescribed.  The patient does not get falsely branded as allergic to amoxicillin.

Helping patients understand the process and complexity of formulating an accurate differential diagnosis is important for a multitude of reasons.  In today’s stressful financial times, patients look for cost saving short cuts, including avoiding tests and follow up visits.

As stated above, tests help refine the DD and follow up visits help verify the DD or help modify it as new information is obtained.  Patients often stay at home believing they know what is wrong based on their own DD, founded on their life’s experience and Google.  Unfortunately, the stakes are high and too many lose.  Some die.  Even worse, patients leave the doctor’s office with a diagnosis and a treatment, only to get worst.  Because the doctor told them what they have and what to do, they stay at home waiting to get better rather than following up when they worsen.

There is an important take home message!  Your doctor’s diagnosis is always provisional.  You may well have what your doctor told you.  You may also have something else, something further down the DD list or maybe not even on it.  It is critical that the patient, like the doctor, keep a fluid list of possibilities.  It is also important that the patient take an active role in helping the physician formulate an accurate differential diagnosis.  Please, don’t hesitate to add your two cents.  It is one place where two cents is still valuable.

Stewart Segal is a family physician who blogs at

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  • Carolyn Thomas

    Excellent example of the perilous journey towards arriving at the DD, Dr. Segal. This reinforces Dr. Jerome Groopman’s “18 Second Rule” in his book “How Doctors Think” – it’s the average amount of time a doctor will listen to a patient’s complaint before interrupting with his/her first stab at the diagnosis.

    For women heart patients, the journey from first seeking help for distressing symptoms to accurate diagnosis/treatment is particularly perilous. Research on cardiac misdiagnoses reported in the New England Journal of Medicine, for example, looked at more than 10,000 patients (48% women) who went to their hospital Emergency Departments with heart attack symptoms. Investigators found that women younger than 55 were SEVEN TIMES more likely to be misdiagnosed and sent home than men of the same age. The consequences of this were enormous: being sent away from the hospital doubled the chances of dying.

    Many, like I myself was, are sent home from the E.R. in mid-heart attack with misdiagnoses ranging from GERD to anxiety or gall bladder problems or menopause (a great all-purpose diagnsosis!) As you correctly say, because a man with the letters M.D. after his name told me very clearly “It is NOT your heart!”, I decided to “stay at home waiting to get better. rather than following up when they worsen.”

    Add to that the confounding tendency of many women to endure increasingly debilitating cardiac symptoms far longer than their male counterparts do (could this be because the men have wives who are insisting that hubby call 911 as soon as they need help?) – and it is small wonder that more women than men die from heart disease each year.

    More on this at “Heart Attack Misdiagnosis In Women” at:

  • pyritedan

    Always interesting the choices we make. In the first place, tender or non tender nodes? Exposure or not exposed to strep or mono.
    This case demonstrates the fallacy of the strep test. Not often it is incorrect, and therefore misleading. A mono test based on the history, probably would not have converted to positive yet.
    The fault lies in the choice of antibiotic. To slavishly follow guidelines once again is the villian. Treat with cephlexion instead of ampicillion, no rash with mono, and you will probably handle the strep, if the patient really has a strep over growth of the mono.
    Get ready for more of this kind of problem, as we get locked into cook book medicine.

  • Joyce Graff

    Excellent! Thank you. Some of my best friends are “zebras”.

    Pheochromocytomas are the worst. Most doctors were told in medical school that pheos are so rare they would never see one. So how does anyone with a pheo ever get a diagnosis? We are working with a patient now who can’t get a doctor to write the order for the test to determine whether his raging pheo symptoms are in fact a pheo. He may need to fly to NIH to get the blood test.

  • BobBapaso

    Unfortunately, insurance companies want “A Diagnosis” and are not supportive of testing for alternatives. Very unfortunately the psychiatric DSM is not supportive of thinking about more than one simplistic formulation.

  • Campykid

    To Joyce (Comment #3), I encountered the opposite situation from your patient: after complaining of chronic palpitations, SOB and dizziness, I was tested for a pheochromocytoma about 11 years ago (very young pysician looking for an uber-zebra). Since it wasn’t a pheo, her DD then shifted to generalized anxiety disorder. Talk about a provisional diagnosis! Nine years later, a Holter monitor finally revealed the symptoms were due to atrial fibrillation/flutter. As I result, I “heartily” endorse the efforts of Carolyn Thomas (Comment #1) to educate physicians and the general public about women’s cardiovascular issues.

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