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 Livewellthy.org.

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