Letter 6: DIAGNOSTIC METHODS AND TRAPS TO AVOID

Methods of Diagnosis

We go to a physician for diagnosis and treatment. Between the two, diagnosis is more important because it leads to treatment. In most situations, physicians arrive at a provisional diagnosis within a few minutes of taking the clinical history.

About 85% of the diagnosis is found in the story of the patient's current complaint.  Except for skin problems, probably about 90% of the diagnosis is provided by physical examination. In a primary care practice, laboratory tests and X-rays are needed only in about 5% of the cases.

For example, it doesn't require a lot of brainpower to figure out that a 5-year-old girl experiencing pain in urination and fever has a urinary tract infection. Most mothers suspect this diagnosis even before seeing a physician. In this instance, the likelihood of arriving at a right diagnosis is about 0.9 or 90%.

However, a 2-year-old girl weighing 18 pounds, way below the 24-pound average for her age, will need extra effort on part of the physician to diagnose. This condition is medically labeled as "Failure To Thrive." The likelihood of getting the right diagnosis before laboratory tests are done is probably only about 0.2 or 20%. 

You might ask, "How do physicians arrive at a diagnosis?"

Most physicians use the "hypothetico-deductive" process. Some use "pattern diagnosis" especially in the diagnosis of skin or genetic disorder. Medical students use the "exhaustive" or "super-complete" history in getting into the differential diagnosis.

The easiest method of diagnosis is pattern diagnosis done by most skin specialist and geneticists. An example of this is Down’s Syndrome (Trisomy 21) and eczema in children. Even without the benefit of clinical history a child with Down’s syndrome and eczema can be diagnosed by just "one look." In pattern diagnosis, experience frequently rules.

Usually, medical students abandon the "exhaustive" or "super-complete" history method once they get into the residency or training program of the specialty of their choice.

By far, the leading method of getting a diagnosis is done by the "hypothetico-deductive" method. From the chief complaint and the first few minutes of the interview, the physician creates a short list of possible illnesses, maybe three or four different ones, that are highly likely based on his or her experience.

A few more targeted questions after the physical examination usually reduces the diagnosis into one or two possibilities. And in some instances, one or two laboratory tests will zero in on the diagnosis.  

Suppose your child is coughing for two weeks without a fever. You visit your primary care physician, who asks a few specific questions such as: Does your child cough after running? Does he cough more at night? Is there difficulty of breathing or wheezing after prolonged playing? Is there runny nose for longer than 10 days? Has the child had bronchitis or pneumonia before? Is there asthma in the family?

If most of your answers are "yes" to these questions, and the physical examination is normal, your physician will most likely give your child an inhaler for asthma.

Based on your physician's experience with hundreds of children who came in with similar symptoms and got better with an inhaler for asthma, he subconsciously estimates the pre-test probability of 0.8 (80%) or more in favor of asthma. Since the inhaler for asthma has very low downside or harm, even if administered to normal children, a physician can accept the probability of being wrong by 0.2 (20%).

Traps to Avoid in Making a Diagnosis

To make you aware of how clinicians can sometimes be wrong with their diagnosis, let me share a couple of case examples.

A clinician makes a diagnosis of a rare disease called Moya-moya in a 25-year-old woman with headaches and weakness of the right arm. The next time the clinician sees a patient with almost similar history and physical findings, there is a good chance he'll remember the patient with Moya-moya and consider it again.  But since it is a rare condition, his chance of being wrong is high in the second patient. This mistake is called “availability heuristic.”

Dr. Smith saw a 42-year-old man who is a gardener, and who had dental work done about four weeks before. He is in great health without any known risk factors for heart disease. Dr. Smith saw several splinter hemorrhages under the nails of both hands and a small ecchymosis on the sole of the left foot that reminded him of Janeway lesion. He heard a grade 2 pulmonic ejection murmur. Because of these findings Dr. Smith made a pre-test estimate of likelihood of Subacute Bacterial Endocarditis at about 3%. He ordered blood cultures.

Dr. Smith was misled by the typical or representative findings of splinter hemorrhages and ecchymosis in SBE. In fact, those findings are also found in trauma from gardening. If Dr. Smith looked at his reference book, the incidence of SBE is about 2 in 100,000 and it is almost always associated with fever (95%), anorexia (98%), and frequently in patients with risk factors. This error in diagnosis is called “representative heuristic.”

Being off by about 20-50% is often acceptable, but being off by a factor of 100 to 1000, as in the examples above, should be avoided. It is a product of flawed reasoning and failure to consider epidemiology.

Leonardo Leonidas, MD, FAAP

Assistant Clinical Professor in Pediatrics (Retired 2008)

Tufts University School of Medicine

Boston

End of CDM Letters of 6