Letter 1: DECISION ANALYSIS AND DIFFERENTIAL DIAGNOSIS

Clinical Decision Analysis

The most common, often considered "standard," method of diagnosis practiced by most students, residents, and even experienced physicians, is the "random decision" or "intuitive thinking" method, which does not really follow any conscious methodology.

Clinical Decision Analysis or CDA, on the other hand, although not yet studied in a controlled trial showing positive outcome, is based on a step-by-step method of thinking that allows clinicians to advise their patients regarding the "best bet" or "best outcome" for any specific condition.

In the same manner that we should wear seat belts, install smoke detectors, recommend car seat for infants and young children, or tell parents to quit smoking, we should use clinical decision analysis in patient diagnosis. CDA keeps the interest of the patient always in the frontal lobes of the clinician.

The CDA method starts with treatment considerations such as harm and improvement, followed by testing and diagnosis with its likelihood ratio and pre-test estimation.

The reason for thinking of treatment first, then testing or diagnosis afterwards, is that decisions about tests can be properly made after the risks and benefits of a treatment have been critically dissected. With the knowledge and understanding of the risks and benefits, harm and improvement, the clinician and patient can discuss if a test is worthwhile doing.

The function of decision analysis, unlike scientific experiment that shows natural truth, is to help a decision maker choose the best option among many alternatives of treatment. Decision analysis does not reduce the uncertainty about the true nature of the patient’s illness, but rather it makes the choices more rational in light of uncertainty.

In the long run, if decision analysis is applied in most of the complicated patients, the probability of making a grave error is less compared to the usual "random" or “intuitive” method.

Differential Diagnosis

There are three components in making a diagnosis. These are Clinical History, Physical Examination, and Laboratory Tests and Imaging.

Among these three, Clinical History is the most important. Depending on the specialty of the physician, History contributes about 80-90% to the diagnosis. Except in Dermatology (Skin Problem), Physical Examination contributes only 10 to 15%.  In Dermatology, about 90% of the diagnosis is done by physical examination. Laboratory Tests and Imaging contributes only about 5 to 10% towards making a diagnosis.

A good Clinical History is dependent on a complete and practical Differential Diagnosis. Without a reasonable and complete differential diagnosis, it is difficult to ask good questions, both pertinent positive and negative ones.

The process is like this: Clinical History starts with a Chief Complaint (CC). The clinician then generates the pertinent questions that will reduce the number of considerations from the possible disease conditions in a complete Differential Diagnosis. The clinician designs the questions so that they rule in or out one or two disease conditions that is brought about the Chief Complaint.

For example: A three-year-old boy came in because of wheezing. With wheezing as the main symptom that prompted the parent to bring her child for examination, a clinician who does not have access to a computer or the Internet will rely on his memory and stock knowledge during the interview.

Most clinicians are capable of mentally listing only up to six or seven different causes of wheezing, like asthma, pneumonia, allergy, bronchitis, foreign body, cystic fibrosis, and bronchomalacia.

With this mental list, a clinician will ask questions such as: Is there a fever? How long has the wheezing been going on? Is your child coughing after running? How long does he cough? Is there a previous history of bronchitis or pneumonia? Is there chronic diarrhea? Is there family history of asthma or cystic fibrosis? When the coughing or wheezing started, did the child choke on a food or small toy? How is your child’s growth? 

Because the average "mental" differential diagnosis is limited to seven possible conditions, the clinician runs out of pertinent questions to ask.

Now, let's look at a clinician who has access to technology. Hearing "wheezing" as the chief complaint, he gets a handheld computer or logs on to the Internet, and types “wheezing” in a search box on Differential Diagnosis.

With just a few strokes on the keyboard, the modern clinician can easily bring up a listing of at least 35 causes of “wheezing” in children. 

From this electronic list, he can easily pinpoint the causes that are serious and more common in the community, or its epidemiology. With this list, he can easily generate pertinent questions.

In the case of the clinician who relies on brute mental recall power, there is a high probability that a not-so-common cause of wheezing, such as vascular ring or heart disease, will be missed during the first clinical encounter. The unfortunate result could be a serious diagnostic error if the right question is not asked.

The current “standard of practice” is 100% dependent on pure mental recall, which is prone to error because of the limitations of the human brain.

With the speedy advancement of technology, we should expect that within the next few years there would be a list of differential diagnosis that includes the pertinent questions to ask, as well as the diagnostic tests or imaging to order for any particular condition. This would be available online and linked to websites with the latest Evidence Based Medicine like Medline, Cohcrane Data Base, UpToDate, or Clinical Evidence. With the use of effective technology, diagnostic errors will certainly be reduced.

End of Letter 1 of 6.