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Diagnostic Likelihood and Laboratory Tests
You 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 diagnosis within a few minutes of taking the clinical history. About 85% of the diagnosis is found in your story of the current complaint. Except in skin problems, only about 10% of the diagnosis is provided by the physical examination. In a primary care practice, laboratory tests and X-rays is needed only in about 5%.
It does not take a lot of brain power to figure out that a 5 year old girl with pain in urination and fever has a urinary tract infection. Most mothers suspect this diagnosis even before coming to physician. In this instance, the likelihood of arriving at a right diagnosis is about 0.9 or 90%. However, a two year girl with a weight of 18 pounds, well below the average of 24 pounds, will need an exerted 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 to 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 rule.
The "exhaustive or super-complete" history method is abandoned by medical students 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 "hypothetico-deductive" method. From the chief complaint and the first few minutes of the interview, your physicians creates a short list of possible illnesses maybe three or four different ones that are highly likely from his or her experience. A few more targeted questions after the physical examination usually reduce the diagnosis into one or two possibilities. And in a some instances, one or two laboratory tests will hit the diagnosis.
Suppose your child is coughing for two weeks without a fever. You visit your primary care physician. A few specific question 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 longer than 10 days? Has there been 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.
From the experience of your physician of seeing hundreds of children with similar symptoms and physical examination who got better with an inhaler for asthma, he or she unconsciously estimate the pre-test probability of 0.8 (80%) or more in favor asthma. Since the inhaler for asthma has very low downside or harm even though given for normal children, a physician can accept the probability of being wrong 0.2 (20%).
In this instance, this physician has exceeded the action threshold for treatment if his or her action threshold is peg at about 0.5 (50%). You might ask what is action threshold? Imagine a horizontal bar about 10 inches long with one inch marks interval. A triangular marker is on top of the bar that can slide from left to right. A physician with the help of a patient or parent can set it wherever they want from 1 to 10. If this threshold marker is set at 5 by conventional reading this will be read as 0.5 threshold. This triangular indicator can be used as a guide for treatment or doing laboratory tests. Depending on the seriousness of the illness being highly considered, the physician and the patient, can move the action threshold from 0.5 to 1.0. It is not reasonable to peg the action threshold to below 0.5.
If the pre-test estimate of the illness is greater than the action threshold it is reasonable to start treatment without tests unless one of the disease being considered is serious. If the pre-test estimate is lesser than or to the left of the action threshold, two options are considered: observe the patient without doing a test or do a diagnostic test. Treatment should be withheld because there is no diagnosis that is likely.
An example of this situation is a two year old girl whose weight is only 18 pounds. This girl is clearly not gaining weight. The medical label is Failure to Thrive and the causes of this condition are many. It is reasonable to estimate the pre-test likelihood is 0.2. It is to the left of the action threshold which is at 0.7 as determined by the physician and parents. In this case, laboratory tests are essential.
Please send me your comment if this article is easy to understand or if it makes sense at all. I am trying to figure out how to explain how Clinicians make decisions.
Leo Leonidas, MD, FAAP
lleonidas@pcpediatrics.org
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