Letter 3: PRE- AND POST-TEST ESTIMATES OF LIKELIHOOD

Pre-Test Estimate of Likelihood

It's important that a physician, using his training, experience, and the epidemiology of the illness being considered, forms a pre-test estimate of disease likelihood in taking care of a patient.

If the pre-test likelihood of an illness being considered is high, and the physician is highly confident, and the illness is not serious, and it is to the right of Action Threshold (AT), treatment can be started without any tests needed. This decision-making process follows the AT concept we discussed previously. As in the AT bar, if the pre-test likelihood is to the right of the 0.5 mid-point, treatment is reasonable. 

Most of the time, clinicians make a diagnosis with very little doubt. In the clinical decision analysis world, this is called "high pre-test estimate of likelihood of the index disease."

This is best illustrated when a clinician makes a diagnosis of chicken pox or Down’s syndrome. Within a few seconds of seeing the patient, even without asking a single question, the clinician can make a diagnosis for which he is highly confident of being right.

However, most patients don't come with classic or typical physical findings or a typical clinical history. With these patients, clinicians are forced to ask many questions that will help bring the likelihood of a target illness as high as possible.

So, how do clinicians come up with a pre-test estimate? Depending on their attitude and thinking style, they arrive at a pre-test estimate by relying on experience, knowledge, judgment, their willingness to ask their superiors or colleagues, and the availability of time to do research using the Internet.

Right now, there are no clinical studies pertaining to how most clinicians organize their thinking process in making a pre-test estimates of likelihood of a target disease (TD), but in general, after listening to the chief complaint and present illness, the clinician first thinks of a list of about three to seven differential diagnosis.

As he continues to ask more questions, he tries to eliminate a few of the differentials until he's left with one or two considerations. He thinks of how specific the findings are for the TD, how many independent findings are seen in the TD, and what risk factors the patient has. Almost at the same time, the clinician's mind recalls his past experiences with similar patients that give him an intuitive “feeling” for the TD's likelihood.

By knowing the community and the epidemiology of a target disease (TD), clinicians can move fast in making pre-test estimates of likelihood.

For example, in the spring of 2003, here in Bangor, Maine, we had a tremendous number of children with strep throat, with petechiae at the soft palate, and typical scarlatina rashes. During that time, I treated a lot of children without even doing “rapid strep” test. If there were no increased number of children with strep in our community, I would be doing many “rapid strep” tests before giving an antibiotic. But since my pre-test estimate of the TD was high, I deleted testing. Once I no longer see numerous children with typical strep throat findings, I will start doing “rapid strep” tests again.

Sometimes, asking what the patients think they have can raise the pre-test estimate of likelihood of a TD. In one study of patients reporting that they thought they had “sinusitis,” more than 30% actually had sinusitis as proven by X-ray and sinus culture. With specific findings of sinusitis, the diagnosis leaped to 50%. (Ann Intern Med. 1992; 117:705-10, Clinical Evaluation For sinusitis: Making the diagnosis by history and physical examination. Williams, JW, et al.).

Most of the time, with just two or three symptoms gathered from the present history, clinicians can make a good pre-test estimate of likelihood. Even parents like you can have a high confidence level on what your child has.

Take for example a 5-year-old girl with pain on urination. Many times a mother would call my office with a complaint that her child has “urinary infection.” Indeed, in most cases the mother is right. A child with pain on urination, with or without fever, has a good chance of having Urinary Tract Infection.

Here are other examples:

Children with cough longer than three weeks without fever, has asthma until proven otherwise, especially if the cough is aggravated by running, playing, or laughing. 

Runny nose longer than 14 days with coughing more prominent at night has a good chance of sinusitis. 

A 7-year-old boy with poor appetite, pain at the right lower side of the belly, and more belly pain on jumping in place, has acute appendicitis until proven otherwise. 

Headache that is on and off, the severity is not increasing, longer than three months, and if there is family history of migraine, should be considered as migraine also.

A teenager with sore throat, tiredness for many days, and big “glands” under the jaw has a high chance of infectious mononucleosis.

A child with frequent bronchitis or recurrent pneumonia, with chronic diarrhea, and poor weight gain has cystic fibrosis until proven otherwise.

Brain tumor should be considered in any child with a headache for at least two weeks if there is vomiting and changes in behavior or personality. If there is blurring or double vision, brain tumor should be in the top list of differential diagnosis.

These shortcuts that experienced physician use is called “heuristics.” Even without the benefit of laboratory tests, there is a high probability of being right after asking just a few questions during the clinical interview.

Post-Test Estimate of Likelihood

Post-test likelihood (PTL) is estimated after the results of laboratory tests or procedures are known. In most instances, the post-test likelihood of discovering an illness will exceed the AT point or be to the right of it.

If PTL remains to the left of AT, and the patient is not getting better, more tests are needed. If the patient is getting better or at least not getting worse, observation is still reasonable.

End Letter 3 of 6