Letter 2: THE ACTION THRESHOLD CONCEPT 

This time let's look at the diagnosis tool called the Action Threshold (AT) concept. AT is a point in medical decision-making that can help physicians decide when to treat, observe, or do a laboratory test or procedure.

How "AT" Works

Imagine a horizontal transparent cylindrical bar 10 inches long. It is calibrated from 1 to 10 with one-inch intervals. Inside this Action Threshold bar is a small red marble that can be moved from left to right and back. If the red marble is positioned at 5, it is read as 0.5 AT.

If the likelihood of an illness is to the right of the AT and it is not a serious illness, treatment can be started without any tests. If the likelihood of an illness is to the left of the AT or less than 0.5, two options are done: observe the patient or do a test to figure out what the illness is.

For purposes of a simple illustration, let's put the AT at 0.5.

Suppose that after the clinician takes the history and physical examination of the patient, he decides that the probability that the patient has a particular disease is at the right of 0.5 AT. If this disease is not generally considered serious like asthma, then he can justify starting treatment without any testing or work-up.

However, if the history and physical examination of the patient puts the probability of the likely condition to the left of the AT or below 0.5, then treatment is not justified and he has to do some testing to ascertain the condition of the patient or observe the patient without any tests but should come back if the symptoms persist or get worse.

A specific example would be a 2-year-old boy whose weight is only 18 pounds, but doesn't show any symptoms. Offhand, this is "failure to thrive." The probability of arriving at a diagnosis during the first visit is only 0.3 which is to the left of the AT of 0.5.

Suppose the patient is a feverish 6-year-old boy with petechiae at the legs and an enlarged spleen. With this combination of findings, the clinician will be considering a more serious illness and, therefore, needs to have the imaginary red marble at the 0.99 mark of the AT bar. This means that the clinician has to resort to more definitive diagnosis before starting treatment because the harm that the treatment can bring is considerable.

AT is a property of any treatment or recommendation for any condition or illness. AT is the likelihood of disease or condition above which -- or to the right of the 0.5 mark -- any treatment or recommendation, on average, provides more improvement than harm, and below which -- or to the left of the 0.5 mark -- will cause more harm than improvement.

"AT" In Sample Cases

When dealing with cancer, where the treatment like chemotherapy has significant serious side effects, the physician should be close to 100% certain of the diagnosis before treatment is started. In this instance, the AT should be near 1.0.

However, in cases like pneumonia, where the treatment itself does not pose great danger, the AT could be lower than 0.5. This would mean that a definite test, like chest X-ray, doesn’t need to be done since the treatment -- giving antibiotics -- is generally a safe bet.

In these two instances, the physician’s reasoning might go like this: “Since the risks in chemotherapy are high, I would not want to expose a healthy patient to this risky treatment. If the patient, indeed, has cancer, she would risk getting the serious side effects, but at least she would have an overall benefit from the chemotherapy. Therefore, because of the treatment's risks to a healthy individual, I need a high certainty on the diagnosis before undergoing treatment.

“In a case like pneumonia, suppose I am wrong with the diagnosis and I fail to treat, and it turns out to be pneumonia, serious complications can occur. If I treat a patient who actually does not have pneumonia, the antibiotic that I give will generally have no serious side effects. All things considered, treatment would be the best action to take, in which case I really do not need to bring up my AT near 0.9.”

The action threshold conditions in these two instances reflect how confident the physician is about the diagnosis before recommending treatment. The confidence level needed to be high in the cancer situation, and moderate in the pneumonia case.

In the daily practice of a clinician, making a diagnosis is “playing the odds.” Some patients are treated with medication or surgery for a particular illness that they do not actually have, while some patients with an illness are not treated at all. This happens because of the inherent qualities of the patient and the variety of manifestations of an illness. 

A common example is appendicitis. Some children with severe belly pain at the right lower side are sometimes operated on for acute appendicitis; when actually, the appendix is normal. On the other hand, some who are not operated on actually have appendicitis about to rupture a few days later, causing complications.

Many children are treated for recurrent bronchitis or pneumonia, when in fact their basic problem is asthma. Since the treatment of bronchitis or pneumonia is relatively safe, clinicians quickly prescribe it. 

Another example is frequent and prolonged colds or a viral infection. Many children are diagnosed to suffer from frequent or prolonged colds, when actually they have allergies to begin with.

In summary: 

If the likelihood of an illness is low, or to the left of the AT (Action Threshold), the clinician should not treat in most instances.

If the likelihood of an illness is high, or to the right of AT, the clinician is betting that the treatment is reasonable even without a test if the target illness is not serious.

If a test or imaging could change a clinician’s diagnostic confidence from either side of the AT, it has high diagnostic value, and therefore, must be requested.

If a test or imaging would not change a clinician’s diagnostic confidence at all, then it should not be requested or even discussed.

Sample Cases From Personal Experience

Last month, a third year medical student and I saw a 6-year-old boy with poor appetite, fever of 101, and fine scattered rashes at the trunk. The boy's throat was red with petechiae (blood spots) at the soft palate.

I explained to the medical student that the pre-test estimate of probability that the boy has scarlatina is high because the typical physical finding is consistent with strep throat.  I also knew that many children in the community are being treated for strep infection. Without doing a strep test I treated the boy with amoxicillin. In this instance, my pre-test estimate of probability of strep throat infection about 0.8 which is to the right of the 0.5 Action Threshold.

Early in the afternoon, we saw a 10-year-old girl with sore throat, no fever, and a history of exposure to someone with strep. However, the examination of the throat was not impressive – red throat, no exudates (white spot), and no petechiae. 

In this situation the pre-test estimate of probability of strep is to the left of the 0.5 AT. In other words, I was not convinced that the girl had a high chance of strep infection. So I did a strep screen test to see if there will be strep in her throat. I decided not to treat because my pre-test estimate of probability of strep is to the left of the action threshold or below 0.5. The girl did not have enough physical findings pointing to strep. The only thing in favor of strep is a history of exposure to the bug and red throat.

End of Letter 2 of 6