In medicine, there are knee jerk responses to working up what illness a patient may have. Indeed, a lot of medicine is practiced from the gut so to speak so these quick decisions can be invaluable in piecing together a picture as complicated as a sick patient.  In the end, these decisions come down to nothing less than educated guess work. Such clinical intuition comes from experience that takes years of dedicated study and practice to hone. An able and experienced clinician can form ideas quickly and decide how to proceed with a systematic workup and come up with an effective treatment plan. Unfortunately, many clinicians based these knee jerk responses on a flawed thought process called availability heuristics.

Availability heuristics describes a process that comes about by the ease of which ideas come to mind. This entails in medicine thinking about diagnoses based upon how easily they pop up in a clinician’s mind and not by what makes medical sense. People are inherently uncomfortable with uncertainty and there is a lot of uncertainty in medicine. The availability heuristic lessens the sense of uncertainty and makes clinicians feel more secure; it makes the uncomfortable feel more comfortable.  Clinicians may feel like they are right and there is a lot to be said about something feeling right but unfortunately not in this case.

Availability heuristics is a type of cognitive bias. Implicit in this bias is that it is flawed and the diagnoses construed under such constraints are misleading. A second problem is that once these ideas (diagnoses) are in mind, they are pursued and defended to the exclusion of contrary evidence supporting alternative diagnoses including the right one. This can result in pursuing wrong diagnostic and treatment pathways. Tests and procedures may be done and treatment started for diagnoses that the patient may not have thereby needlessly exposing a patient to whatever risks and complications these procedures and treatments may entail. This may also result in delays in establishing and treating the actual diagnosis. Availability heuristics is hard wired and widely entrenched in medicine and it is a real challenge for clinicians to not fall into it.

Pulmonary embolism is blood clot in the lungs. It is a serious and life threatening disease if identified in a timely manner is readily treatable. This is a common disease and it most often starts with blood clots forming in the veins of the legs that subsequently break off and travel to lodge in the arteries within the lungs. In its worse form, it can cause sudden death. The risk factors for this include common disease states and immobility associated with illness and hospitalization. Getting this right can be life-saving. So, this is hard wired to be in the top of any clinician’s mind to look out for in any patient showing even the slightest symptoms of this serious and life threatening disease. This is also probably the top availability heuristic that health practitioners fall prey to.

The diagnosis of this is pretty straight forward in simple cases. A young women with few or no medical problems who is on birth control pills (a known risk factor for blood clots) shows up with a racing heart and shortness of breath has a relatively high likelihood of having a pulmonary embolism. The knee jerk to think of this diagnosis is appropriate. The testing is also relatively straight forward. A simple blood test called a D-dimer can be ordered. This test is different than most other laboratory medical tests in that its utility rests on, not on whether it is positive, but on it being negative. A negative D-dimer test virtually excludes the diagnosis of a pulmonary embolism. If the test is positive then a confirmatory test such as a spiral CT scan of the chest can easily clinch the diagnosis. It is safe to say that in our hypothetical case of the young woman on birth control pills with shortness of breath, a negative D-dimer should have the clinician look for other reasons for these symptoms other than blood clots in the lungs.  This scenario with the diagnosis easily and quickly coming to mind but making sense medically does not fall into the availability heuristic.

It gets really challenging with cases that are not so simple. The following is a plausible scenario. An elderly man with a history of congestive heart failure presents with shortness of breath and a racing heart. He also may have obesity, high blood pressure, diabetes, kidney disease and have swelling in both of his legs and all this in addition to a sedentary life style.  A knee jerk thought based on the known history of heart failure should be an acute flare up of this underlying chronic illness. Assuming a decent history has been done, a focused physical exam would produce findings that go along with heart failure. A third heart sound may be heard, small crackly sounds called crackles may be heard when listening to his lungs. His neck veins may be sticking out or distended and his blood pressure may be high. A chest x-ray may show a partial whiting out of the normally black lungs as they should appear normally on x-rays. All of these findings support his shortness of breath is from an acute exacerbation of congestive heart failure. Within minutes, he receives an injection of a diuretic which is part of the definitive treatment, makes a large quantity of urine and starts feeling better. Other medications used in treating heart failure that when given to patients like this even when they are really gasping for breath, can drastically turn them around in a matter of minutes. The key here is that the initial thought of a congestive heart failure exacerbation that is strongly supported by the history and physical examinations, is acted upon by initiating appropriate treatment and is further supported by the patient responding to treatment; that is, he starts feeling a whole lot better.

However, the patient may have instead of, or in addition to congestive heart failure, a pulmonary embolism. He certainly has multiple risk factors for this disorder so it’s reasonable to also consider this possibility. That he would have both disorders is unlikely and a whole other subject matter (Okham’s Razor). Anyway, a D-dimer is obtained and is elevated reinforcing the evidence for a blood clot in the lungs and we are off and running with the availability heuristic. The clinician concerned about this gives a blood thinner to initiate treatment just in case he does have a pulmonary embolism and orders a spiral CT scan. The patient may still be found to have all of those findings supporting congestive heart failure and may even get a diuretic injection but still gets the blood thinner and the CT scan ordered. By the time he is going to the scanner for the spiral CT, he is feeling much better. He gets the CT and to everyone’s relief it shows no clots in the lungs. After he is admitted to the hospital perhaps also reassured that he has no blood clots in the lungs, he develops a massive bleed from his GI tract. Remember he received the blood thinner just in case he had a clot in his lungs and a major complication of blood thinners is, well, bleeding. His breathing worsens with the strain on his heart from bleeding; he is transferred to the ICU, placed on a ventilator, receives several blood transfusions to replace the blood loss, and undergoes an upper GI endoscopy in which a previously unknown duodenal ulcer is found to be the source of the bleed. The following day after he is doing better from all this, it is noted his kidneys are failing. The intravenous contrast given for the spiral CT scan is toxic to the kidneys. The fact that he has high blood pressure, diabetes and is in congestive heart failure places him at especially high risk for this complication. So, you can see where this went.

The D-dimer is only useful if it is negative. The clinician acting under this flawed thought process interprets the positive D-Dimer as supporting the diagnosis of a pulmonary embolism. Because so many other things beside a blood clot can make it positive, it is not useful when it is positive. This test is sensitive for clots but not specific. It just so happens that sick patients like the hypothetical one above will be guaranteed to have a positive D-dimer and because of its nonspecific nature, it is essentially useless. This really begs the question as to why in such instances it is even ordered. The next issue is that the test being positive inappropriately reinforces the cognitive bias of the pulmonary embolism and then forces the issue of treating and testing for it. The clinician is worried and starting treatment for this is reassuring. But treating and testing have their own sets of risks and complications as the above scenario illustrates.  The last point is an especially important one and it is that the patient was feeling better being treated for an acute exacerbation of congestive heart failure. The clinician disregarded this crucial point because of the focus on pulmonary embolism to the exclusion of findings supporting the actual diagnosis.  Again, most of medicine is guess work. Clinicians see patients and guess what is wrong and then act on those guesses. Could the above patient have had a blood clot? Absolutely, but it is more unlikely that he does not. If treating upon a diagnostic hunch results in the patient improving than this, in itself, provides strong support that the diagnosis suspected is the right one.

The alternative happy ending scenario is that the clinician’s knee jerk thought acting on the heart failure hunch gives the diuretic along with the other things to treat this and waits to see if the patient improves. The patient does and the blood thinner is never ordered and the CT scan with the intravenous dye toxic to the kidneys is never done. The patient is admitted to the hospital and treated for his heart failure, first by using diuretics and the slowly adding other drugs that ease the strain on his heart. Over the next few days, his breathing becomes comfortable, the swelling in his legs has gone down, his blood pressure is well controlled, and he is feeling so much better. Could the patient still have had a pulmonary embolism? Absolutely, but it is exceedingly unlikely. If he had a clot in his lungs causing his severe symptoms he would not have responded to treating heart failure. Because he did the idea of the clot in the lung becomes a non-issue. He is better.

This cognitive bias or availability heuristic happens frequently. It happens in small community hospitals and at even the most prestigious and revered medical centers in the country. The patients are varied along with the suspected diagnoses but the results are the same: missing correct diagnoses at the expensive of inappropriate testing and procedures with all their attendant risks and complications and delays both in establishing the real diagnosis and in the institution of definitive treatment. This can end up disastrously. How do we as health care providers avoid this? Realizing this cognitive bias and its ensnaring traps is the first step in avoiding it. The rest is just good medical practice which takes years of study and practice to master.