| Experienced clinicians begin the process of making a | | | | The physical examination provides another source of |
| diagnosis upon first laying eyes on a patient, and | | | | facts to distinguish among still-viable possibilities. If my |
| probability is one of the main tools they use in this | | | | patient has migraine or medication-overuse headaches, |
| process. A glimpse "behind the scenes" from the point | | | | she might have tender muscles in her scalp and neck |
| of view of a diagnosing physician might help to explain | | | | but should not have a blind spot in her visual fields, |
| an otherwise mysterious process. | | | | slurring of her speech or clumsiness on just one side |
| The diagnostic process can begin even before laying | | | | of her body. These findings, if present, would cause |
| eyes on the patient. As an exercise (and to prove a | | | | the probabilities of migraine and medication overuse |
| point) I ask medical students who are with me in the | | | | headaches to be revised downward. By contrast, the |
| office to diagnose the patient we haven't seen yet | | | | probability of a brain disease - like a tumor, for |
| who is still in the waiting room. Of course, they look at | | | | example - that started with a low anchor probability |
| me like I'm crazy. But I tell them that we already know | | | | would get revised upwards. |
| a lot about the patient and can make some educated | | | | If a blood test or a scan is ordered, it is again with the |
| guesses. For example, we might already know that | | | | idea that the test has been individualized to discriminate |
| the patient is a 34-year-old woman referred by a | | | | between competing diagnoses and re-adjust their |
| family doctor because of headaches. | | | | relative probabilities. |
| So what have other women in their thirties referred to | | | | There is an important principal in medical diagnosis |
| me for headaches ended up having as their diagnosis? | | | | called Bayes' theorem. In a nutshell, Bayes' theorem |
| In my neurology practice, as well as in those of most | | | | states that the probability of a diagnosis after a new |
| other headache specialists, about a third (33%) have | | | | fact is added depends on what its probability was |
| migraine, another third have medication-overuse | | | | before the new fact was added. Another way of |
| headaches (in which the treatment has become the | | | | saying this is that the same "yes" answer on |
| problem instead of its solution), and the remaining third | | | | history-gathering, reflex result on physical exam or |
| fall into an "everything else" category that includes | | | | dark spot on an MRI scan has different implications in |
| tension-type headaches, arthritis of the neck or | | | | different people. The meaning of each depends on its |
| jaw-joints, sinus disease, tumors, etc. So before seeing | | | | context. Yet another implication of Bayes' theorem is |
| the patient I'm already able to identify the two most | | | | that one can't skip past the history and examination by |
| likely diagnoses and assign an initial probability for each. | | | | ordering a test in isolation and expect it to make an |
| These starting-point likelihoods are called "anchor" | | | | accurate diagnosis. A test is an answer to a question. |
| probabilities. During the subsequent history, examination | | | | If there was no question, how could the test be an |
| and supplemental testing (if necessary) the anchor | | | | answer? |
| probabilities will undergo a series of upward and | | | | Let's say that at a particular point in time we have |
| downward adjustments according to what the patient | | | | completed the diagnostic process for a patient. Then |
| has to say and what does or does not turn up on her | | | | what? By the end of the diagnostic process the |
| physical examination and testing. The physician | | | | doctor might have a diagnosis that is nearly 100% likely, |
| individualizes the questions asked and items examined | | | | but in other cases, the working diagnosis (number one |
| so that the outcome of each query forces one | | | | choice) might still be just 70% or 80% probable, with a |
| diagnosis to be more likely and another to be less likely. | | | | number two choice less likely, but still on the radar |
| Thus, diagnosis is a dynamic and sequential process. | | | | screen. It might make some patients uncomfortable to |
| We invite the woman into the examining room and | | | | realize that the diagnostic process does not lead to |
| listen to her story. In the headache example given, one | | | | 100% certainty in every case, but a doctor wouldn't be |
| key piece of data is how many days per month she | | | | doing a patient any favors by pushing the analysis past |
| takes an as-needed medication - for example, aspirin, | | | | the outcome that the available information leads to. |
| acetaminophen or a prescription drug. If she takes | | | | When a diagnosis is not 100% likely at the time of initial |
| as-needed medicine more days than not and has been | | | | evaluation, the patient's course of symptoms over time |
| doing so for a matter of months, then the initial 33% | | | | provides yet another form of data that can lead to |
| anchor probability of medication-overuse headaches | | | | revision of diagnostic probabilities. Fortunately, in cases |
| gets adjusted upward and the initial anchor probability | | | | involving uncertainty, even just narrowing down the list |
| of uncomplicated migraine moves downward. This, of | | | | of diagnoses to a small number of concrete |
| course, is just a single distinguishing feature, and cannot | | | | alternatives allows the doctor and patient to discuss |
| be relied upon to tell the whole story. The physician | | | | reasonable options and make sensible choices. |
| gathers many such data points to refine the diagnosis. | | | | |