Making a diagnosis is a process of managing uncertainty.
When we are ill, our body responds with “symptoms and signs”. A symptom is a tangible feeling such as pain. A sign is a detectable physical alteration. The body is wonderfully complex in function (biochemistry, pathology, embryology, genetics, for examples) but relatively limited in how it responds to disease with symptoms and signs.
This last statement is a crucial diagnostic point. No matter how intricate the underlying perturbations in our body’s functions, our responses to the perturbations may feel and look similar. For example, you may have a sign of an elevated temperature, fever, but that sign may have myriad causes; you may have a minor viral illness or, alternatively, a life threatening bacterial infection. The sign, by itself, may not point to a specific cause, just that a cause must be entertained.
Making a diagnosis, then, is the search for a specific cause among diseases that produce sometimes-similar symptoms or signs. (Again, a symptom is a sensation; a sign is a palpable or visual physical abnormality). Making a diagnosis is, in essence, a sorting out process. You must identify the certain cause from a list of potential other causes (uncertainty). The table above depicts the process.
The sorting out process starts with your description of symptoms and signs. Your account of the start and course, or progress of your symptoms/signs generates a preliminary diagnostic list, called the, “differential diagnosis”. The differential diagnosis list is an important step and I will show you how you can communicate your symptoms/signs so you get the best differential diagnosis list to begin your diagnostic search.
After the preliminary differential diagnosis list, diagnostic tests are used as tools for sorting out the true cause from the list. Tests make, or refute specific diagnoses based on the frequency, or percent chance of the test’s result occurring in each diagnosis on the differential diagnosis list (more later). In other words, making a diagnosis is a mathematical sorting out based on a test’s results. More than a single test may be needed (rarely; more later) and each test’s result alters not only the list of items, but the likelihood of each potential cause.
In the table (figure at beginning of this chapter), the differential diagnosis list is depicted as a set of possible diagnoses, A, B, and C. In truth, A, B, and C will not be equally likely. There is uncertainty, not only in terms of the potential diagnoses on the list, but, also, in the likelihood of each potential diagnosis on the list. These elements of uncertainty are managed by knowing how tests weed out the more likely from less likely potential causes.
In summary, it is the natural state of affairs that uncertainty exists when making a diagnosis, and tests are used to reduce the uncertainty. Often, the steps forward from a longer list of potential causes to the correct singular cause are, “one-step forward, two-steps back”. In fact, diagnostic decision-making is often a path of first proposing and considering diagnoses that ultimately are shown not to be present.
The uncertainty in the process of making a diagnosis is indisputable and requires us to accept that no diagnostic search will be or can be perfect, or uniform. The diagnostic search will be better, however, after you learn how diagnoses are made and then become a willing participant in making your diagnostic decisions.
