The lost art of the differential diagnosis

It used to be the case that formulating a differential diagnosis (possible causes for what might be wrong with a patient) and implementing a plan of how the differential diagnosis was going to be investigated, was one of the key arts of being a doctor. Many people often make the mistake of thinking the role of a doctor is to write prescriptions but in fact the key role is to work our what is wrong with the patient and then set in motion a plan to make them better if possible.

Nowadays it is all too common to see only a single proposed diagnosis, or a reiteration of clinical signs (e.g. sepsis), without any alternative reasoning mentioned. This is potentially dangerous as it stops the continued evaluation and assessment of the patient. If the initial diagnosis is incorrect this single approach may actually lead to patient harm, a missed diagnosis and long term irreversible damage. An old saying in medicine was “the most dangerous patient was the one with the known diagnosis” as any new problem or symptoms this patient presented with would be assumed to be part of the existing problem and so would be missed!

An example of how this might go wrong would be a patient admitted with back pain and difficulty passing urine. Without a proper history and thorough examination this patient could easily be labelled as having pyelonephritis with kidney pain. This would lead to the commencement of antibiotics to treat of severe UTI and possibly a referral to urology for inpatient care. However the patient doesn’t respond to these initial antibiotics within 48 hours so the team calls a microbiologist saying “their patient with pyelonephritis is not responding, what should they change the antibiotic to?” Unless the microbiologist questions the diagnosis, another 48 hours might elapse before anyone questions what is happening. Does this sound familiar? 

The differential diagnosis for severe back pain may also include potentially life-threatening conditions such as a dissecting aneurysm and pancreatitis. Failing to consider  these when admitting a patient could be disastrous. And what if the patient had severe back pain due to an epidural abscess? The pain may well be lumbar (i.e. near the renal angle) and if there was compression of the spinal cord they may well have difficulty passing urine due to compromise of the nerve supply to the bladder sphincter muscle.

Investigating these differential diagnoses would initiate clinical examination, pulse and blood pressure in all 4 limbs, amylase blood test and possibly a CT scan of chest to eliminate dissecting aneurysm and pancreatitis. Examination of the patient reveals that the pain is worse on movement, especially lying down and on palpation of the spine itself. The patient was catheterised because they weren’t passing urine and this confirmed a large residual of urine in the  bladder, which tested as negative on dipstick and microscopy. Further questioning elicited the fact that the patient can’t open their bowels properly and a PR examination reveals a change in anal sphincter tone. Clearly this patient doesn’t have pyelonephritis! 

The fact that a differential diagnosis includes other potential sources ensures doctors are not blinkered into following one course. Like a game of Cluedo differential diagnosis is a process of elimination, or reducing the probability, of life-threatening, common and uncommon conditions systematically. As differential diagnoses are ruled out a quick adjustment shows the most likely cause, in this case a spinal epidural abscess and allows correct urgent management to be implemented. Spinal epidural abscess is a surgical emergency and when present for more than 24
hours, the neurological damage may be irreversible. In this case, if the team had waited till the first diagnosis, pyelonephritis, was no longer probable, it
would have lead to long term irreversible damage in the  patient.

My personal method for formulating a differential is to use the following list in order, and then to fit a system based approach afterwards:

1. Life-threatening conditions
2. Common conditions
3. Uncommon conditions

Then the systems respiratory, cardiovascular, neurological, genitourinary, gastrointestinal, skin, bone and joint can be examined. But remember this is a dynamic process so
as the story changes or as further results come back from investigating, the differential diagnosis can be modified until the final confirmed diagnosis is reached.

By using a differential diagnosis, patients are managed safely and more effectively and doctors become very good at diagnosing, which is what all good doctors do best.