The diagnostic process

A complex task

Differential diagnosis

Generating a differential diagnosis — that is, developing a list of the possible conditions that might produce a patient's symptoms and signs — is an important part of clinical reasoning. It enables appropriate testing to rule out possibilities and confirm a final diagnosis.

The list might be prioritized by likelihood and urgency.

Courts view the formulation and documentation of a differential diagnosis as evidence of a physician's competence, prudence, and thoughtfulness.

Case: A 58-year-old man with back pain
Close up of hand rubbing lower back (suggesting back pain)


An obese Caucasian male, with a history of hypertension and smoking, complains of severe lower back pain that has lasted four days.

The back pain is accompanied by occasional vomiting and radiates intermittently to both lower quadrants of the abdomen. The increased severity of back pain had awoken him on the morning he sees his doctor.

Vital signs are normal except for a mild elevation of the systolic blood pressure. Dr. A assesses the patient at 0500 hours and finds no significant physical abnormalities. Femoral pulses are strong and symmetrical. A flat plate X-ray of the abdomen is read and later confirmed as normal. A complete blood count (CBC) is normal.

Background continued

The preliminary diagnosis by Dr. A is musculoskeletal back pain. Narcotic analgesics are administered.  

At shift change the patient's care is transferred to Dr. B, who reviews the patient and agrees with the previous diagnostic impression of mechanical back pain.

Dr. B subsequently discharges the patient with a prescription for analgesics and the instruction to find a family doctor for follow-up care.


Two days later, the patient is found dead at home.

An autopsy reveals a ruptured abdominal aortic aneurysm (AAA) with 3,000 cc of blood in the retroperitoneal space.

The patient's family threatens a legal action against Dr. B, alleging failure to diagnose the condition and failure to provide adequate discharge instructions.

Think about it

What can we learn from this case?

Lessons learned

  • Leaking AAA may mimic renal colic, mechanical back pain, and diverticulitis/gastroenteritis.
  • In particular, AAA might be considered in the differential diagnosis of an older patient with symptoms suggestive of renal colic.
  • Severe radiating pain is a common symptom. Syncope and vomiting may also be associated with AAA.
  • A patient with persistent symptoms may warrant a new evaluation. As appropriate, alternative diagnoses including the "worst case" possibility should be considered.

Lessons learned continued

  • Patients with pain require analgesia. Even if appropriate doses of narcotics control the patient's pain, it may still be prudent to review the patient to determine if the diagnosis is being masked by the analgesia. When appropriate doses of narcotics fail to control pain, the patient's diagnosis should be reassessed.
  • In appropriate clinical circumstances, the medical record should indicate that the diagnosis with the worst prognosis, in this case AAA, was considered and was pursued if reasonable to do so.
  • The rationale for not investigating should also be clearly documented.