The differential diagnosis is the initial set of possible diagnoses a doctor must make before settling on a series of tests or a definitive diagnosis.

In the differential, all diseases that cause the symptoms seen in the patient are considered. They are then eliminated by:

  • Considering which of them would have symptoms not seen in the patient that would be present if that particular disease were present;
  • Considering which of the remaining diagnoses is the most likely; and
  • Considering which of the less likely diagnoses might be the most serious to the patient's immediate health.

For example, the patient comes in saying "I have a pain in my leg". Some of the possible diagnoses are:

  • Traumatic blunt force injury
  • Tendonitis
  • Normal compliction of pregnancy
  • Drug seeking behavior
  • Venomous snake bite
  • Infected dog bite
  • Infarction and necrosis

The first four are common, the last three far less so. However, the less common diagnoses can result in almost immediate death, while the more common ones are far less serious.

Once a differential is established, the doctor must eliminate the diagnoses that are incorrect, usually by ordering tests to rule out more symptoms. Less invasive tests are usually ordered first unless the differential points to a diagnosis that is very likely, but can only be confirmed by an invasive test.

The physician may also try treatment to make the differential. However, this can be risky as the wrong treatment can often be worse than no treatment at all. Antibiotics, for example, are usually a safe choice, but must be avoided in patients who have an allergy to them. A CT Scan can be very useful, but exposes the patient to a fair amount of radiation.

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