Sine qua non

Last revised by Daniel J Bell on 30 Aug 2019

"Sine qua non" is a phrase used in radiology, and more widely in clinical medicine, to refer to a symptom, sign, radiology finding, etc., which is absolutely necessary for a diagnosis to be made. 

For example, if one is querying a thoracic aortic dissection then the presence of a visible dissection flap is a sine qua non for the diagnosis to be true, i.e. the presence of a flap is absolutely necessary to make the diagnosis.

Caution is required, however because:

  1. as any experienced clinician knows diseases can rarely present in a previously unreported way. Therefore a sine qua non today, might not be one tomorrow.
  2. absence of a finding on imaging does not always equate to absence of the finding in reality

The pronunciation of the word 'sine' is SI-NAY; in classical Latin all letters are pronounced, i.e. the 'e' is not silent.

History and etymology

"Sine qua non" is a Latin term which literally means "without which not". 

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