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In medicine, a symptom is generally a subjective experience of a disease, in contrast to a sign, which is something that can be detected or measured. For example, stomach ache, lower-back pain, and fatigue are symptoms and can only be experienced (or sensed) and reported by the patient. A sign, on the other hand, might be something such as blood in the stool, a skin rash that can be recognised by the doctor, or a high body temperature. Sometimes, a sign may not be noticed by the patient, but is meaningful and helpful for the doctor for diagnosis. For example:

A rash – this could be a sign, symptom, or both:

  • If the patient notices the rash it is a symptom.
  • If the doctor, nurse or anyone else (but not the patient) notices the rash it is a sign.
  • If both the patient and doctor notice the rash it is both a sign and a symptom.

A light headache – this can only be a symptom:

  • A light headache can only be a symptom because it is only ever detected by the patient.
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