Symptom

Overview
A symptom is a medical sign indicating the nature of the disease. It is usually subjective, observed by the patient, and not measured.

Loose definition
A symptom may loosely be said to be a physical condition which indicates a particular illness or disorder.

An example of a symptom in this sense of the word would be a rash. However this is also known as a sign as explained in below.

Possible causes of a symptom
Some symptoms, such as nausea, occur in a wide range of disease processes, whereas other symptoms are fairly specific for a narrow range of illnesses. For example, a sudden loss of sight in one eye has a significantly smaller number of possible causes.

Misleading symptoms
Some symptoms can be misleading to the patient or the medical practitioner caring for them. For example, inflammation of the gallbladder often gives rise to pain in the right shoulder, which may understandably lead the patient to attribute the pain to a non-abdominal cause such as muscle strain, rather than the real cause.

Symptoms and diagnosis
The terms chief complaint, presenting symptom, or presenting complaint is used to describe the initial concern which brings a patient to a doctor. The symptom that leads to a diagnosis is called a cardinal symptom.

Symptom vs sign
A symptom can more simply be defined as any feature which is noticed by the patient. A sign is noticed by the doctor or others. It is not necessarily the nature of the sign or symptom which defines it, but who observes it.

The same feature may be noticed by both doctor and patient, and so is at once both a sign and a symptom. A sign or a symptom may be one, the other, or both, depending on the observer(s).

Some features, such as pain, can only be symptoms. A doctor cannot feel a patient's pain. Others can only be signs, such as a blood cell count measured by a doctor or a laboratory.

Engineering definition
In engineering, symptom may be used to refer to an undesired effect occurring in a system. To eliminate the effect, a root cause analysis is performed which traces the symptom to its cause and again through the cause's cause and so on until the subsystem is identified that can be changed to eliminate the symptom.