A sign has the potential to be objectively observed by someone other than the patient, whereas a symptom does not. There is a correlation between this difference and the difference between the medical history and the physical examination . Symptoms belong only to the history, whereas signs can often belong to both. Clinical signs such as rash and muscle tremors are objectively observable both by the patient and by anyone else. Some signs belong only to the physical examination, because it takes medical expertise to uncover them. (For example, laboratory signs such as hypocalcaemia or neutropenia require blood tests to find.) A sign observed by the patient last week but now gone (such as a resolved rash) was a sign, but it belongs to the medical history, not the physical examination, because the physician cannot independently verify it today.
Polycythemia (elevated red blood cell count) is a rare blood disease in which the body produces too many red blood cells. Causes of polycythemia are either primary (acquired or genetic mutations) or secondary (diseases, conditions, high altitude). Examples of primary polycythemia include:
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