In medical school I learned to record four vital signs to assess my patients: blood pressure, heart rate, respiratory rate, and temperature. Later, health regulators required the measurement of a “fifth vital sign” to determine pain. The patient was asked to rate their pain on a scale from 1 to 10. Once we knew the pain level, we treated the pain, often with opioids. This caused problems because pain is not a vital sign, pain is a symptom.
Pain cannot be objectively measured and any measurement is inherently unreliable. For example, a 91-year-old grandma may rate her pain a four out of 10, yet visibly be in severe pain.