From the WHO:

1. Use validated automated monitors or, if not available, calibrated aneroid devices.
2. Measure blood pressure in a quiet place.
3. Follow the protocol below:

• Don’t have a conversation. Talking or active listening adds up to 10 mmHg
• Support arm at heart level. Unsupported arm adds up to 10 mmHg
• Put the cuff on bare arm. Thick clothing adds up to 5-50 mmHg
• Use correct cuff size. Cuff too small adds up to 2-10 mmHg
• Support feet. Unsupported feet adds up to 6 mmHg
• Keep legs uncrossed. Crossed legs adds up to 2-8 mmHg
• Have empty bladder. Full bladder adds up to 10 mmHg
• Support back. Unsupported back adds to mmHg

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