Regarding white coat hypertension, the shot isn’t always necessary. Some people’s blood pressure just goes up from the stress of the clinical setting.
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Blood pressure is highly variable even moment by moment. A single data point at your yearly visit is basically meaningless. Even your posture can shift it. Legs in a different position? Too much tension in one arm over the other? Seated awkward? Have gas? Unfortunately, it's also very difficult to get a 24 hour monitor unless you have another suspected condition (such as kidney problems).