The diagnosis of abnormalities in arterial pressure may require serial measurement. Since arterial pressure varies throughout the day, measurements should be taken at the same time of day to ensure the readings taken are comparable. Suitable times are:

* immediately after awakening (before washing/dressing and taking breakfast/drink), while the body is still resting,
* immediately after finishing work.

It is sometimes difficult to meet these requirements at the doctor’s office; also, some patients become nervous when their arterial pressure is taken at the office, causing readings to increase (this phenomenon is called white coat hypertension). Taking blood pressure levels at home or work with a home blood pressure monitoring device may help determine a person’s true range of arterial pressure readings and avoid false readings from the white coat hypertension effect. Long term assessments may be made with an ambulatory blood pressure device that takes regular arterial pressure readings every half an hour throughout the course of a single day and night.

Aside from the white coat effect, arterial pressure readings outside of a clinical setting are usually slightly lower in the majority of people. The studies that looked into the risks from hypertension and the benefits of lowering the arterial pressure in affected patients were based on readings in a clinical environment.

Arterial pressure exceeding normal values is called arterial hypertension. In itself it is only an acute problem; see hypertensive emergency. But because of its long-term indirect effects (and also as an indicator of other problems) it is a serious worry to physicians diagnosing it.

All levels of arterial pressure put mechanical stress on the arterial walls. Higher pressures increase heart workload and progression of unhealthy tissue growth (atheroma) that develops within the walls of arteries. The higher the pressure, the more stress that is present and the more atheroma tend to progress and the heart muscle tends to thicken, enlarge and become weaker over time.

Persistent hypertension is one of the risk factors for strokes, heart attacks, heart failure, arterial aneurysms, and is the leading cause of chronic renal failure. Even moderate elevation of arterial pressure leads to shortened life expectancy. At severely high pressures, mean arterial pressures 50% or more above average, a person can expect to live no more than a few years unless appropriately treated.

In the past, most attention was paid to diastolic pressure; but nowadays it is recognised that both high systolic pressure and high pulse pressure (the numerical difference between systolic and diastolic pressures) are also risk factors. In some cases, it appears that a decrease in excessive diastolic pressure can actually increase risk, due probably to the increased difference between systolic and diastolic pressures (see the article on pulse pressure).