Proper diagnosis of hypertension requires a thorough medical history, a physical examination, and laboratory tests. Blood pressure levels determined by a doctor to be lower than 120 mm Hg systolic and 80 mm Hg diastolic should be rechecked within two years. Pressures between 120 and 139 mm Hg systolic or 80 and 89 mm Hg diastolic should be rechecked within one year. Hypertension is diagnosed when the average blood pressure reading is 140/90 mm Hg or higher on at least two separate doctor visits.
When blood pressure levels are consistently 140/90 mm Hg or above, the next step is to determine whether the hypertension is primary or secondary. Although secondary hypertension is uncommon, secondary causes of high blood pressure should always be considered, since they are correctable in many cases and their identification may spare the patient antihypertensive medication—at least in the short term.
Historically, doctors focused on diastolic blood pressure for the diagnosis and treatment of hypertension. But today the focus is on both systolic and diastolic blood pressure, since systolic pressure is an important determinant of hypertension complications, particularly in people older than age 50.
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Hypertension is discovered most often during a routine visit to the doctor. The instrument used to evaluate blood pressure in a doctor's office is called a sphygmomanometer and typically consists of an inflating bulb, an inflatable cuff, and a mercury column gauge.
Blood pressure is measured by wrapping the cuff around the upper arm and determining how much pressure is needed to compress the brachial artery—the major artery in the arm. The amount of pressure needed is equivalent to the height of the mercury in the gauge. Thus, blood pressure is expressed in millimeters of mercury, or mm Hg.
Because of concerns about mercury contamination of the environment, the Environmental Protection Agency is encouraging doctors to switch to aneroid or electronic blood pressure devices that use dial or digital gauges, respectively, to indicate blood pressure levels. Some experts are uneasy about these devices, but when used properly they can be as accurate as mercury sphygmomanometers.
Regardless of the type of device used to measure blood pressure, the following steps will help ensure accurate results:
Ambulatory Blood Pressure Monitoring. Ambulatory blood pressure monitors automatically measure and record blood pressure over a 24- to 48-hour period. Such measurements may be useful in the diagnosis of white coat hypertension. There is also some evidence that ambulatory monitoring may be helpful in identifying people with drug-resistant hypertension, hypotension caused by blood pressure medication, episodic hypertension, or borderline hypertension (systolic blood pressure 130 to 139 mm Hg or diastolic pressure 85 to 89 mm Hg).
In ambulatory monitoring, an inflatable cuff is worn around the arm and connected to a blood pressure monitor about the size of a Walkman. At predetermined times—typically every 15 to 30 minutes during the day and every 30 to 60 minutes during the night—the cuff inflates automatically and takes blood pressure readings that are stored in the monitor and later interpreted by a doctor.
Precise diagnosis of a secondary cause of hypertension—such as a kidney disorder, sleep apnea, or an adrenal tumor—usually requires special laboratory tests and procedures. Because these tests are expensive and inconvenient, they are not performed on everyone. Instead, they are done only when a thorough medical history and physical examination—or the results of routine laboratory tests—raise a strong suspicion of a secondary cause of hypertension.