High blood pressure is diagnosed based on the results of a blood pressure test. The test yields two numbers: systolic and diastolic.
This is the first (or top) number. It represents the pressure inside the arteries when the heart contracts to pump blood—in other words, when the heart beats.
This is the second (or bottom) number. It represents the pressure inside the arteries when the heart is resting and refilling with blood—in other words, between heartbeats.
Blood pressure numbers are often written as systolic pressure/diastolic pressure; for example, 120/80. The unit of measurement for blood pressure is millimeters of mercury (mmHg).
Blood Pressure Categories in Adults
The National Heart, Lung, and Blood Institute divides blood pressure levels into several categories. Below are the numbers that define each of these categories in adults.
Systolic pressure less than 120 mmHg and Diastolic pressure less than 80 mmHg
Systolic pressure 120-139 mmHg or Diastolic pressure 80-89 mmHg
Stage 1 high blood pressure
Systolic pressure 140-159 mmHg or Diastolic pressure 90-99 mmHg
Stage 2 high blood pressure
Systolic pressure 160 mmHg and above or Diastolic pressure 100 mmHg and above
When systolic and diastolic pressures fall into different categories, the higher one is used. For example, a blood pressure reading of 165/85 is considered stage 2 high blood pressure.
Isolated systolic hypertension refers to high blood pressure in which only the systolic number is high. It occurs in about two-thirds of people over age 60 who have high blood pressure. This condition should be taken as seriously as high blood pressure in which both numbers are elevated, because it can cause just as much harm if left untreated.
Diagnosis in Adults With Diabetes or Kidney Disease
Having diabetes or chronic kidney disease along with high blood pressure compounds the risk of serious complications. Therefore, the threshold for diagnosing high blood pressure is lower for people with these conditions. In adults with diabetes or chronic kidney disease, a systolic pressure of 130 or above or a diastolic pressure of 80 or above is considered high.
Diagnosis in Children and Teens
Blood pressure is measured the same way in children and teens as it is in adults. However, the younger and smaller the child, the lower the numbers normally are. To diagnose high blood pressure, the blood pressure numbers for a particular child or teen are compared to average blood pressure readings for young people of the same age, gender, and height.