Hypertension is diagnosed when your blood pressure consistently reads at or above 130/80 mmHg in the United States, or at or above 140/90 mmHg under European guidelines. A single high reading isn’t enough. Diagnosis requires multiple readings taken on separate occasions, or confirmation through monitoring at home or with a wearable device over several days.
Blood Pressure Categories
US guidelines, set by the American Heart Association and American College of Cardiology, define four categories based on the two numbers in a blood pressure reading. The top number (systolic) reflects pressure when your heart beats. The bottom number (diastolic) reflects pressure between beats.
- Normal: below 120/80 mmHg
- Elevated: systolic 120 to 129 with diastolic below 80
- Stage 1 hypertension: 130 to 139 systolic, or 80 to 89 diastolic
- Stage 2 hypertension: 140/90 mmHg or higher
European guidelines use a higher cutoff. The 2024 European Society of Cardiology guidelines define hypertension as 140/90 mmHg or above, with a broad “elevated” category covering readings from 120/70 up to that threshold. This means someone with a reading of 135/85 would be diagnosed with Stage 1 hypertension in the US but classified as having elevated blood pressure in Europe. The distinction matters if you’re comparing information from different health systems.
How a Reading Is Taken Correctly
The way your blood pressure is measured affects the result more than most people realize. A poorly positioned arm or a wrong-sized cuff can throw off a reading by 10 to 20 points, enough to change a diagnosis. The CDC recommends sitting in a comfortable chair with your back supported for at least five minutes before any measurement. Both feet should be flat on the floor, legs uncrossed, and the arm with the cuff resting on a table at chest height. You shouldn’t talk during the reading.
Cuff size is another common source of error. A cuff that’s too small will give a falsely high reading, and one that’s too large will read low. The American Medical Association provides specific sizing based on arm circumference: a small adult cuff fits arms of 22 to 26 cm, a standard adult cuff fits 27 to 34 cm, a large adult cuff fits 35 to 44 cm, and an extra-large cuff fits 45 to 52 cm. If your arm is on the larger side and you’ve been measured with a standard cuff, your readings may have been artificially inflated.
Why One Reading Isn’t Enough
Blood pressure fluctuates throughout the day based on stress, caffeine, physical activity, and even the anxiety of being in a doctor’s office. That’s why a diagnosis is never made from a single visit. Your doctor will typically take at least two readings at each appointment and want to see elevated numbers on at least two separate occasions before confirming hypertension.
Two specific patterns make this confirmation step essential. White coat hypertension is when your blood pressure reads high at the clinic but is normal at home, a pattern that affects roughly 1 in 5 people with elevated office readings. The opposite, masked hypertension, is when your numbers look fine in the clinic but run high the rest of the time. In one study of people with borderline or Stage 1 readings, about 21% of those who appeared to have normal blood pressure in the office actually had masked hypertension. Both patterns can only be caught by measuring blood pressure outside the clinic.
Home Blood Pressure Monitoring
Home monitoring is one of the most reliable ways to confirm a diagnosis. The recommended protocol calls for taking two readings in the morning (after waking, before any medications) and two in the evening (after dinner, before bed), for a total of four readings per day. Ideally you do this for seven consecutive days, though a minimum of three days can be sufficient. Your doctor will average the results, discarding the first day’s readings since those tend to be unreliable as you get used to the process.
A validated upper-arm monitor is the standard tool. Wrist monitors are less accurate. When reviewing your home averages, readings of 135/85 mmHg or higher during waking hours are considered hypertensive, which is slightly higher than the office threshold because home readings tend to run a few points lower than clinic readings.
Ambulatory Blood Pressure Monitoring
Ambulatory monitoring (often called ABPM) involves wearing a small cuff on your arm for a full 24 hours while going about your normal life. The device inflates automatically every 15 to 30 minutes during the day and every 30 to 60 minutes at night, building a complete picture of your blood pressure across an entire day-night cycle.
The international consensus thresholds for diagnosing hypertension through ABPM are a 24-hour average of 130/80 mmHg or higher, a daytime average of 135/85 mmHg or higher, or a nighttime average of 120/70 mmHg or higher. This method is particularly useful for detecting masked hypertension, identifying people whose blood pressure doesn’t drop at night (a pattern linked to higher cardiovascular risk), and resolving uncertainty when office and home readings disagree.
Lab Tests After Diagnosis
Once hypertension is confirmed, your doctor will order a set of baseline tests. These aren’t used to diagnose the high blood pressure itself but to check whether it has already affected your organs and to assess your overall cardiovascular risk. The standard workup includes blood tests for kidney function, blood sugar, cholesterol levels, and potassium. A urine test checks for protein, which can signal early kidney damage. Most people will also get an electrocardiogram (ECG) to look for signs that the heart has been working harder than normal.
These results help your doctor determine how aggressively to treat your blood pressure. Someone with Stage 1 hypertension and normal labs may start with lifestyle changes alone, while the same reading in someone with signs of kidney stress or diabetes would typically prompt medication right away.
When Doctors Look for an Underlying Cause
About 5 to 10% of people with hypertension have a specific, identifiable cause, known as secondary hypertension. Most people don’t need to be screened for this, but certain red flags prompt a deeper investigation:
- Age of onset under 40
- Sudden onset or sudden worsening after blood pressure was previously well controlled
- Resistant hypertension, meaning blood pressure stays above 140/90 despite taking three different medications including a water pill
- Low potassium on blood work, which can point to excess aldosterone production
- Symptoms like episodes of headache, sweating, and rapid heartbeat, which may suggest a rare adrenal gland tumor
- Snoring and daytime sleepiness, which suggest obstructive sleep apnea, one of the most common reversible causes
Certain medications and substances can also drive blood pressure up. Anti-inflammatory painkillers, decongestants, some antidepressants, oral contraceptives, and stimulants like cocaine and amphetamines are all known contributors. Your doctor will review your medication and supplement list as part of the diagnostic process. If a secondary cause is found and treated, blood pressure often improves substantially or normalizes entirely.