Masked hypertension: normal readings in clinic but high blood pressure elsewhere

Blood pressure varies through the day and between settings. Most people are diagnosed with high blood pressure after repeated measurements. NICE guideline NG136 uses the term masked hypertension for the situation in which clinic and non-clinic blood pressure results are conflicting: the clinic reading looks acceptable but readings elsewhere, such as at home or during a 24-hour recording, are high.

It is the opposite of white-coat hypertension, which NICE Clinical Knowledge Summaries describe as blood pressure that is unusually raised during consultations with clinicians but normal when measured in other non-threatening situations. In masked hypertension the clinic reading under-states the problem, so a person can be told their blood pressure is fine when, for much of their day, it is not.

The sources read for this page do not explain why the pattern occurs in a particular person. The practical point is that a normal clinic reading does not always settle the question.

Because routine clinic checks can miss it, masked hypertension is usually suspected when readings elsewhere differ from clinic readings, or when there is evidence of damage that blood pressure may explain. Two forms of out-of-clinic monitoring are used.

The thresholds differ by setting. NICE says hypertension is confirmed in people with a clinic reading of 140/90 mmHg or higher and an ABPM daytime average or HBPM average of 135/85 mmHg or higher. The NHS says you are usually considered to have high blood pressure if your reading is 140/90 or higher when checked by a healthcare professional, or 135/85 or higher when checked at home. The British Heart Foundation says that if your blood pressure is over 135/85 mmHg when you measure it at home you may be diagnosed with high blood pressure.

NICE adds that the corresponding ABPM and HBPM measurements are 5 mmHg lower than clinic measurements. These figures are guidance ranges, and your clinician will interpret your readings alongside your age, other conditions and overall risk.

High blood pressure usually causes no symptoms, so people with masked hypertension generally feel well. The NHS says that if blood pressure is too high it puts extra strain on the blood vessels, heart and other organs such as the brain, kidneys and eyes, and untreated it can increase the risk of heart disease, heart attacks, strokes, heart failure, kidney disease and vascular dementia. The British Heart Foundation adds that high blood pressure can make arteries stiff or narrow.

The sources read for this page do not give separate risk figures for masked hypertension compared with other forms, so this page does not quote any. NICE recommends that when hypertension is suspected, investigations for target organ damage and a formal cardiovascular risk assessment are carried out while waiting for confirmation of the diagnosis.

If your clinic readings are normal but your home readings are repeatedly high, tell your GP or practice nurse and bring your record. The NHS says that many pharmacies offer free blood pressure checks for people aged 40 or over, and that if you are 40 or over and have not had a check for more than 5 years you should get one. In England, the BHF mentions a Blood Pressure @home scheme that supports home monitoring, which you can ask your GP about.

• Use a validated upper-arm monitor with a cuff of the right size, and sit quietly with your arm supported before measuring.

• Take readings at the same times each day, ideally morning and evening, and record all of them.

• Do not change or stop prescribed medicines because of home readings without speaking to your clinician.

• Seek urgent help if you have chest pain, which may be a heart attack: the NHS says to call 999 for sudden chest pain that does not go away or pain that spreads to the arm, neck, jaw, stomach or back.

Once hypertension is confirmed by ABPM or HBPM, management is the same as for other hypertension. NICE says to offer lifestyle advice, and the NHS lists a healthy balanced diet, at least 150 minutes of exercise a week, losing weight if overweight, cutting down on salt, keeping to no more than 14 alcohol units a week, limiting caffeine and not smoking. A GP will usually prescribe medicine if blood pressure is very high, if you are at higher risk of heart attack or stroke, or if it is still high after lifestyle changes.

NICE says people with hypertension who choose to self-monitor should be advised to use HBPM, with training and information on what to do if they are not achieving the target. For adults under 80 the clinic target is below 140/90 mmHg, with an ABPM or HBPM target of below 135/85 mmHg, according to NICE Clinical Knowledge Summaries.

On the NHS, blood pressure checks and ambulatory monitoring are available through GP surgeries and some pharmacies, and the NHS Health Check offers a blood pressure check in England for eligible adults. Private clinics and pharmacies also offer checks and 24-hour monitoring. Whichever route you use, take your results to your own GP so that they are recorded and can guide treatment.