Ambulatory Blood Pressure Monitoring: The Gold Standard for Diagnosing Hypertension

Diagnosing Hypertension in Primary Care

Hypertension is the most common condition seen in adult primary care practices. To help identify this medical condition, the U.S. Preventative Services Task Force (USPSTF) issued a Grade A recommendation stating that out-of-office measurements via ambulatory blood pressure monitor or home blood pressure monitor should be obtained prior to a hypertension diagnosis.¹ As a result, ambulatory blood pressure monitoring is now considered to be the reference standard for measuring blood pressure versus in-office and home measurements.

Limitations of Measuring Blood Pressure in Clinical Settings

There are several limitations when it comes to measuring blood pressure in clinical settings. Unfortunately, even with an automatic blood pressure device, poor technique is common. Errors can be introduced by:²

• Using the wrong cuff size
• Placing the cuff over the patient’s clothing
• Improper patient position (e.g., feet not on the floor, arm not at heart level)
• Failure to allow five minutes of rest
• Patient conversation
• Full bladder
• Other observer biases

When using a manual blood pressure device, additional errors can be introduced. Potential errors when using a manual sphygmomanometer include:²

• Letting air out of the cuff too rapidly
• Terminal digit bias (i.e., rounding to nearest 5 or 10 mmHg)

Even when performed correctly, office blood pressure measurement has limited reliability due to the small number of readings recorded.² In addition, office blood pressure measurement has substantial variability that can come from physiologic and external factors causing measurements to fluctuate.³ For example, clinical blood pressure can be influenced by white-coat hypertension which may result in a misdiagnosis.⁴

As a result, the typical practice is to base a hypertension diagnosis off repeated measurements at the same visit or over different visits.3 Unfortunately, repeat measurement protocols are rarely followed.³ Therefore, two main out-of-office measurement methods have been identified to avoid basing diagnosis off limited office measurements:⁴

1. Ambulatory blood pressure monitoring
2. Home blood pressure monitoring

What is Ambulatory Blood Pressure Monitoring?

Ambulatory blood pressure monitoring requires the patient to wear a monitor for a 24-hour period. A trained nurse or medical assistant fits the monitor on the patient and provides basic instructions:

• Do not remove the cuff
• Avoid strenuous activity
• Try to relax your arm when the device is taking a reading

The monitor is programmed to take a blood pressure reading automatically at desired intervals (e.g., every 30 minutes throughout the 24-hour period). If a patient’s measurements are greater than or equal to the following cut points, they can be diagnosed as having elevated ambulatory blood pressure:⁵

Although ambulatory blood pressure monitoring is the reference standard, there are still some difficulties with this method:

• Lack of availability as there are few providers of this service and it requires clinical staff training
• Recommended to be worn during a work day over a 24-hour period
• Limits some physical activity (e.g., swimming, running)
• Can be a nuisance
• The patient may have to repeat the session due to low-quality data or an insufficient number of readings

Ambulatory Blood Pressure Monitoring Research

A great deal of evidence has accumulated in the past decade regarding ambulatory blood pressure monitoring (ABPM). Results demonstrate ABPM is superior to office blood pressure in predicting cardiovascular outcomes.⁴ As a result, ABPM is now considered to be the noninvasive gold standard.⁴

In 2011, the British Medical Journal (BMJ) published an important systematic review. This review explored the relative effectiveness of clinic and home blood pressure monitoring compared with ABPM regarding the diagnosis of hypertension. The authors found seven studies that compared clinic measurements and three studies that compared home measurements to ABPM. The results concluded neither clinic nor home blood pressure monitoring were adequate as a single diagnostic test.⁶

The Lancet cost-effectiveness study provided additional research that supports ABPM. This paper was a modeling study that examined the cost-effectiveness of options for diagnosing hypertension in primary care. The Markov model was used to simulate a hypothetical primary-care population of 40 years and older with a screening BP >140/90 mmHg and risk-factor prevalence equivalent to the general population. The authors found ABPM is cost-saving for all groups and resulted in more quality-adjusted life years for men and women older than 50 years of age.⁷

Because of the BMJ systematic review and the Lancet modeling cost-effectiveness study, the National Institute for Health and Care Excellence (NICE) of the United Kingdom released new guidelines in 2011. These guidelines declared that “if clinic blood pressure is 140/90 mmHg or higher, use ABPM to confirm the diagnosis of hypertension.”

Recommendation by the U.S. Preventative Services Task Force

In 2015, the U.S. Preventative Services Task Force (USPSTF) published an important systematic review. Within this review, the author discussed the diagnostic and predictive accuracy of different blood pressure methods for cardiovascular events. The systematic review identified nine studies that evaluated the predictive value of 24-hour ABPM on long-term health outcomes. These studies met the following requirements:

• Followed a cohort of patients over time and reported the hazard or risk ratio
• Had to have at least two blood pressure measurement methods at baseline
• Outcomes had to be overall mortality or CVD events

Out of the identified studies, four found that every 10 mmHg increase in ambulatory systolic blood pressure was significantly associated with an increased risk for fatal and nonfatal stroke.8 Additionally, six studies found that every 10 mmHg increase in ambulatory systolic blood pressure was associated with increased risk for fatal and nonfatal cardiovascular events, with hazard ratios ranging from 1.11 to 1.42.⁸ The systematic review concluded that “…ambulatory blood pressure monitoring consistently and statistically significantly predicted stroke and other cardiovascular outcomes independently of office blood pressure monitoring.”⁸

Based on the systematic review, the USPSTF recommended that in adults aged 18 years or older, measurements should be obtained outside the clinical setting for diagnostic evaluation before starting treatment. The USPSTF stated that “…ambulatory blood pressure monitoring is the best method for diagnosing hypertension…the USPSTF recommends ABPM as the reference standard for confirming the diagnosis of hypertension.”

What is Home Blood Pressure Monitoring?

Home blood pressure monitoring allows blood pressure to be measured and recorded throughout the day in the patient’s natural environment. Home blood pressure monitoring is another method used to determine the misclassification of hypertension in the clinic.9 Home blood pressure averages correlate reasonably well with daytime ambulatory averages and therefore is a better predictor of cardiovascular outcomes than office blood pressure.¹⁰ This method may be more feasible than ABPM due to its affordability and availability.¹⁰

Evidence has demonstrated home monitoring is a stronger predictor of hypertension than office blood pressure monitoring.⁹ In addition, this evidence also shows that home monitoring is not as strong compared to ambulatory monitoring.⁹ As a result, home blood pressure monitoring is useful for ruling in hypertension but not as useful for ruling out the condition.⁹

Although home blood pressure monitoring is preferred over office blood pressure monitoring, there are still some difficulties with this method:

• Relies on proper techniques specifically by the patient
• Measurements are dependent on patient effort and engagement
• Trustworthiness and quality of data
• Data gaps (i.e., readings may not be recorded during sleep)

Home Blood Pressure Monitoring Research

The Annals of Internal Medicine included a systematic review of home blood pressure monitoring. This study provided data that showed the association between home blood pressure monitoring and the prediction of cardiovascular events or mortality.

Notably, there are fewer studies that link home blood pressure monitoring to cardiovascular outcomes or mortality when compared to ABPM. Due to the lack of evidence linking home blood pressure monitoring to cardiovascular outcome and mortality, the USPSTF only recommends home blood pressure monitoring when ABPM is not available.

Key Takeaway

Ambulatory blood pressure monitoring is superior to office blood pressure in predicting cardiovascular events. This method is now recommended by the USPSTF to rule out white-coat hypertension prior to diagnosis. Additionally, the USPSTF recommends initial therapy for those with extremely elevated clinic blood pressure or organ damage.

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