Definition of Uncontrolled Blood Pressure Based on Number of Measurements
Uncontrolled blood pressure is not defined by counting individual "spikes" in a 24-hour period, but rather by the average blood pressure values obtained from multiple measurements over time. The guidelines focus on mean values from ambulatory or home monitoring, not the frequency of elevated readings.
How Uncontrolled BP is Actually Defined
For 24-Hour Ambulatory Blood Pressure Monitoring (ABPM)
Uncontrolled hypertension is diagnosed when the 24-hour average BP is ≥130/80 mmHg, daytime average is ≥135/85 mmHg, or nighttime average is ≥120/70 mmHg 1. The diagnosis requires:
- A minimum of 70% useable BP recordings (typically ≥27 measurements over 24 hours) 1
- Preferably seven nocturnal readings 1
- Emerging data suggest ≥8 wake readings and ≥4 sleep readings may be adequate if more cannot be obtained 1
The key point is that you calculate the mean of all valid readings, not count how many individual readings exceed a threshold 1.
For Home Blood Pressure Monitoring (HBPM)
Uncontrolled hypertension is defined as an average HBPM ≥135/85 mmHg 1. The proper protocol requires:
- Two readings per session, 1-2 minutes apart 1
- Twice daily (morning and evening) for a minimum of 3 days, ideally 7 days 1
- Discard the first day's readings and average all remaining measurements 1
- This yields a minimum of 12 readings for clinical decision-making 1
Again, the diagnosis is based on the average of all readings, not the number of elevated individual measurements 1.
Why Individual "Spikes" Are Not Used
Blood pressure fluctuates continuously throughout the day due to neural, mechanical, and humoral factors 1. Patterns of BP rather than isolated measurements are important in confirming the diagnosis 1. The guidelines explicitly state that:
- Prior to using mean ABPM values, raw BP values at each measurement should be reviewed for possible outlier or erroneous values 1
- Individual spikes may represent measurement artifacts, activities, or normal physiologic variation 1
Clinical Implications
Masked Uncontrolled Hypertension
Among patients with treated and controlled clinic BP (<140/90 mmHg), 31.1% have masked uncontrolled hypertension based on 24-hour ABPM averages 2. This condition is most often due to poor control of nocturnal BP, with elevated nighttime readings accounting for twice as many cases as daytime elevation alone (24.3% vs 12.9%) 2.
Office BP Thresholds
The office BP threshold of 130/80 mmHg better predicts uncontrolled out-of-office BP than 140/90 mmHg in apparent treatment-resistant hypertension 3. Among subjects with office BP <140/90 mmHg, 66% had uncontrolled out-of-office BP, while among those with office BP <130/80 mmHg, 55% had uncontrolled out-of-office BP 3.
Common Pitfalls to Avoid
- Do not diagnose uncontrolled hypertension based on a single elevated reading or a few "spikes" 1
- Do not rely solely on clinic BP measurements, as they may miss masked uncontrolled hypertension, particularly nocturnal hypertension 2
- Ensure adequate number of valid readings before calculating averages—at least 70% of expected values for ABPM 1
- Review individual readings for outliers before accepting the calculated mean 1