Why are the first day's readings discarded when using home blood pressure monitoring to confirm a diagnosis of hypertension in a 46-year-old patient with elevated office blood pressure readings?

Why First Day Readings Are Discarded in Home Blood Pressure Monitoring

The first day's readings should be discarded because they are systematically higher and less reliable than subsequent measurements, reducing the accuracy of blood pressure assessment and potentially leading to overdiagnosis of hypertension. 1

The Evidence Behind Discarding Day 1

Improved Correlation with Ambulatory Blood Pressure

• There is consensus that correlations with ambulatory blood pressure monitoring are more reliable when the first day's home BP readings are discarded. 1
• The American Heart Association, American Society of Hypertension, and Preventive Cardiovascular Nurses Association jointly recommend following European Society of Hypertension guidelines: take at least 2 morning and 2 evening readings daily for 1 week, but discard the first day's readings, yielding 12 total readings for clinical decisions. 1

The First Day Phenomenon

• Blood pressure measurements on the first day tend to be systematically higher than subsequent days, likely reflecting patient anxiety, unfamiliarity with the device, or improper technique during initial use. 2
• This "first measurement effect" can artificially elevate the average home BP and lead to false-positive diagnoses of hypertension or inappropriate treatment intensification. 2

Recommended Protocol

Standard Measurement Schedule

• Patients should measure BP twice daily (morning before medications and evening) for 7 consecutive days, with 2 readings at each session separated by 1 minute. 1
• After excluding the first day, this provides 12 readings (6 days × 2 sessions per day) on which to base clinical decisions. 1
• The diagnostic threshold for hypertension is ≥135/85 mmHg based on the average of these 12 readings. 3, 4

Minimum Requirements

• While 7 days is ideal, a minimum of 3-4 days (after excluding day 1) provides a reliable estimate of home BP, though with slightly increased variability. 2
• At least 4.5 consecutive days of monitoring are required for reliable diagnosis of home hypertension (κ-statistic 0.88). 2

Clinical Implications

Avoiding Diagnostic Errors

• Including first-day readings increases the risk of misclassifying patients as hypertensive when they have white coat hypertension or normal BP. 1
• Home BP monitoring already tends to be lower than office BP due to absence of the white coat effect; proper technique including discarding day 1 ensures the most accurate assessment. 1

Important Caveat

• Recent evidence suggests that even with proper training and adherence to guidelines (including discarding day 1), home BP monitoring may actually underestimate hypertension prevalence compared to daytime ambulatory BP monitoring, with fewer patients reaching the hypertension threshold via home monitoring. 5
• This underscores that while discarding day 1 improves reliability, home BP monitoring has inherent limitations and may miss some cases of true hypertension that would be detected by ambulatory monitoring. 5

Practical Application

• Patients should be instructed to record all readings they take, but clinicians should calculate the average using only days 2-7. 1
• Using devices with memory storage is encouraged to ensure accurate reporting and prevent patients from selectively reporting lower readings. 1