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There are different types of blood pressure monitors based on how the blood pressure is obtained. The most common types are auscultatory and oscillometric method. Both methods rely on an inflatable cuff and measure the signal as the cuff signal is decreased. In the auscultatory method, sounds known as Korotkoff sounds can be heard in the stethoscope. The sound appears at the systole and disappears at the diastole. This is the most accurate non-invasive method of measuring blood pressure and often use a gold standard that other blood pressure monitors have to compare with. The oscillometric method uses pressure sensors to capture the oscillometric pulses and then compute the results. There are many algorithms available and each manufacturer has its own algorithms. By nature of the technology, the results are not as accurate.
There are several international standards that certify a oscillometric blood pressure monitor. The most common ones are AAMI (Association for the Advancement of Medical Instrumentation) and BHS (British Hypertension Society). If you examine the standards in details, you would appreciate the inaccuracy or accuracy of oscillometric method.
AAMI standard: "For systolic and diastolic pressures, treated separately, the mean difference of the paired measurements of the test system and the comparison system shall be ± 5 mmHg or less, with a standard deviation of 8 mmHg or less. The comparison measurements shall be obtained by the auscultatory method or the intra-arterial method..." This means 68% of the readings would have an error under 8mmHg and 27% of the readings would have an error between 8 and 16mmHg.
BHS standard: Instead of using standard deviation, the BHS specifies multiple ranges and the maximum number of readings that can fall within each range. It assigns a grade to each criterion and a grade of A or B is required to get the certification.
|Grade||≤ 5mmHg||≤ 10mmHg||≤ 15mmHg|
|D||Worse than C|
Both standards are similar and they recognize the issues associated with the technology.
When you compare your oscillometric blood pressure monitor with an auscultatory monitor, a difference in 10mmHg or more is not uncommon. If two oscillometric monitors give similar reading, you can be sure both are pretty accurate. However, if they differ by 20mmHg or more, you cannot draw any conclusion as both can be 10mmHg off the true value.
If an accurate reading is important to you, get an auscultatory blood pressure monitor; however it is difficult to use.