Mercury sphygmomanometers use the Korotkoff sound method, while electronic blood pressure monitors use the oscillometric method. The Korotkoff sound method relies on a doctor using a stethoscope to listen to the sound of blood flowing through the blood vessels, while the oscillometric method measures the vibrations on the blood vessel wall caused by blood flow. Although the measurement principles differ, in theory, the results should be the same. In practice, the accuracy of the Korotkoff sound method can be affected by the variability in a doctor's hearing speed, potentially leading to significant errors if the blood pressure readings are not noted immediately during measurement. Electronic blood pressure monitors eliminate this human error.

Additionally, the emotional state of the person being measured can influence the results. Some individuals may experience a degree of anxiety in the presence of a doctor, leading to higher measurements, while they might be more relaxed at home, resulting in relatively lower measurements, which may be closer to their actual blood pressure.