Why do the readings on my digital BP monitor vary?

A variety of reasons. First of all, blood pressure itself is a dynamic rather than static vital sign. Both systolic and diastolic pressures can vary from 10mmHg to 20mmHg or more throughout the course of the day. In some people, a lot more than that. BP is generally lower in the morning and increases throughout the day. Eating, drinking, smoking, exercise and even talking all influence blood pressure. Instrument repeatability will also result in measurement variance. As an example, if the instrument has a stated tolerance of 3% and you measured your blood pressure at the morning when it was 120/70 and the evening when it was 130/80, the instrument might read 115/68 for the morning read, and 135/83 for the evening read. The initial impression is that the pressure varied by nearly 20mmHg when in fact, part of the variance was due to normal fluctuations in BP, and in part was due the tolerances of the instrument. For best results, you should measure your blood pressure at the same time each day, preferably first thing every morning. If time permits, take three readings, each 1-2 minutes apart. If any one reading looks out of line, discard it. Otherwise average the results and record the average of the three (or two if one was discarded). Doing this will smooth out the readings and produce more consistent and meaningful results. Our 6016, 6022 and 6023 monitors feature an average mode setting that will automatically display the average of three successive readings.