If you’ve made the measurements yourself using home monitoring equipment or the machines commonly found in grocery stores and pharmacies, the changes you’ve seen might actually be related to errors and/or variations in the measurement process itself. While home blood pressure monitoring can be an effective and useful tool in some situations, you need proper training to ensure you're using the correct technique; performing the measurements without this training might explain the variation you see. Drugstore machines -- the kind that require you to sit in a chair and put your arm through a cuff -- are notorious for being poorly calibrated and fairly inaccurate. These measurements should never be used as an indication that something is wrong, or as reassurance that everything is OK.
Once you’ve verified that the fluctuation is real, understand that some fluctuation in blood pressure is completely normal. Changes of 25 to 30% during the day are not abnormal -- they reflect the fact the body is a dynamic, changeable organism. Many normal things can have large effects on blood pressure. Walking 20 feet can raise systolic blood pressure by 10 to 15 points. Your stress level, how tight your shoelaces are, what you had for breakfast, and how well you slept last night can all change your blood pressure, too.
If you’re sure that the measurements are accurate, and the blood pressure swings happen even when you’re relaxed, well-rested, and have no reason that can account for the changes, go see a doctor. Chances are that everything is OK and that the variations are just a natural part of your body’s cycle. Even if the fluctuation is not completely normal, the odds are that the exaggerated variance is the result of mild to moderate high blood pressure that hasn’t yet been diagnosed; that should fade with proper treatment and management.
Sometimes, wide fluctuations in blood pressure can be a signal that something else is wrong. Blood pressure swings in the absence of explainable baseline high blood pressure can indicate problems with the kidneys, heart, or blood vessels. They can also be a sign of hormone problems and certain types of tumors, such as pheochromocytoma. All of these conditions are rare.
If you’re concerned, the bottom line is that you should be evaluated by a physician. He’ll be able to tell you if the changes are real, what they mean, and what to do about them. Even if some abnormality is discovered, being in the care of a doctor already puts you on the right track.
Kaplan, NM. Clinical hypertension 8th ed. Philadelphia, Lippincott Williams & Wilkins, 2002.