While time flies by, it may also tinker with our blood pressure.

Results of blood pressure tests may vary by up to 40 percent depending when blood pressure is checked, new research shows. That's a significant difference, say Cynthia Thomas, DO, MPH, and colleagues.

"Awareness of the effects of these temporal factors should be weighed when diagnosing [high blood pressure] and making therapeutic decisions," says Thomas in a news release.

She cautions that the results need to be confirmed by other studies. Thomas' findings were presented in San Francisco at the American Society of Hypertension's 20th Annual Scientific Meeting and Exposition.

Timing Counts

Thomas' study was based on medical records of about 25,000 people. Over a three- to seven-year period, those people took nearly 205,000 blood pressure tests at clinics in Pennsylvania's Geisinger Health System, where Thomas works.

Blood pressure had already been shown to vary with the time of day and season of year. Thomas and colleagues wanted to see how much of a difference timing made in blood pressure. They noted four different blood pressure levels: systolic blood pressure of more than 120 or 140, and diastolic blood pressure of more than 80 or 90.

Systolic blood pressure is the first or "top" number; diastolic blood pressure is the second or "bottom" number. High blood pressure is systolic blood pressure of 140 or more and/or diastolic blood pressure of 90 or higher. "Prehypertension" is systolic blood pressure of 120-139 and/or 80-89.

Highs and Lows of Blood Pressure

The lowest blood pressure readings were seen in the morning and at midday, the study shows. Summertime tests also showed lower blood pressure.

In contrast, blood pressure was highest at the end of the day and during winter.

"For example, the probability of a systolic blood pressure greater than 140 mmHg for a measurement taken in July at 1 p.m. was 21.1 percent, compared with 29.6 percent if the blood pressure was taken in January at 7 p.m., a difference of 40 percent," write researchers.

Age, sex, race, and history of high blood pressure didn't change the pattern. The findings also weren't influenced by blood pressure treatment status, write the researchers.

"Our research shows that seasonal and hourly variations in blood pressure can be quite substantial, and should be taken seriously by health care professionals treating patients for [high blood pressure]," write the researchers.

High Blood Pressure Is Common

Nearly one in three U.S. adults has high blood pressure, but almost a third of them don't know it, says the American Heart Association.

High blood pressure that's not treated can lead to stroke, heart attack, heart failure, or kidney failure, making it a "silent killer," says the AHA.

A quick, noninvasive test is all it takes to check blood pressure. A healthy diet and active lifestyle can help; some people may also need medicine to get their blood pressure under control.

By Miranda Hitti, reviewed by Brunilda Nazario, MD

SOURCES: American Society of Hypertension 20th Annual Scientific Meeting and Exposition, San Francisco, May 14-18, 2005. American Heart Association: "High Blood Pressure." WebMD Medical Reference from Healthwise: "High Blood Pressure (Hypertension)."