Doctors call it the "white coat syndrome."

Even the healthiest of patients can become anxious during a routine physical examination, or while sitting in the waiting room of a doctor's office. The anxiety can cause sweaty palms, change the heart rate, boost blood pressure.

"You'd be surprised how many people think the pulse rate and the blood pressure are one fixed rate," says Dr. Richard Rubin, a cardiologist in Chevy Chase. "They're not. They go up and down."

Not only does blood pressure rise with exertion and anxiety -- for example, while a person is jogging or stuck in a traffic jam or in a strenuous argument -- but it goes through a daily cycle. It tends to be higher in the morning and late afternoon, and lower during sleep.

"Everybody goes through a pretty large swing during the day," says Dr. Aram V. Chobanian, director of the cardiovascular institute at Boston University Medical Center and chairman of the American Heart Association's council on high blood pressure research. "It's not unusual to see a 30-point difference in systolic pressure the higher number in any one individual throughout the day."

Such fluctuations can make a diagnosis of high blood pressure, or hypertension, somewhat tricky.

"Anything that causes the patient to be upset or nervous is going to make the blood pressure higher," says Bill Sanders, a spokesman for the National Heart, Lung and Blood Institute. No doctor, he adds, should base a firm diagnosis of hypertension on just one abnormally high reading of a patient's blood pressure.

Technology has provided medicine with portable machines to record 24-hour medical data that used to be available only from a one-shot reading in a doctor's office or continuously in a hospital.

A device called a Holter monitor -- a portable tape recorder that fits on the belt and attaches to electrodes connected to the chest -- records a continuous 24-hour electrocardiogram, or graph of the electrical activity and beating of the heart. A similar machine fitted to an inflatable cuff can record a patient's blood pressure automatically throughout the day.

Several companies around the nation, including Medifacts Ltd. in Potomac, Md., provide such portable blood pressure monitors, giving doctors access to a day-long recording of blood pressure.

Ten percent of the patients who are initially diagnosed as having high blood pressure "are hypertensive only in the medical environment," says Dr. Bruce Garrett, a kidney disease and hypertension specialist and acting medical director of Medifacts.

"Your heart beats about 100,000 times a day," Garrett says. "Each beat generates a blood pressure. Your doctor takes one measurement, and from that he makes a huge statement about your blood pressure."

An above-normal blood pressure reading caused by the "white coat syndrome" in a doctor's office, says cardiologist Rubin, can result in some patients being inaccurately labeled as having hypertension. These patients may even be put on medication unnecessarily.

Rubin recently saw a 21-year-old patient, whose Type-A parents hounded him into going to the doctor's office to have his blood pressure checked. His blood pressure, recorded in the office, was borderline-high. But a 24-hour monitoring of his blood pressure revealed that it was normal at other times.

"I was really thinking of starting him on a lifelong medicine at age 21," Rubin says. "But sure enough, away from his parents and the anxiety of the doctor's office, his blood pressure was 120 over 60 normal for his age ."

"All the available information would strongly suggest that the cardiovascular risks from high blood pressure are related to the average level of blood pressure, not the peaks," Chobanian says.

"The reality is, our instrumentation is very crude," says Garrett. "We're in a technological era and we can do better. This high blood pressure is an extremely pervasive disease, and we haven't hit the nail on the head yet by a long shot."

The 24-hour monitor, Garrett says, is useful in providing additional data for two main groups of patients: those whose "borderline" hypertension may be a result of the white coat syndrome, and those whose high blood pressure does not seem well controlled by medication.

"Before piling drug on drug on drug," Garrett says, "the physician wants to get an idea of what's going on."

Since Medifacts opened last fall, more than 100 physicians in the Washington metropolitan area have used its 24-hour blood pressure monitoring service, says company president Sandra Garrett, who is Bruce Garrett's wife. The service is available to patients only through a physician.

A 24-hour blood pressure monitor is not cheap. Most doctors charge between $200 and $300 for use of the portable machine, compilation of the data and interpretation of the results, the Garretts say. The charge is covered by most insurance companies, but not by Medicare, according to Sandra Garrett.

Most doctors would agree that the use of a 24-hour monitoris a waste of money for patients whose office readings are normal and who have no symptoms or history of hypertension.

"The units are quite useful, but in selective cases," says Dr. Sam Fox, director of the preventive cardiology program at Georgetown University Medical Center. "It's expensive technology, and you don't want to overdo it."

The monitors consist of a tape recorder attached by a cord to a cuff that fits around the upper arm and inflates automatically as often as every six minutes or as infrequently as every hour. The device weighs about 3.5 pounds, and the recorder can be worn on the belt or placed on the floor during sleep.

The machine automatically records both blood pressure and pulse. At the end of the 24-hour period, Medifacts compiles the data and sends the doctor a detailed computer printout of the results. It also keeps an anonymous record of the data, for possible use by researchers studying the health effects of fluctuations in blood pressure.

Patients using the monitor are asked to keep a brief diary, so that doctors can correlate changes in blood pressure with the day's events or activities.

Another way around the white coat syndrome is for patients who need to monitor their blood pressure regularly to buy a standard blood pressure cuff for use at home -- now available for less than $100.

Coin-operated machines that record heart rate or blood pressure also are increasingly common in shopping malls and supermarkets around the country. Most doctors agree that the machines may help increase patients' awareness of the importance of monitoring their own health, but they caution against reading too much into a single machine measurement without interpretation from a doctor.

"If you've just run up the steps," Rubin says, "it may tell you you're grossly unfit and scare the hell out of you." And a machine could easily overlook an irregular heart beat even while it recorded a normal pulse.

The American Heart Association has expressed "major concerns" about the reliability, durability and maintenance of such coin-operated machines and their use without medical counseling. "An imprecise blood pressure reading," cautions an AHA statement, "could cause unnecessary anxiety or unwarranted complacency for the user."