Had Matthew Goodwin’s blood pressure been checked at age 3, when it should have been, he most likely would have been spared a medical emergency a year later. He spent two weeks in the pediatric intensive care unit while doctors tried to determine why his blood pressure had spiked off the charts following minor surgery.

Matthew turned out to have a congenital kidney disorder that caused severe hypertension and required surgeons to remove one of his kidneys and implant it in a new location, a procedure known as a kidney autotransplant. Now a 15-year-old honors student from Prairieville, La., Matthew takes pressure-lowering medication and adheres closely to a low-sodium diet loaded with fresh fruits and vegetables and no soft drinks.

Knowing his experience is not unique and that millions of young Americans with elevated blood pressure don’t know they have it, Matthew, who hopes to become a pediatric nephrologist, volunteers for the National Pediatric Blood Pressure Awareness Foundation, a nonprofit advocacy group his mother, Celeste Goodwin, created to educate parents and professionals about the importance of regular blood pressure checks in children and adolescents.

Boosting this effort, in August the American Academy of Pediatrics published a 74-page report updating its guidelines for screening and managing high blood pressure in youngsters. The report includes a much-simplified chart to help practicing physicians readily determine whether a child’s blood pressure is within normal parameters.

Just as children’s blood pressure cannot be measured with a cuff meant for an adult-size arm, it also cannot be interpreted on an adult scale.

For children, the definition of normal blood pressure is not based on a simple reading of systolic over diastolic blood pressure, like 110 over 70 or 120 over 80. Rather, because normal blood pressure varies with a child’s age, gender and height, doctors typically needed to consult pages of charts to determine whether a patient’s pressure was normal or not.

Now, the new guidelines provide a short, easy-to-use table based only on a child’s gender and age, from 3 to 18, that doctors can readily consult and even hang in the office where they and parents can see it.

Study after study has demonstrated that elevated blood pressure is correctly diagnosed in less than one-quarter of the children who have it. For example, the largest study, published last year in the journal Pediatrics, covered more than 1.2 million patients ages 3 to 18. Electronic health records showed that 398,079 had their blood pressure measured at three or more visits. Of these, readings in 12,138 children met the criteria for hypertension and 38,874 for pre-hypertension (now called elevated blood pressure). But only 23.2 percent with hypertension and 10.2 percent with pre-hypertension were diagnosed as such and duly noted for follow-up in their health records.

Dr. David Kaelber, a pediatrician and internist at the MetroHealth System in Cleveland who directed the study and was co-chairman of the team that developed the new guidelines, said in an interview, “It’s great to have guidelines, but they’re only as good as the people who actually follow them.”

You may wonder why a diagnosis of high or elevated blood pressure in children is so important. Although children don’t usually suffer the consequences of high blood pressure in the pediatric years, Dr. Kaelber said that, unless properly treated, it can result in early heart attacks, strokes and kidney disease.

Elevated blood pressure in children predicts high blood pressure in adults, a leading risk factor for heart attacks and strokes. Already in young adults who’ve had elevated blood pressure as children, “you can see two kinds of cardiovascular damage,” Dr. Joseph T. Flynn, lead author of the new guidelines, said in an interview. “There is thickening of the left ventricle, the heart’s main pumping chamber, and thickening of blood vessel walls that is clearly tied to atherosclerosis and heart attacks in adults.”

Because high blood pressure often runs in families, detecting it in a child may prompt a check of the parents and other family members and end up saving their health and lives, Dr. Flynn said.

And as demonstrated by Matthew Goodwin’s experience, in about 20 percent of cases there is an underlying and correctable cause for high blood pressure in children. Failing to make the correct diagnosis can be disastrous. As Mrs. Goodwin put it bluntly, “Matthew could have died.”

The prevalence of elevated blood pressure and frank hypertension in children and adolescents has risen dramatically in recent decades, largely the result of increasing overweight and obesity in the young. An estimated 17 percent of children aged 2 to 19 are obese, and approximately two million children have hypertension.

Other contributors to the prevalence of hypertension in children include a growing dependence on fast foods, processed foods and snacks that are high in salt, causing excess sodium and fluid in the blood and an added strain on the delicate blood vessels that feed the kidneys.

The new guidelines from the pediatric academy urge health care practitioners to measure and record blood pressure at every well-child or preventive care visit, whether or not the child is overweight or there appears to be anything wrong. If the blood pressure reading is recorded in an electronic health record, the computer does the necessary calculation to determine if the reading is normal.

To prevent this exam from being overlooked, Dr. Kaelber suggested that parents gently nudge the practitioner by asking, “By the way, how is my child’s blood pressure doing?”

If, based on the chart, blood pressure is above normal, the guidelines offer clear follow-up procedures, for example, when to recheck the child, whether to prescribe lifestyle measures like dietary changes and physical activity or drug treatment, and when to look for a possible underlying cause of blood pressure that is persistently elevated despite treatment.

A diagnosis of high or elevated blood pressure should be based on three consecutive elevated readings. Dr. Kaelber noted that an underlying cause of hypertension is more common in children than in adults, especially in children younger than 9.

Dr. Flynn, chief of pediatric nephrology at Seattle Children’s Hospital, said his team, among others, solves a major problem in getting an accurate blood pressure assessment by using an ambulatory blood pressure monitor for 24 hours on every child with a high reading. In 30 percent to 40 percent of cases in which office blood pressure readings are consistently elevated, the cause is so-called “white-coat hypertension” – meaning the child is simply stressed from seeing a doctor. The monitor, which is a little bigger than a smartphone, takes many measurements during the child’s normal waking and sleeping hours.

The monitor has the added advantage, Dr. Flynn said, of greatly reducing the number of children who must undergo further testing to determine why their blood pressure is high, saving both money and undue stress on the family and the health care system.