By the time Rena Barrow-Wells gave birth to her fourth baby in 2020, she was well-versed in caring for a child with cystic fibrosis. She was also experienced in fighting for a diagnosis of the disease, which runs in families and can severely damage the lungs and digestive system.

Nineteen years earlier, her first son, Jarrod, displayed classic symptoms of cystic fibrosis as a newborn — failure to gain weight; a stubborn, phlegmy cough; and frequent, oily stools. But instead of identifying the cause of her son’s illness, doctors at the New Orleans emergency room where she took Jarrod blamed his poor growth on his mother, who is Black and was a teenager at the time. Ms. Barrow-Wells said that doctors had accused her of starving her son, placed the two of them in a room with video surveillance and reported her to child protective services.

Doctors discharged Jarrod two weeks later without a diagnosis. It would take four years — and dozens of additional visits to doctors and emergency rooms — for Jarrod to be diagnosed with cystic fibrosis. By then, she said, the disease had interfered with his body’s ability to absorb nutrients for so long that he was profoundly malnourished. Children who are diagnosed after they are 2 months old are also at greater risk of serious lung problems and death.

People from minority communities have struggled to receive lifesaving care for cystic fibrosis for decades, in large part because many doctors were traditionally taught that the disease almost exclusively affects white people, said Dr. Jennifer Taylor-Cousar, a professor of medicine and pediatrics at the University of Colorado Anschutz Medical Campus. Several factors have fueled that misconception, she said, including limited testing and scientific research that focused on white patients and failed to identify the full range of affected populations, as well as bias against patients of color, some of whom were blamed for their symptoms, as happened with Ms. Barrow-Wells and Jarrod.

Still, Ms. Barrow-Wells had reason to hope in 2020 that things would be easier for her newborn son, Jahsir. The way cystic fibrosis was diagnosed had changed dramatically since Jarrod was born. Since 2010, all states have screened newborns for the disease, an effort that promised to help all babies with cystic fibrosis receive a diagnosis and treatment in the critical first weeks of life, before the disease causes serious organ damage.

A newborn screening should have quickly confirmed what Ms. Barrow-Wells soon recognized — that Jahsir’s salty skin and foul-smelling diapers were signs that he, too, had cystic fibrosis.

But her baby’s screening came back negative. Ms. Barrow-Wells, who now lives in Lawrenceville, Georgia, begged doctors to test her baby’s sweat to see if it had the high levels of salt that are a hallmark of the disease. She reminded them that her older son’s diagnosis meant the baby was at high risk.

But Jahsir’s pediatrician pointed to the negative test result as proof that her son was fine, she said, and told her to stop worrying.

Experts say that universal testing has given some doctors a false sense of security: While early detection has undoubtedly transformed diagnosis for the disease, state screening programs still miss children with cystic fibrosis, particularly those who aren’t white.

Jahsir was diagnosed with cystic fibrosis at 3 months old. “A person shouldn’t have to go through so much,” Ms. Barrow-Wells said.

A New Genomic Era, But Lingering Disparities

Cystic fibrosis is a progressive disease caused by defects in a single gene. It makes the mucus that coats the inside of the body thick and sticky. Instead of protecting the airways, the digestive tract and other organs and tissues, the mucus in people with cystic fibrosis causes blockages, damage and infections.

Nearly 40,000 Americans have cystic fibrosis, about 15 percent of whom are people of color, according to data from a Cystic Fibrosis Foundation patient registry.

The disease occurs when people inherit two defective copies of the gene, called CFTR — one from the mother and one from the father. People who inherit a single defective copy don’t develop the disease but can pass on the mutation.

Only a few decades ago, people with cystic fibrosis often died in childhood or adolescence. Scientific advances and public health initiatives, including newborn screening, have transformed the prognosis and quality of life for people living with it. Half of the babies born with cystic fibrosis in 2021 can expect to live to age 65 or older, according to the C.F.F.

Universal screening has helped doctors diagnose babies an average of three months sooner, said Dr. Clement Ren, the director of the Cystic Fibrosis Center at Children’s Hospital of Philadelphia. It has also provided a clearer picture of who actually has cystic fibrosis: The share of newly diagnosed patients who are Black or Hispanic increased from 9 percent in 2000 to 14 percent in 2020. (This includes newborns as well as people who aren’t diagnosed until later in life.)

Dr. Ren said he recently diagnosed a Hispanic baby born in New Jersey who came to his hospital in Philadelphia, the closest cystic fibrosis center. “She was two weeks old,” Dr. Ren said. “That is the goal: to diagnose babies within the first month of life.”

The screening process starts before newborns leave the hospital, with a blood test for a protein that is commonly elevated in babies with cystic fibrosis. If a sample shows high levels, the newborn is then tested for genetic mutations. Scientists have identified hundreds of mutations that can drive the disease.

But the genetic tests now used are largely based on older clinical trials that enrolled mostly white children, focusing primarily on genetic mutations found in people of European ancestry, said Dr. Meghan McGarry, a pediatric pulmonologist at Seattle Children’s Hospital. As a result, they can miss babies with less common mutations that originated in Africa, Asia and Latin America.

Some states screen for more mutations than others. Wisconsin’s screening program is so robust — it tests for 689 mutations — that Montana, Kansas and Connecticut now send their newborn samples to the state.

New Jersey, which for years only screened for one mutation, now tests for 139; Pennsylvania and Florida have also expanded their screenings.

But in Mississippi, where nearly 38 percent of the population in 2020 was Black, newborns are still tested for only a single genetic variant that is primarily found in white people.

For many states, the biggest barrier is cost. The more mutations included in a screening panel, the greater the expense, said Dr. Rachel Linnemann, the director of the Children’s Healthcare of Atlanta and Emory University Cystic Fibrosis Care Center.

In Georgia, where Ms. Barrow-Wells lives, the cystic fibrosis screening program only tests for 39 variants. She is working with state officials to increase that number. Although Jahsir, now 3, is receiving therapy to protect his lungs and improve his nutrition, her older son has significant scarring on his lungs and pancreas. Five years ago, he needed emergency surgery for a collapsed lung caused by years of damage. Ms. Barrow-Wells said she felt fortunate that Jarrod, now 23, was able to take a new type of cystic fibrosis drug that is prolonging life for many patients.

The Shortfalls of Screening

Even states with robust screening programs can miss babies with rare genetic mutations. Cambrey Vasconez White, 34, requested genetic testing when she was pregnant. Her husband had a cousin with cystic fibrosis, meaning that he might carry one of the genetic mutations that causes the disease. Ms. White, who is Hispanic, wanted to know if she did, too.

In California, where Ms. White lives, the screening program tests for 75 mutations, including ones more commonly found among Hispanic people. Ms. White’s test came back negative, as did the newborn screening for her son, Rowland, born in 2021.

Ms. White didn’t believe the result: Rowland screamed and cried after breastfeeding, a common symptom of cystic fibrosis, which interferes with digestion and can cause stomach pain. Rowland also failed to gain weight. When he began to fall off his growth curve, Ms. White said, she pushed for more testing.

Some pediatricians are reluctant to order additional testing for babies who screen negative, even if they have signs of the disease, Dr. McGarry said, because they don’t realize that the screenings can be inaccurate or that nonwhite babies can inherit the disease.

“Parents are told, ‘Your child had a normal newborn screen. You are crazy.’ It’s becoming a barrier to care. That’s the opposite of what it was meant to be,” she said.

Doctors eventually agreed to further testing, and Rowland was diagnosed with cystic fibrosis at 6 months old. Rowland and both of his parents had rare mutations not included in California’s panel.

Rowland’s experience underscores why doctors can’t rely on screening alone, said Dr. Susanna McColley, a professor of pediatrics in pulmonary and sleep medicine at the Northwestern University Feinberg School of Medicine. She said that children who have symptoms or a sibling with the disease should be referred to a cystic fibrosis center for a definitive diagnostic test, called a sweat test, regardless of their screening results.

Before universal screening, doctors “used to do a lot more sweat tests for kids with poor growth, recurrent respiratory symptoms or both,” Dr. McColley said.

To better educate doctors and patients, the Cystic Fibrosis Foundation plans to release new screening guidelines this fall that promote racial equity. And both the foundation and the Centers for Disease Control and Prevention are funding efforts to improve screening — work Dr. McGarry said is sorely needed for families like Ms. Barrow’s.

“It can take a lot of persistence, determination and knowledge from the person with cystic fibrosis or the parent to get that diagnosis,” Dr. McGarry said. “Not every family has that.”