African Americans with sickle cell trait - generally thought to be benign, unlike sickle cell disease - may still be at increased risk of kidney disease, a new study suggests.

Based on data for more than 2,000 people, researchers found that about 19 percent of those with sickle cell trait had kidney disease, compared to about 14 percent of people without the trait.

The study team calculates that sickle-cell trait raises the risk of kidney disease by nearly 60 percent. They reported their results in JAMA and during a presentation at Kidney Week in Philadelphia.

“We set out to do this, because there has been some uncertainty and controversy around the health consequences of sickle cell trait, which is a genetic condition related to sickle cell disease,” said one of the study’s authors, Dr. Alexander Reiner from the Fred Hutchinson Cancer Research Center in Seattle.

Sickle-cell disease is an inherited disorder in which red blood cells contain an abnormal type of hemoglobin. The defective cells frequently take on a sickle- or crescent-shape and can block small blood vessels, which can lead to tissue damage or even stroke.

People with sickle cell disease have inherited two defective genes, one from each parent, whereas those with sickle cell trait inherit only one problematic gene and one healthy one - which helps to offset the effect of the malfunctioning gene.

Sickle cell disease affects about one of every 500 blacks, according to the U.S. Centers for Disease Control and Prevention. Sickle cell trait affects one in 12 blacks.

“Even though sickle cell trait has been considered to be relatively benign, it has been known that kidney complications, such as having blood in the urine, was more common in people with sickle cell trait versus people without sickle cell trait,” said the study’s lead author, Dr. Rakhi Naik from Johns Hopkins University in Baltimore.

Naik told Reuters Health that the red blood cells of people with sickle cell trait would look completely normal under a microscope.

“The theory is that under very highly stressful situations (such as a low oxygen levels) . . . there may be some localized sickling that happens in the kidney that is leading to functional impairment,” she said.

For the new study, the researchers combined data from five large U.S. studies that included 15,975 self-identified African Americans - 1,248 of them with sickle cell trait - who were followed for 12 to 26 years.

At the outset, people with sickle cell trait were 57 percent more likely overall to have chronic kidney disease than people without a defective gene. People who didn't have kidney disease at the start were 80 percent more likely to develop it if they had the sickle trait.

Kidney disease tended to appear later in life among those with sickle cell trait than it tends to do in sickle cell disease, Naik noted.

The findings were similar in each of the studies included in the analysis. Researchers accounted for other genetic factors that are tied to chronic kidney disease among African Americans and found that they did not explain the link.

The new study concludes, however, that together, other genetic factors, socioeconomic factors and sickle cell trait may explain the overall increased risk of kidney disease seen among African Americans, Reiner said.

“Exactly how much of the increased burden of kidney disease sickle cell trait explains or is explained by other factors is still an open question that will require additional study,” he said.

Testing for sickle cell disease is common in the U.S., Naik said. “As a byproduct, people with sickle cell trait are identified,” she added.

Without additional research, it’s difficult to say whether people with sickle cell trait should be screened for kidney disease, Naik said.

“They may want closer monitoring of kidney function if someone has been identified to be a sickle cell trait carrier,” Reiner said. He added that there is no specific treatment or intervention to prevent kidney failure or further decline in kidney function.

He said these findings may lead to greater awareness of the potential health impact of sickle cell trait.

“It’s a reasonably common condition in the Africa American community,” Reiner said. “It may be important in terms of greater community awareness and education, which may be important for general health.”

Additionally, he said, the findings may spur future research into how sickle cell trait might affect other systems in the body.