BOSTON (Reuters) - Aggressively lowering blood pressure did little to prevent kidney damage in blacks, unless protein in their urine showed evidence of damage in the first place, researchers reported on Wednesday.

Doctors had hoped to show that dropping blood pressures to 130/80 or below would significantly lower rates of kidney problems for African-Americans, who have higher-than-average rates of high blood pressure.

But the updated findings from a long-running study, reported in the New England Journal of Medicine, show that aggressive blood pressure control did little more than standard blood pressure treatment geared toward getting the number down to 140/90.

The exception was when the telltale protein signature was initially present. In those cases, the progression of kidney disease was reduced by about 25 percent when blood pressure was lowered more, according to the team led by Dr. Lawrence Appel of Johns Hopkins University in Baltimore.

"This has always been a hot topic: Is a lower blood pressure goal better at preserving kidney function than the standard goal? The answer is a qualified yes, notably in people who have some protein in their urine," Appel said in a statement.

All 1,094 volunteers in the study, the first to probe the effect of aggressive treatment in blacks, had mild to moderate chronic kidney disease at the start of the test, but only one third had traces of protein in the urine.

In the United Sates, about $49 billion a year is spent on treating chronic kidney disease and another $23 billion caring for those with kidney failure. Roughly one third of those cases are caused by high blood pressure.

It has been known since 2002, when initial results from the study were reported, that aggressive therapy did not make any short-term difference in most volunteers.