A quick test to diagnose kidney failure has been developed, US doctors say.
The study identified the protein in children following heart surgery
Researchers identified a protein which indicates renal failure within hours during tests on children who developed the condition after heart surgery.
The Cincinnati Children's Hospital team believes the test - which it claims works on adults - will save lives, as traditional diagnosis can take days.
Experts said the study was interesting, but there was no proof it would work on anyone other than child heart patients.
While many conditions can cause kidney failure, it is more common among patients who have undergone major surgery, or suffered strokes or severe infections.
In the UK 8,000 people a year suffer acute renal failure - only about 300 of these are children - with about half dying.
Doctors currently diagnose the condition by checking levels of a chemical called creatinine in the blood.
But in the study of 71 child heart bypass surgery patients, 20 of whom developed kidney failure, the team looked for high levels of the chemical neutrophil gelatinase-associated lipocalin in the urine and blood, the Lancet reported.
Kidney failure was identified within hours.
Lead researcher Dr Prasad Devarajan said: "Despite major achievements in treating children and adults with kidney failure, little has changed in the last four decades to diagnose kidney failure early enough before it progresses into a chronic or deadly condition."
He said even though the research was on children the test would work on adults as well.
But Dr Stefan Herget-Rosenthal, of the University of Duisburg-Essen in Germany, warned the findings should be treated with caution.
"Although impressive, it should be noted that the data apply solely to a small number of children with initially normal renal function who developed acute renal failure after cardiothoracic surgery in a single centre."
Kidney physician Professor Stewart Cameron, of the National Kidney Research Fund, said the test may be "useful".
But he added: "My concerns were that it was done on a rather narrow population - children undergoing heart operations - a number of whom would be expected to develop acute renal failure.
"It needs to be repeated and extended to see if it applies also to children and adults with other causes of renal failure.
"Certainly identifying such patients a day or two earlier would be helpful, but again it needs to be demonstrated that doing this makes any difference to what happens to them in the long run."