Guesswork on dosing for a lifesaving heart medication has been virtually eliminated, researchers at the University of Illinois at Chicago said.

The research team developed a new formula to help physicians prescribe the proper amount of digoxin, the university said in a news release. Digoxin helps a weakened heart work more efficiently by strengthening the force of the heart muscle's contractions and helping restore a normal heart rhythm.

Digoxin can be difficult to dose and has little margin for error, said Jerry Bauman, interim dean of the UIC College of Pharmacy and lead investigator.

"The therapeutic range for digoxin in heart failure has recently changed to become lower and narrower," Bauman said. "However, dosing methods have not been modified to reflect this change."

Medical records of adult patients with a steady bloodstream level of digoxin were reviewed for six months. Researchers devised an equation using the patients' blood digoxin level, the digoxin dose, kidney function and ideal body weight.

When compared to methods often used to estimate the initial digoxin dose, the new formula was found to be superior. Using the new method, investigators developed a chart to help doctors choose the right dose.