The finding by the Children's Hospital in Boston in collaboration with George Washington University, could also prevent unnecessary hospital admissions and medication.

Fevers are one of the main reasons children are admitted to hospital. While many fevers turn out to be minor or short-lived, a proportion can be due to serious bacterial infections such as meningitis or pneumonia.

Medical professionals often have to perform a battery of tests to find the source behind the fever. In infants less than three months of age this can include blood and urine tests, lumbar punctures and precautionary antibiotic treatment.

The researchers found a new diagnostic marker called procalcitonin could help identify infants at high risk of serious bacterial infections such as urinary tract and bloodstream infections.

The study, published in the October issue of the American medical journal Pediatrics, is the first to examine procalcitonin as a tool for evaluating infant fever in an emergency.

The researchers used a novel procalcitonin test, approved by the Food and Drug Administration, in 234 feverish babies under three months of age, of whom 18 per cent had definite or possible serious bacterial infections confirmed by independent clinical criteria.

Procalcitonin detected all cases of the infection but also proved sensitive enough to establish a threshold value that would identify infants at low risk of serious infection.

Acting chief of emergency medicine at the Children's Hospital, Boston, Richard Bachur said the research was sparked by ''inefficient'' fever management in infants.

The researchers are planning a wider study to confirm results on a larger scale.

If the test proves valuable, Dr Bachur hopes it will become a standard tool for infant fever evaluation.