When a physician tries to determine whether or not a patient's cough or sore throat is caused by a bacterium or a virus, they're correct only 50% of the time.

The findings show that a physicians' ability to diagnose such ailments is "low and no more reliable than tossing a coin.

The report suggests that it is not uncommon for patients to receive antibiotics unnecessarily, or conversely, that patients who might benefit from antibiotics don't get them. Antibiotics can fight bacteria, but are useless in treating infections caused by a virus.

In the study, the researchers looked at 250 adults over 21 years of age with a fever that had lasted less than a week and either a cough, sore throat, hoarseness or a head cold. A family doctor or emergency department physician interviewed and examined patients before diagnosing the infection as bacterial or viral. Blood was taken from patients at that point, and again 3 to 4 weeks later.

The investigators tested the blood for antibodies to a number of different bacteria and viruses. Because antibodies to infectious agents can persist in the body for years, the researchers diagnosed the illness based on changes in antibody level between the first and second blood test. Such a technique would be inappropriate for diagnosing an illness before treatment.

The researchers found that doctors correctly ruled out bacteria as a cause of infection only 60% of the time and correctly identified bacteria as the cause of symptoms only 50% of the time.

British Journal of General Practice 2001;51;998-1000