A University of Minnesota study published yesterday indicates that Minnesota doctors were right only 60 percent of the time when diagnosing rheumatic fever in patients under age 20.

In the remaining 40 percent of the cases, the report said, doctors either concluded that the patient had the disease when a subsequent review of clinical and laboratory data indicated he did not, or concluded the patien did not have it when subsequent review indicated he did.

In addition, the report said, in nine out of 10 cases doctors did not prescribe the most effective method of treatment, choosing antibiotics given orally rather than by injection.

Rheumatic fever is an infectious disease that can lead to serious heart ailments.

The report said its findings suggested "over-reporting of acute rheumatic fever in Minnesota." But it also said that some doctors failed to report suspected cases of the disease to the state health department as the department requires, "making the registry an inaccurate reflection of the number of cases occuring in the state." It called for reevaluation of existing assessment methods.

The report was published in the April issue of Minnesota Medicine, the journal of the Minnesota State Medical Association. It was written by a three-member team of University of Minnesota doctors who studied a sample of 60 cases reported to the Minnesota health department from May 1975 to June 1976.

One of the authors, Dr. Diane Dahl, a resident in pediatrics at the university, conceded that diagnosing rheumatic fever is difficult. She also said that she assumed the doctors gave the antibiotic orally instead of by injection because of "patient preference."

The study found that the oral treatment, which is meant to lessen the risk of a second attack of rheumatic fever and heart damage, did not meet the standards of the American Heart Association. It has said that the best prevention method is a series of monthly penicillin shot.

The authors of the report were Drs. F. Blanton Bessinger and Edward L. Kaplan, cardiologists.