The importance of disease prevalence in transporting clinical prediction rules. The case of streptococcal pharyngitis.

Because clinical prediction rules often are applied in new settings to calculate the probability of a disease, we evaluated the accuracy of three rules for predicting streptococcal pharyngitis in 310 patients. Use of the rules led to overestimations of disease probability in 47%, 82%, and 93% of the patients. When we used receiver-operating characteristic curve analysis, no rule lost power to discriminate streptococcal from nonstreptococcal causes of pharyngitis. The overestimations in disease probability likely were caused by differences in disease prevalence between our setting (5%) and the settings in which they were developed (15% to 17%). All rules led to accurate predictions when they were adjusted for the disease prevalence found in our setting using a likelihood ratio formulation of Bayes' theorem. The value of prediction rules, like that of other diagnostic tests, is affected by differences in disease prevalence in different settings. Failure to recognize and adjust for these differences may cause poor decision making or the premature dismissal of valid rules.

The accuracy of experienced physicians' probability estimates for patients with sore throats. Implications for decision making.

Ten physicians recorded their treatment decisions and estimated probabilities of streptococcal infection for patients with sore throats. Of 308 throat cultures, 15 (4.9%) were positive for group A streptococci. The physicians overestimated the probability of a positive culture for 81% of their patients and their estimates and treatment decisions were strongly associated. Of 104 patients treated before culture results were available, only eight had positive cultures. Probability overestimation may have been due to neglect of the low culture-positive rate, assignment of undue importance to weakly predictive or highly intercorrelated clinical features, and a value-induced bias, occurring when features important for treatment are erroneously linked to the likelihood of disease. Cognitive limitations in information processing may limit the effectiveness of pharyngitis management protocols that require subjective estimates of disease probability.

Incidence of beta hemolytic streptococcal pharyngitis in adolescent with infectious mononucleosis.

Reports on the incidence of beta-hemolytic group A streptococci (BHGAS) in the pharynx of patients with infectious mononucleosis (IM) have varied from 3% to 33%. To ascertain the rate of infection, we prospectively performed serial throat cultures and determined anti-streptococcal antibody titers on 45 students with confirmed IM by Epstein-Barr virus-specific serology. One hundred healthy control students had throat cultures for comparison. The rate of recovery of BHGAS was similar in patients with IM (4%) and controls (3%). No students with IM had a fourfold rise of anti-streptococcal antibodies. We conclude that routine culture for BHGAS and/or treatment with antibiotic agents is not indicated in all patients with IM.

Screening for chest disease in college students: policies of student health services regarding the use of routine screening chest radiographs and tuberculin skin tests.

We conducted a survey of college health services to determine their policies regarding the use of screening chest radiographs and tuberculin skin tests. Pre-enrollment chest radiographs are prescribed for all incoming students by 24 per cent of the 531 respondents and for health professions students, foreign students, and intercollegiate athletes only by an additional 20 per cent, 19 per cent, and 6 per cent, respectively. Periodic chest radiography is conducted for all students by 7 per cent of the respondents and for health professions students, foreign students, and intercollegiate athletes only by an additional 22 per cent, 16 per cent, and 8 per cent, respectively. Pre-enrollment tuberculin skin tests are prescribed for all incoming students by 52 per cent of the respondents and for health professions students, foreign students, and intercollegiate athletes only by an additional 48 per cent, 29 per cent, and 9 per cent, respectively. Periodic tuberculin skin testing is conducted for all students by 27 per cent of the respondents and for health professions students, foreign students, and intercollegiate athletes only by an additional 48 per cent, 23 per cent, and 16 per cent, respectively. We estimate from these data that 723,000 incoming students in the United States received screening chest radiographs in 1979 with estimated charges totaling between $7 million and $27 million. There may be 0.05 to 0.33 induced cases of lung cancer, leukemia, thyroid cancer, and female breast cancer over a 20-year period among this group of students exposed to ionizing radiation.