Strep throat epidemiology and demographics

Sensitivity & Specificity
A study of 729 patients with pharyngitis in which 17% had a positive throat culture for group A streptococcus, identified the following four best predictors of streptococcus :

1. Lack of cough 2. Swollen tender anterior cervical nodes 3. (Marked) tonsillar exudates. Although the original study did not specify the degree of exudate, 'marked exudate' may be more accurate. A subsequent study of 693 patients with 9.7% having positive cultures found that 'marked exudates' had a sensitivity and specificity of 21% and 70% while 'pinpoint exudates' were nonspecific with sensitivity and specificity of 22% and 45%. 4. History of fever 5.hurts when swallowing.
 * sensitivity = 56%
 * specificity = 74%
 * sensitivity = 65%
 * specificity = 69%
 * sensitivity = 78%
 * specificity = 45%

When these findings are counted in a patient, the probabilities of positive cultures in the original study (prevalence=17%) are :
 * 4 findings -> 55.7%
 * 3 findings -> 30.1 – 34.1%
 * 2 findings -> 14.1 – 16.6%
 * 1 findings -> 6.0 - 6.9%
 * 0 findings -> 2.5%

The probabilities can also be computed with the following equation: X = −2.69 + 1.04 (exudtons) + 1 (swolacn) - 0.95 (cough) + 0.89  (fevhist)