Table 4.

Sensitivity, specificity, likelihood ratios for positive and negative results, and pre-test and post-test probabilities for positive and negative results, defining a “new prescription” as one issued 21, 28, and 35 days after a previous one

	21 days	28 days	35 days
Sensitivity	0.987	0.980	0.976
Specificity	0.787	0.903	0.987
Likelihood ratio +	4.63	10.18	77.73
Likelihood ratio −	0.02	0.02	0.02
Pre-test probability +	0.805	0.805	0.805
Post-test probability +	0.948	0.976	0.997
Pre-test probability −	0.195	0.195	0.195
Post-test probability −	0.003	0.005	0.006

The likelihood ratio incorporates both the sensitivity and specificity of the test and provides a direct estimate of how much a test result will change the odds of having a certain property. The likelihood ratio for a positive result (LR+) tells you how much the odds of the property increase when a test is positive. The likelihood ratio for a negative result (LR−) tells you how much the odds of the property decrease when a test is negative. The pre-test probability is the proportion of prescriptions that do or don’t have the target characteristic (“first” or “not first” antibiotic). The post-test probability is the proportion of prescriptions with that particular test result that have the target characteristic