Bayes' formula is used to calculate the probability of prevalence of certain diseases in a given population or probability of certain diagnosis with specific signs and symptoms.

It can be used to calculate probability when the prevalence and the sensitivity and specificity of the test are known

Formula used:

• Positive = ((Pretest × Sensitivity) ÷ ((Pretest × Sensitivity) + ((100 - Pretest) × (100 - Specificity)))) × 100
• Negative = ((Pretest × (100 - Sensitivity)) ÷ (((100 - Pretest) × Specificity) + (Pretest × (100 - Sensitivity)))) × 100

Positive: Probability of patient having the condition given a positive test result

Negative: Probability of patient not having the condition given a negative result

Sensitivity: The probability of positive test given a patient with the condition

Specificity: The probability of negative test given a patient without the condition

Pretest: Prevalence

References:

Spitalnic S. Test Properties 2: Likelihood Ratios, Bayes' Formula, and Receiver Operating Characteristic Curves. Hospital physician; 2004.