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Calculator: Post-test probability from pre-test probability, sensitivity, and specificity


Calculator: Post-test probability from pre-test probability, sensitivity, and specificity

 
Input
 
Pre-test probability  
Sensitivity  
Specificity  

 
Results
 
Important: Inputs must be complete to perform calculation.

 
 
Post-test probability of positive result  
Post-test probability of negative result  
 
Decimal precision  

 
 

 
Notes
  • Post-test probability of positive result is the probability that the disease is present when the test result is positive.
  • Post-test probability of negative result is the probability that the disease is present when the test result is negative.
  • Pre-test probability is usually equivalent to the prevalence of a disease or finding in a particular population.

 
Equations used
 
Post_Odds_Pos = (Pre-test probability * Sensitivity) / ((1 - Pre-test probability) * (1-Specificity))
Post-test probability of positive result = Post_Odds_Pos / (1 + Post_Odds_Pos)
Post_Odds_Neg = (Pre-test probability * (1-Sensitivity)) / ((1-Pre-test probability) * Specificity)
Post-test probability of negative result = Post_Odds_Neg / (1 + Post_Odds_Neg)

 

 
References
  1. Brown DD, LeBlond RF, DeGowin RL. Principles of diagnostic testing. In: DeGowin’s Diagnostic Examination, 9th ed, McGraw-Hill Professional, 2009. p.798.
  2. Perera R, Heneghan C. Making sense of diagnostic test likelihood ratios. ACP J Club 2007; 146:A8.
Only digits 0 to 9 and a single decimal point (".") are acceptable as numeric inputs. Attempted input of other characters into a numeric field may lead to an incorrect result.

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