Pre- and post-test probability: Difference between revisions

Revision as of 19:56, 28 January 2014

Revision as of 16:35, 24 February 2014

Line 24:

| Direct quotients from reference group ||<span style="color:DarkGreen;">Most straightforward: Predictive value equals probability ||<span style="color:DarkRed;"> Usually low: Separate reference group required for every subsequent pre-test state ||<span style="color:DarkGreen;"> Available both for [[binary classification|binary]] and [[Continuity (mathematics)|continuous]] values ||

|-

! By [[Likelihood-ratio test|likelihood ratio]]

| Derived from [[sensitivity and specificity]] || Post-test odds given by multiplying pretest odds with the ratio ||<span style="color:DarkGreen;"> Theoretically limitless ||<span style="color:DarkGreen;"> Pre-test state (and thus the pre-test probability) does not have to be same as in reference group ||<span style="color:DarkRed;"> Requires [[binary classification|binary]] values

|-

Line 88:

In these cases, the ''prevalence'' in the reference group is not completely accurate in representing the ''pre-test probability'' of the individual, and, consequently, the ''predictive value'' (whether ''positive'' or ''negative'') is not completely accurate in representing the ''post-test probability'' of the individual of having the target condition.

In these cases, a posttest probability can be estimated more accurately by using a [[Likelihood-ratio test|likelihood ratio]] for the test. ''Likelihood ratio'' is calculated from [[sensitivity and specificity]] of the test, and thereby it does not depend on prevalence in the reference group,<ref name=cebm/> and, likewise, it does not change with changed ''pre-test probability'', in contrast to positive or negative predictive values (which would change). Also, in effect, the validity of ''post-test probability'' determined from likelihood ratio is not vulnerable to [[sampling bias]] in regard to those with and without the condition in the population sample, and can be done as a [[case-control study]] that separately gathers those with and without the condition.

Estimation of post-test probability from pre-test probability and likelihood ratio goes as follows:<ref name=cebm>[http://www.cebm.net/index.aspx?o=1043 Likelihood Ratios], from CEBM (Centre for Evidence-Based Medicine). Page last edited: 1 February 2009. When used in examples, the general formulas are taken from reference, while example numbers are different</ref>