Pre- and post-test probability: Difference between revisions

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Revision as of 05:06, 5 June 2016

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It is possible to do a calculation of likelihood ratios for tests with continuous values or more than two outcomes which is similar to the calculation for dichotomous outcomes. For this purpose, a separate likelihood ratio is calculated for every level of test result and is called interval or stratum specific likelihood ratios.<ref>{{cite journal | doi = 10.1067/mem.2003.274 | author vauthors= Brown MD, Reeves MJ. | year = 2003 | title = Evidence-based emergency medicine/skills for evidence-based emergency care. Interval likelihood ratios: another advantage for the evidence-based diagnostician | url = | journal =Ann Emerg Med | volume = 42 | issue = 2| pages = 292–297 | pmid = 12883521 }}</ref>

====Example====

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=====Interference with test=====

''Post-test probability'', as estimated from the ''pre-test probability'' with ''likelihood ratio'', should be handled with caution in individuals with other determinants (such as risk factors) than the general population, as well as in individuals that have undergone previous tests, because such determinants or tests may also influence the test itself in unpredictive ways, still causing inaccurate results. An example with the risk factor of [[obesity]] is that additional abdominal fat can make it difficult to palpate abdominal organs and decrease the resolution of [[abdominal ultrasonography]], and similarly, remnant [[barium contrast]] from a previous radiography can interfere with subsequent abdominal examinations,<ref>[https://books.google.com/books?id=CQuBkXDspBkC&pg=PA750 Page 750] (Chapter 10) in: {{cite book |authorauthor1=Dunning, Marshall Barnett; |author2=Fischbach, Frances Talaska |title=A manual of laboratory and diagnostic tests [electronic resource] |publisher=Wolters Kluwer Health/Lippincott Williams & Wilkins |___location=Philadelphia |year=2009 |pages= |isbn=0-7817-7194-3 |oclc= |doi= |accessdate=}}</ref> in effect decreasing the sensitivities and specificities of such subsequent tests. On the other hand, the effect of interference can potentially improve the efficacy of subsequent tests as compared to usage in the reference group, such as some abdominal examinations being easier when performed on underweight people.

=====Overlap of tests=====

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If only one risk factor of an individual is taken into account, the post-test probability can be estimated by multiplying the relative risk with the risk in the control group. The control group usually represents the unexposed population, but if a very low fraction of the population is exposed, then the prevalence in the general population can often be assumed equal to the prevalence in the control group. In such cases, the post-test probability can be estimated by multiplying the relative risk with the risk in the general population.

For example, the [[Incidence (epidemiology)|incidence]] of [[breast cancer]] in a woman in the United Kingdom at age 55 to 59 is estimated at approximately 280 cases per 100.000 per year,<ref name=cancerresearchuk>[http://info.cancerresearchuk.org/prod_consump/groups/cr_common/@nre/@sta/documents/generalcontent/cases_crude_breast1_xls.xls Excel chart] for ''Figure 1.1: Breast Cancer (C50), Average Number of New Cases per Year and Age-Specific Incidence Rates, UK, 2006-2008'' at [http://info.cancerresearchuk.org/cancerstats/types/breast/incidence/ Breast cancer - UK incidence statistics] at Cancer Research UK. Section updated 18/07/11.</ref> and the risk factor of having been exposed to high-dose [[ionizing radiation]] to the chest (for example, as treatments for other cancers) confers a relative risk of breast cancer between 2.1 toand 4.0,<!--

--><ref name="acs bc facts 2005-6">{{cite web |author=ACS |year=2005 |title=Breast Cancer Facts & Figures 2005–2006 |url=http://www.cancer.org/downloads/STT/CAFF2005BrFacspdf2005.pdf |format=PDF|accessdate=2007-04-26 |archiveurl = https://web.archive.org/web/20070613192148/http://www.cancer.org/downloads/STT/CAFF2005BrFacspdf2005.pdf <!-- Bot retrieved archive --> |archivedate = 2007-06-13 |authorlink= American Cancer Society}}</ref> compared to unexposed. Because a low fraction of the population is exposed, the prevalence in the unexposed population can be assumed equal to the prevalence in the general population. Subsequently, it can be estimated that a woman in the United Kingdom that is aged between 55 and 59 and that has been exposed to high-dose ionizing radiation should have a risk of developing breast cancer over a period of one year of between 588 and 1.120 in 100.000 (that is, between 0,6% and 1.1%).