Sensitivity, Specificity, Predictive Values, Pre/Post-test Probability, and Likelihood Ratios explained

Sensitivity, Specificity, Predictive Values, Pre:Post-test Probability, and Likelihood Ratios explained

The purpose of this post is to explain the concept of sensitivity, specificity, predictive values, and likelihood ratios.

Screening Tests

Screening tests (surveillance tests) are tools use to assess the likelihood that a patient may have a certain disease.  They are not definitive, but if positive, will heighten suspicion that would warrant use of a gold standard diagnostic test to rule in or rule out a certain diagnosis.  The goal of screening tests is to reduce the morbidity and mortality in a population group (Maxim, Niebo, & Utell, 2014). Examples of screening tests include routine EKGs, PSA, PAP smears, and mammograms.  For example, a male with an elevated PSA may have prostate cancer, BPH, or prostatitis. Positive results of screening tests need to be compared to the established gold standard test that is regarded as definitive.  In this case, a prostate biopsy is considered a definitive test, as it will reveal the etiology of the elevated PSA. Screening tests are less invasive and less costly, whereas the gold standard test may be more invasive, expensive, or too late (discovered during an autopsy).  Ideally, gold standard tests, such as coronary angiography, breast biopsy, or colposcopy should have 100% sensitivity and specificity. However, in reality, this may not be the case, as it may be the best test given the clinical picture at the time (Maxim, Niebo, & Utell, 2014).  

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