<P> A quantification of a target substance, a cell type or another specific entity is a common output of, for example, most blood tests . This is not only answering if a target entity is present or absent, but also how much is present . In blood tests, the quantification is relatively well specified, such as given in mass concentration, while most other tests may be quantifications as well although less specified, such as a sign of being "very pale" rather than "slightly pale". Similarly, radiologic images are technically quantifications of radiologic opacity of tissues . </P> <P> Especially in the taking of a medical history, there is no clear limit between a detecting or quantifying test versus rather descriptive information of an individual . For example, questions regarding the occupation or social life of an individual may be regarded as tests that can be regarded as positive or negative for the presence of various risk factors, or they may be regarded as "merely" descriptive, although the latter may be at least as clinically important . </P> <P> The result of a test aimed at detection of an entity may be positive or negative: this has nothing to do with a bad prognosis, but rather means that the test worked or not, and a certain parameter that was evaluated was present or not . For example, a negative screening test for breast cancer means that no sign of breast cancer could be found (which is in fact very positive for the patient). </P> <P> The classification of tests into either positive or negative gives a binary classification, with resultant ability to perform bayesian probability and performance metrics of tests, including calculations of sensitivity and specificity . </P>

Which of the following tests is the most common test used to identify blood in the field