a distribution among persons without any mycobacterial infection (dashed line), usually not exceeding 1 or 2 mm;
a distribution among persons with tuberculous infection (full line), with an expected close to normal distribution peaking at 16 to 17 mm;
a distribution among persons with infection due to environmental mycobacteria (dashed-dotted line), probably peaking at 4 to 8 mm.

The table shows the four possibilities that arise when a test (here the tuberculin skin test) with a simple categorical definition into "positive" and "negative" (defined by a cut-off point) is used to identify the presence or absence of a condition (here tuberculous infection).

The number with a false negative result is denoted as "c", the number with a false positive result as "b".

The sensitivity of the test is the proportion of persons with tuberculous infection correctly identified with a positive test: "a/(a+c)".

The specificity of the test is the proportion of persons without tuberculous infection correctly identified with a negative test: "d/(b+d)".

The predictive value of a positive test is the proportion among all with a positive result who actually have tuberculous infection: "a/(a+b)".

The predictive value of a negative test is the proportion among all with a negative result who do not have tuberculous infection: "d/(c+d)".

At the cut-off defined to denote a positive or a negative test in this example, a certain number of persons (c) has a negative test, but has actually tuberculous infection. That is the limitation in the sensitivity of the test.

At the cut-off defined to denote a positive or a negative test in this example, a certain number of persons (b) has a positive test, but has actually no tuberculous infection. That is the limitation in the specificity of the test.

As one moves the cut-off point to the left, sensitivity increases, and specificity decreases.

As one moves the cut-off point to the right, sensitivity decreases, and specificity increases.

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