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Test combinations

If one uses more than one test, they can be combined in two ways: in parallel or sequentially.

Parallel tests:

Two or more independent tests with good specificity are undertaken at the same time. The diagnosis is positive when at least one of the tests gives a positive result.
The goal is to increase the sensitivity of the diagnosis and/or to optimise the predictive value for a negative result.

Sequential tests:

First test: screening test (with higher sensitivity), i.e. with few false-negative results.
Second test: confirmatory test (with higher specificity), i.e. with few false-positive results.
The goal is to optimise the positive predictive value.

The graph below shows the combination of a screening test with a confirmatory test. Note the different values for sensitivity (A) and specificity (B).

Advantage: the predictive value is much higher than when performing the second test alone (0.98 in comparison to 0.83 for test 2)!

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