Introduction
A diagnostic test should be compared to a gold standard. The ideal gold standard is a widely accepted and widely used definition of the disease – for instance, the presence of above knee thrombus on ultrasound.
All diagnostic tests have performance characteristics compared to the gold standard. The table below shows how they are reported.
| Gold standard positive | Gold standard negative | Total | |
| Test positive | a | b | a+b |
| Test negative | c | d | c+d |
| Total | a+c | b+d | a+b+c+d |
This is the proportion of people who have the disease and who test positive. A test that is 100% sensitive picks up all the cases. If this test is negative, it effectively rules out a condition (SnOUT = sensitive test rules OUT).
This is the proportion of people who are disease-free who test negative. A test that is 100% specific excludes all the well people. If this test is positive, it effectively rules in a condition (SpIN = specific test rules IN).
This is the proportion of people who test positive who actually have the disease.
This is the proportion of people who test negative who actually don’t have the disease.