In the normal course of hormone production in the human body, hormones are produced in related proportions to one another. For example, there is a "normal" proportion (ratio) between a man's test and estrogen. This type of proportional relationship has been established for many of our hormones.
One of the common methods used to assess whether or not a person is using exogenous testosterone is the ratio of testosterone to epitestosterone.
The normal ratio for this should be somewhere between 1:1 - 2:1.
For testing purposes, if a person has test:epitest ratio great than 4:1, we would flag him and say he must be using exogenous test because his ratio is beyond the "normal" expected ratio.
Some liberal testing ranges allow a ratio of test:epitest to be as high as 6:1 before claiming a positive test.
20:1 is off the chart high.