The MDC is the net concentration that has a specified chance of being detected. It is an estimate of the detection capability of a measuring protocol and is calculated before measurements are taken.
The detection limit is the lowest net response level, in counts, that you expect to be see with a fixed level of certainty, customarily 95%. The MDC is the detection limit expressed as an activity concentration. If the activity concentration in a sample is equal to the MDC, then there is a 95% chance that radioactive material in the sample will be detected.
See, Chapter 6: Field Measurement Methods and Instrumentation on the MARSSIM Manual download page.