Presumptive tests, in medical and forensic science, analyze a sample and establish one of the following:
#The sample is definitely not a certain substance.
#The sample probably is the substance.
For example, the Kastle–Meyer test will show either that a sample is not blood or that the sample is probably blood but may be a less common substance. Further chemical tests are needed to prove that the substance is blood.
Confirmatory tests are the tests required to confirm the analysis. Confirmatory tests cost more than simpler presumptive tests so presumptive tests are often done to see if confirmatory tests are necessary.  
Similarly, in medicine, a presumptive diagnosis identifies the likely condition of a patient, and a confirmatory diagnosis is needed to confirm the condition.