Presumptive tests are used at crime scenes to determine if a sample could be evidence such as blood, semen, or drugs. They are quick, cheap, and easy but not confirmatory. Confirmatory tests are used in a lab to confirm that a sample is a specific type of evidence and are more expensive and time-consuming. Investigators may use lights or chemical reagents like luminol or phenolphthalein to detect blood evidence not visible to the naked eye at a crime scene.