Forensic biology encompasses the examination of evidentiary items from crime scenes for biological fluids, often identifying the specific biological fluid present and developing a DNA profile that can be used to link a suspect to a crime. Blood identification consists of visual examination, presumptive tests based on the catalytic activity of hemoglobin, and confirmatory tests based on antigen-antibody interactions. Issues encountered in blood identification include the occurrence of false positive and false negative results. Many causes of these results are well-known but more recently three substances resulting in false negatives with catalytic color tests, chemiluminescent reagents, and immunoassays have been explored. Quebracho extract (a common leather tannin), sodium percarbonate (the main component of detergents containing active oxygen) and vitamin C-containing beverages were all found to produce false negative results at varying degrees with each of the tests mentioned. Increased knowledge of potential negative interfering agents by forensic investigators can help ensure that probative evidence is properly collected and thoroughly analyzed from a crime scene.