To diagnose E. coli O157, laboratory testing of a stool specimen from an ill individual is almost always necessary. The lab test is usually a stool culture, which can readily detect E. coli O157:H7 because its unique growth characteristics can be used to differentiate it from most other types of E. coli, including the many harmless strains that commonly inhabit the human intestine. However, stool cultures cannot differentiate non-O157 STEC from the harmless E. coli and, thus, were not a useful diagnostic tool for them. National guidelines recommend that when STEC is a possible cause of illness, a stool sample should both be tested for Shiga toxin (to detect non-O157 STEC or O157) and cultured specifically for E. coli O157:H7. Serotype-specific enzyme immunoassays are also available for the detection of E. coli O157 directly from stool samples. In addition, some clinical laboratories perform H7 serology and toxin testing to confirm the presence of E. coli O157:H7.