The usual test to diagnose E. coli O157:H7 is a stool culture , often with selective culture methods, and many labs also use Shiga toxin testing or PCR-based NAAT as part of the workup. In some settings, the organism is first screened by immunoassay or PCR , then confirmed by culture and serotyping.

How it’s diagnosed

  • Stool culture: The classic method; the lab grows the bacteria from a stool sample and looks for O157:H7.
  • Shiga toxin test: Because O157:H7 is a shiga-toxin-producing E. coli, many labs test stool for the toxin or the toxin genes.
  • PCR / NAAT: Some labs use nucleic acid amplification tests to detect E. coli O157:H7 or shiga-toxin genes quickly.
  • Confirmatory typing: If a culture is positive, the isolate may be confirmed with serologic agglutination or other identification steps.

Practical note

A stool sample is the key specimen, and testing is best done early in the illness, before the organism disappears from the sample. If you need a one- line answer for a post or caption, use: “E. coli O157 is usually diagnosed with a stool culture, often paired with Shiga toxin testing or PCR.”

TL;DR

Stool culture is the main diagnostic test, with Shiga toxin assays and PCR/NAAT commonly used for screening or confirmation.