Stool Polymerase Chain Reaction (PCR) Testing of Clostridium Difficile (C-Diff) Toxin A and Toxin B
Stool PCR testing detects the DNA genes inside the bacteria that can make toxins, not the active toxins themselves. High amounts on a PCR test mean there is more bacterial DNA detected, which does not prove that active toxin proteins are being made or causing illness.
How PCR Tests Detect C. Diff
- Gene Detection: The test looks for the specific DNA code (genes) for toxin A or toxin B.
- Bacterial Presence: It tells you that toxigenic C. diff bacteria are present in the gut.
- Not the Toxin: It does not measure the actual poison (toxin) proteins floating in the stool.
How PCR Tests Detect Toxin A vs. Toxin B
The core difference between PCR testing for the two toxins lies in the specific genetic sequences the test target, as most dangerous C. diff bacteria carry the genes to produce both toxins. However, variant bacterial strains exist that only have the gene for one of them.
- Toxin A (tcdA gene): Known as the enterotoxin, this gene codes for a toxin that primarily targets and disrupts the fluid balance in the intestinal walls.
- Toxin B (tcdB gene): Known as the cytotoxin, this gene codes for a highly potent toxin that directly destroys structural mucosal cells in the colon.
Do Different Strains Produce Different Toxins?
- Most Strains Produce Both: The majority of pathogenic C. diff bacteria are A⁺B⁺, meaning they possess the DNA instructions to manufacture both Toxin A and Toxin B simultaneously.
- The Mutant Variant (A⁻B⁺): There are clinically significant, mutant strains of C. diff that are Toxin A-negative but Toxin B-positive. These bacteria completely lack the genetic sequence to make Toxin A, but they still produce a highly virulent Toxin B that causes severe diarrhea and colitis.
- Rare Variations: It is rare to find a strain that is Toxin A-positive but Toxin B-negative (A⁺B⁻), because Toxin B is generally essential for the bacteria to cause a successful infection.
- Laboratory Testing: Some clinical laboratories utilize molecular PCR tests that explicitly target the Toxin B gene (tcdB), or multiplex assays that screen for both genes simultaneously. This guarantees that no matter which strain of toxigenic C. diff is present in your gut, the test will successfully flag its DNA.
High Amounts of C-Diff Bacteria vs. Active C-Diff Infection
- Microbe Count, Not Toxin: A higher or faster PCR signal points to a larger amount of bacterial DNA, but it still measures the microbe's genes rather than the physical toxin levels.
- Colonization vs. Infection: A person can carry these bacteria and have a positive PCR test without being sick, because the genes might not be actively turning on to make poison.
- Clinical Context: Providers must match the test result with clinical presentation and symptoms (e.g., watery diarrhea, abdominal pain and cramping, nauseas, fever, blood or pus in stool, loss of appetite, dehydration) to decide if it is a true infection needing treatment.