Testing for Clostridium difficile toxin with real-time polymerase chain reaction (PCR) can improve the laboratory diagnoses of C difficile–associated diarrhea, compared with A/B enzyme immunoassay (EIA), according to results from a study presented at a Association for Molecular Pathology meeting.
In fact, real-time PCR demonstrated a sensitivity of 100% and a specificity of 96.9%, whereas the A/B EIA was found to generate both false-positive and false-negative results.
“Although C difficile–associated diarrhea has increased in prevalence and severity, the inaccuracy of conventional C difficile toxin EIA for diagnosis is well documented in the literature,” said lead investigator Cynthia Essmyer, MD, medical pathologist at Saint Luke’s Hospital in Kansas City, Missouri. (1)
The Microbiology Laboratory medical pathologist at a University Hospital located in NC shared the following information to answer one of the many questions: “Why won’t the lab run a test on hard formed stool?”
The lab will not accept a stool specimen that is hard/formed stool due to the fact that the PCR test is regulated by the testing company and a hard/formed stool can produce a false positive result. Soft, unformed or watery stools are all accepted for the PCR testing.
- ACCEPTABLE SPECIMEN: FecesCollect specimen as follows:
1. Collect feces in a clean, dry container or bedpan not contaminated with urine, residual soap, or disinfectants.
2. Transfer appropriate volume of feces to a clean dry tightly capped specimen cup.
3. Submit double-bagged specimen cup immediately to laboratory.
1. Patients should be passing at least 3 unformed or watery stool specimens in a 24-hour period. Most patients have more than 3 episodes of watery unformed stools per day.
2. Soft specimen is defined as specimen assuming the shape of its container (unformed).
3. Formed or hard fecal specimens will not be tested.
4. Repeat testing following a negative test during the same episode of diarrhea is NOT recommended for at least 7-days because of high sensitivity (between 98-99% and with a 99% negative predictive value).
* There may be an alternate send out micro. test for hard formed stool and this information can be obtained at the laboratory being utilized. Have a healthcare provider contact the lab prior to delivering a specimen to the lab location.
When a PCR result is NEGATIVE: the Wait time between testing is 7 days to retest.
When a PCR result is POSITIVE: the Wait time between testing is 14 days to retest.
Q: Is there a wait time after completing antibiotic treatment (greater than 14 days since first positive result) to retest?
A: No, since the PCR is testing micro-organisms the antibiotic will not interfere with testing.
Q: Can a stool specimen be kept in a refrigerator and if so for how long?
A specimen can be rejected by the lab if Specimen is received > 2 hours after collection unless submitted on ice or cold pack or with note that specimen stored in refrigerator prior to transport.
Q: How long does it take to receive lab results?
A: Routine turn around time is one day.
Source: (1) Medscape
other: University Hospital Microbiology Laboratory in NC