Early Repeat C. difficile Testing Rarely Useful

SAN FRANCISCO — Repeating a negative stool cytotoxicity assay for Clostridium difficile rarely returns a different result if done within the first 3 days, according to a retrospective study involving more than 10,000 patients.

This is true even during a C. difficile epidemic, Dr. Joe Dylewski said in an interview at the annual Interscience Conference on Antimicrobial Agents and Chemotherapy. Dr. Dylewski of St. Mary's Hospital, Montreal, presented his results during a poster session.

The study involved 10,882 patients and 16,665 stool specimens submitted for toxin testing between January 2001 and March 2008. Dr. Dylewski divided the samples into two time periods to reflect an epidemic of C. difficile–associated diarrhea that began in April 2002.

Overall, the positivity rate of the cytotoxicity assay was 27% during the epidemic and 15% during the nonepidemic period. Among samples that initially tested negative, fewer than 1% changed to positive if tests were repeated within the first day. Repeat testing of negative results yielded positive results in 6% during day 2 and in 3% during day 3.

After day 3, the percentage of positive results after initial negative findings rose to 7%, 9%, 12%, and 13% on days 4, 5, 6, and 7, respectively.

“Repeat testing during the first 3 days is probably not beneficial even during an epidemic,” Dr. Dylewski said in the interview at the meeting, sponsored by the American Society for Microbiology. He was particularly critical of repeat testing on the same day of the initial test, which occurred in 698 cases. It takes about 2 days to return a negative result from the C. difficile cytotoxicity assay, so most of these repeat stool samples were sent to the lab before results from the initial sample could have been received.

He attributed these decisions to an erroneous belief among physicians that a second test always is necessary. But even that doesn't explain why he sometimes received three samples from the same patient within that first day. “The order is 'C. diff. times three,'” he said. “So [the patient has] three bowel movements on the same day, and they send all three [samples] down on the same day.”

Of the 698 cases of same-day repeat testing, the initial test was positive in 225 cases and negative in 473. On repeat testing, no results switched from positive to negative, and only two (0.4%) switched from negative to positive.

“It makes sense to retest after 3 days if the patient is still symptomatic and your index of suspicion is high and you haven't started empiric therapy,” he said.

If, after 3 days, the result is still negative but the index of suspicion remains high, there are two ways to proceed. One is to start empiric therapy, and the other is to refer the patient to colonoscopy.

“It's a little bit different when you're in an epidemic situation … because patients get really sick really quickly,” Dr. Dylewski said. During an epidemic, patients with an initial negative result must be followed closely if the index of suspicion is high. “You still need to follow them, potentially treat empirically, or repeat the test after a certain number of days, because you can be wrong, but it's not often.”

But even during an epidemic, he advised, “it should not be routine to say, 'I'm going to do multiple specimens in order to detect it.'”

Dr. Dylewski reported no conflicts of interest related to his study.

SAN FRANCISCO — Repeating a negative stool cytotoxicity assay for Clostridium difficile rarely returns a different result if done within the first 3 days, according to a retrospective study involving more than 10,000 patients.

This is true even during a C. difficile epidemic, Dr. Joe Dylewski said in an interview at the annual Interscience Conference on Antimicrobial Agents and Chemotherapy. Dr. Dylewski of St. Mary's Hospital, Montreal, presented his results during a poster session.

The study involved 10,882 patients and 16,665 stool specimens submitted for toxin testing between January 2001 and March 2008. Dr. Dylewski divided the samples into two time periods to reflect an epidemic of C. difficile–associated diarrhea that began in April 2002.

Overall, the positivity rate of the cytotoxicity assay was 27% during the epidemic and 15% during the nonepidemic period. Among samples that initially tested negative, fewer than 1% changed to positive if tests were repeated within the first day. Repeat testing of negative results yielded positive results in 6% during day 2 and in 3% during day 3.

After day 3, the percentage of positive results after initial negative findings rose to 7%, 9%, 12%, and 13% on days 4, 5, 6, and 7, respectively.

“Repeat testing during the first 3 days is probably not beneficial even during an epidemic,” Dr. Dylewski said in the interview at the meeting, sponsored by the American Society for Microbiology. He was particularly critical of repeat testing on the same day of the initial test, which occurred in 698 cases. It takes about 2 days to return a negative result from the C. difficile cytotoxicity assay, so most of these repeat stool samples were sent to the lab before results from the initial sample could have been received.

He attributed these decisions to an erroneous belief among physicians that a second test always is necessary. But even that doesn't explain why he sometimes received three samples from the same patient within that first day. “The order is 'C. diff. times three,'” he said. “So [the patient has] three bowel movements on the same day, and they send all three [samples] down on the same day.”

Of the 698 cases of same-day repeat testing, the initial test was positive in 225 cases and negative in 473. On repeat testing, no results switched from positive to negative, and only two (0.4%) switched from negative to positive.

“It makes sense to retest after 3 days if the patient is still symptomatic and your index of suspicion is high and you haven't started empiric therapy,” he said.

If, after 3 days, the result is still negative but the index of suspicion remains high, there are two ways to proceed. One is to start empiric therapy, and the other is to refer the patient to colonoscopy.

“It's a little bit different when you're in an epidemic situation … because patients get really sick really quickly,” Dr. Dylewski said. During an epidemic, patients with an initial negative result must be followed closely if the index of suspicion is high. “You still need to follow them, potentially treat empirically, or repeat the test after a certain number of days, because you can be wrong, but it's not often.”

But even during an epidemic, he advised, “it should not be routine to say, 'I'm going to do multiple specimens in order to detect it.'”

Dr. Dylewski reported no conflicts of interest related to his study.

SAN FRANCISCO — Repeating a negative stool cytotoxicity assay for Clostridium difficile rarely returns a different result if done within the first 3 days, according to a retrospective study involving more than 10,000 patients.

This is true even during a C. difficile epidemic, Dr. Joe Dylewski said in an interview at the annual Interscience Conference on Antimicrobial Agents and Chemotherapy. Dr. Dylewski of St. Mary's Hospital, Montreal, presented his results during a poster session.

The study involved 10,882 patients and 16,665 stool specimens submitted for toxin testing between January 2001 and March 2008. Dr. Dylewski divided the samples into two time periods to reflect an epidemic of C. difficile–associated diarrhea that began in April 2002.

Overall, the positivity rate of the cytotoxicity assay was 27% during the epidemic and 15% during the nonepidemic period. Among samples that initially tested negative, fewer than 1% changed to positive if tests were repeated within the first day. Repeat testing of negative results yielded positive results in 6% during day 2 and in 3% during day 3.

After day 3, the percentage of positive results after initial negative findings rose to 7%, 9%, 12%, and 13% on days 4, 5, 6, and 7, respectively.

“Repeat testing during the first 3 days is probably not beneficial even during an epidemic,” Dr. Dylewski said in the interview at the meeting, sponsored by the American Society for Microbiology. He was particularly critical of repeat testing on the same day of the initial test, which occurred in 698 cases. It takes about 2 days to return a negative result from the C. difficile cytotoxicity assay, so most of these repeat stool samples were sent to the lab before results from the initial sample could have been received.

He attributed these decisions to an erroneous belief among physicians that a second test always is necessary. But even that doesn't explain why he sometimes received three samples from the same patient within that first day. “The order is 'C. diff. times three,'” he said. “So [the patient has] three bowel movements on the same day, and they send all three [samples] down on the same day.”

Of the 698 cases of same-day repeat testing, the initial test was positive in 225 cases and negative in 473. On repeat testing, no results switched from positive to negative, and only two (0.4%) switched from negative to positive.

“It makes sense to retest after 3 days if the patient is still symptomatic and your index of suspicion is high and you haven't started empiric therapy,” he said.

If, after 3 days, the result is still negative but the index of suspicion remains high, there are two ways to proceed. One is to start empiric therapy, and the other is to refer the patient to colonoscopy.

“It's a little bit different when you're in an epidemic situation … because patients get really sick really quickly,” Dr. Dylewski said. During an epidemic, patients with an initial negative result must be followed closely if the index of suspicion is high. “You still need to follow them, potentially treat empirically, or repeat the test after a certain number of days, because you can be wrong, but it's not often.”

But even during an epidemic, he advised, “it should not be routine to say, 'I'm going to do multiple specimens in order to detect it.'”

Dr. Dylewski reported no conflicts of interest related to his study.